tag:blogger.com,1999:blog-44462926663983443822014-11-19T09:00:13.430-08:00Machined LearningsI'm all about that Bayes.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.comBlogger147125tag:blogger.com,1999:blog-4446292666398344382.post-15398681945928619582014-11-16T17:36:00.000-08:002014-11-19T09:00:13.488-08:00Large Scale CCAThere's plenty of unlabeled data, so lately I've been spending more time with unsupervised methods. <a href="http://lowrank.net/nikos/index.html">Nikos</a> and I have spent some time with <a href="http://en.wikipedia.org/wiki/Canonical_correlation">CCA</a>, which is akin to SVD but assumes a bit of structure on the data. In particular, it assumes there are two (or more) views of the data, where a view is basically a set of features. A <a href="http://www.stat.berkeley.edu/~jordan/688.pdf">generative interpretation</a> is that the features are conditionally Gaussian distributed given an unobserved latent variable. The numerical linear algebra interpretation is that we are trying to solve the following optimization problem: given two views $\mathbf{A} \in \mathbb{R}^{n \times d_a}$ and $\mathbf{B} \in \mathbb{R}^{n \times d_b}$, the CCA projections $\mathbf{X}_a \in \mathbb{R}^{d_a \times k}$ and $\mathbf{X}_b \in \mathbb{R}^{d_b \times k}$ are the solution to \[<br />\begin{aligned}<br />\mathop{\mathrm{maximize}}_{ \mathbf{X}_a , \mathbf{X}_b }& \mathop{\mathrm{Tr}} \left( \mathbf{X}_a^\top \mathbf{A}^\top \mathbf{B} \mathbf{X}_b \right), \nonumber \\<br />\mathrm{subject\ to}\;& \mathbf{X}_a^\top \mathbf{A}^\top \mathbf{A} \mathbf{X}_a = n \mathbf{I}, \\<br />\;& \mathbf{X}_b^\top \mathbf{B}^\top \mathbf{B} \mathbf{X}_b = n \mathbf{I}.<br />\end{aligned}<br />\] <br />For “small data”, CCA can be solved using SVD, and we have good randomized methods for SVD which work great in the distributed context. So why don't we have good randomized methods for CCA? Basically, the constraints make CCA into something like a generalized eigenvalue problem, albeit with two denominators. In fact, for two view data, CCA can be reduced to a pair of generalized eigenvalue problems, \[<br />\mathbf{A}^\top \mathbf{B} (\mathbf{B}^\top \mathbf{B})^{-1} \mathbf{B}^\top \mathbf{A} \mathbf{X}_a = \mathbf{A}^\top \mathbf{A} \mathbf{X}_a \Lambda_a,<br />\] with an analogous problem to find $\mathbf{X}_b$. We have <a href="http://arxiv.org/abs/1307.6885">randomized square-root free algorithms</a> for generalized eigenvalue problems, so problem solved, right? Yes, with important caveats. First, the spectrum is unfavorable so the randomized range finder will require many passes or lots of oversampling. Second, range finding involves computing the action of $ (\mathbf{B}^\top \mathbf{B})^{-1}$ on $\mathbf{B}^\top \mathbf{A} \Omega$ and vice versa, which is a least squares problem (and in practice <a href="http://arxiv.org/abs/1407.4508">need not be computed extremely accurately</a>). Third, the pair of generalized eigenvalue problems share significant state so interleaving the operations is beneficial. With these observations, we ended up with something that was very close to a classic algorithm for computing CCA called <a href="http://www4.ncsu.edu/~mtchu/Research/Lectures/natalk_multvariate.pdf">Horst iteration</a>, but with the Halko-style flair of “oversample, early-stop, and then polish up with an exact solution in the smaller subspace.” We've had good luck with this method, which is on github as <span style="font-family: monospace;"><a href="https://github.com/pmineiro/cca/blob/master/alscca.m">alscca.m</a></span>.<br /><br />Furthermore, it turns out that you can sometimes avoid least squares entirely: during range finding you can approximate the inverse covariance as scaled identity matrix, and compensate with (lots of) additional oversampling. If you would have used a large regularizer then this works well, and the overhead of oversampling is compensated for by the simplicity of the computation (especially in the distributed context). Essentially we are restricting the optimization to the top spectrum of $\mathbf{A}^\top \mathbf{B}$, and this <a href="http://arxiv.org/pdf/1411.3409v1.pdf">can yield good results</a>. This is available on github as <span style="font-family: monospace;"><a href="https://github.com/pmineiro/cca/blob/master/rcca.m">rcca.m</a></span>.<br /><br />CCA is versatile: one application is to <a href="http://papers.nips.cc/paper/4193-multi-view-learning-of-word-embeddings-via-cca.pdf">create word embeddings</a>, similar in spirit to <a href="https://code.google.com/p/word2vec/">word2vec</a>. As a demo, we took the American English Google n-grams corpus and used CCA to create embeddings. Matlab on a commodity desktop takes about an hour to produce the embedding, which is faster than the many hours it takes to download the data. The code to reproduce can be found on <a href="http://github.com/pmineiro/cca">github</a> (warning: you'll need about 40 gigabytes of memory, 50 gigabytes of disk space, and bandwidth to download the n-grams). You can verify that the embedding satisfies the “ultimate test” of word embeddings: <span style="font-family: monospace;">king - queen $\approx$ man - woman</span>.<br />Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-58662548270693945712014-10-21T19:11:00.000-07:002014-10-21T19:11:24.180-07:00A Posthumous RebutalA recently published piece by Isaac Asimov titled <a href="http://www.technologyreview.com/view/531911/isaac-asimov-mulls-how-do-people-get-new-ideas/">On Creativity</a> partially rebuts <a href="http://www.machinedlearnings.com/2014/10/costs-and-benefits.html">my previous post</a>. Here's a key excerpt:<br /><blockquote>To feel guilty because one has not earned one’s salary because one has not had a great idea is the surest way, it seems to me, of making it certain that no great idea will come in the next time either.<br /></blockquote>I agree with all of Azimov's essay. It resonates truth according to my experience, e.g., I'm most productive collaborating with people in front of whom I am not afraid to look stupid.<br /><br />So how to square this with the reality that research is funded by people who care, to some degree, about ``return on investment''?<br /><br />I'm not entirely sure, but I'll make a pop culture analogy. I'm currently enjoying the series <a href="http://en.wikipedia.org/wiki/The_Knick">The Knick</a>, which is about the practice of medicine in the early part of the 20th century. In the opening scene, the doctors demonstrate an operation in a teaching operating theatre, using the scholarly terminology and methods of the time. The patient dies, as all patients did at that time, because the mortality rate of placenta previa surgery at the time was <a href="https://twitter.com/AtTheKnick/status/506594742399160320">100%</a>. Over time procedures improved and mortality rates are very low now, but at the time, doctors just didn't know what they were doing. The scholarly attitude was one way of signalling ``we are trying our best, and we are striving to improve''.<br /><br />We still don't know how to reliably produce ``return on investment'' from industrial research. Azimov's point is that many mechanisms proposed to make research more productive actually do the opposite. Thus, the way forward is unclear. The best idea I have at the moment is just to conduct myself professionally and look for opportunities to provide value to my employer, while at the same time pushing in directions that I think are interesting and which can plausibly positively impact the business within a reasonable time frame. Machine learning is highly practical at this particular moment so this is not terribly difficult, but this balancing act will be much tougher for researchers in other areas.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-12003574907319054992014-10-16T21:35:00.001-07:002014-10-21T08:38:17.857-07:00Costs and Benefitstl;dr: If you love research, and you are a professional researcher, you have a moral obligation to make sure your benefactor both receives some benefit from your research and is aware of the benefit.<br /><br />I love research. Great research is beautiful in at least two ways. First, it reveals truths about the world we live in. Second, it exhibits the inherent beauty of peak human performance. A great researcher is beautiful in the same way a great artist or athlete is beautiful. (Noah Smith <a href="http://www.bloombergview.com/articles/2014-10-13/nobel-for-charles-barkley-of-economics">apparently agrees</a>.) Unfortunately, a <a href="http://www.statista.com/statistics/197401/nfl-regular-season-home-attendance-of-the-seattle-seahawks-since-2006/">half million people</a> will not pay for tickets to watch great researchers perform their craft, so other funding vehicles are required.<br /><br />Recent events have me thinking again about the viability of privately funded basic research. In my opinion, the history of Xerox PARC is deeply troubling. What?! At it's peak the output of Xerox PARC was breathtaking, and many advances in computation that became widespread during my youth <a href="http://www.computerworld.com/article/2515874/computer-hardware/timeline--parc-milestones.html">can be traced to Xerox PARC</a>. Unfortunately, Xerox did not benefit from some of the most world-changing innovations of their R&D department. Now a generation of MBAs are told about <a href="http://www.apqc.org/blog/why-cisco-builds-buys-and-partners-its-way-innovation">the Cisco model</a>, where instead of having your own research department, you wait for other firms to innovate and then buy them.<br /><blockquote><a href="http://books.google.com/books?id=jX7RXTi8MTEC&pg=PA195&lpg=PA195">... it continues to buy small, innovative firms rather than develop new technology from scratch ...</a></blockquote>To be clear my employer, Microsoft, still shows a strong commitment to basic research. Furthermore, recent research layoffs at Microsoft were not related to research quality, or to the impact of that research on Microsoft products. <i>This post is not about Microsoft, it is about the inexorable power of incentives and economics.</i> <br /><br />Quite simply, it is irrational to expect any institution to fund an activity unless that organization can realize sufficient benefit to cover the costs. That calculation is ultimately made by people, and if those people only hear stories about how basic research generates benefits to other firms (or even, competitors!), appetite will diminish. In other words, benefits must not only be real, they must be recognizable to decision makers. This is, of course, a deep challenge, because the benefits of research are often not recognizable to the researchers who perform it. Researchers are compelled to research by their nature, like those who feel the need to scale Mount Everest. It so happens that a byproduct of their research obsession is the advancement of humanity.<br /><br />So, if you are a professional researcher, it follows logically that as part of your passion for science and the advancement of humanity, you should strive to make the benefits of your activity salient to whatever institutions support you, because you want your funding vehicle to be long-term viable. Furthermore, let us recognize some great people: the managers of research departments who constantly advocate for budget in the boardroom, so that the people in their departments can do great work.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-9165457879043960272014-09-24T09:20:00.000-07:002014-10-21T08:38:33.035-07:00Sub-Linear DebuggingI have a post on sub-linear debugging on <a href="http://blogs.technet.com/b/machinelearning/">Microsoft's machine learning blog</a>.<br /><blockquote>Online learning algorithms are a class of machine learning (ML) techniques that consume the input as a stream and adapt as they consume input. They are often used for their computational desirability, e.g., for speed, the ability to consume large data sets, and the ability to handle non-convex objectives. However, they come with another useful benefit, namely “sub-linear debugging”.<br /></blockquote>If you are interested in hitting the <a href="http://haltandcatchfire.wikia.com/wiki/Doherty_Threshold">Doherty threshold</a> in Machine Learning, read the <a href="http://blogs.technet.com/b/machinelearning/archive/2014/09/24/online-learning-and-sub-linear-debugging.aspx">whole thing</a>!Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-83746909512343107372014-08-26T19:47:00.000-07:002014-10-21T08:39:01.287-07:00More Deep Learning Musings<div class="separator" style="clear: both; text-align: center;"><a href="http://4.bp.blogspot.com/-sLgBLS3bJO8/U__Kx6PnkRI/AAAAAAAAAWM/Gz3cL5jiwTo/s1600/godeeper.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://4.bp.blogspot.com/-sLgBLS3bJO8/U__Kx6PnkRI/AAAAAAAAAWM/Gz3cL5jiwTo/s320/godeeper.jpg" /></a></div><br />Yoshua Bengio, one of the luminaries of the deep learning community, gave multiple talks about deep learning at ICML 2014 this year. I like Bengio's focus on the statistical aspects of deep learning. Here are some thoughts I had in response to his presentations.<br /><br /><h4>Regularization via Depth</h4>One of Bengio's talking points is that depth is an effective regularizer. The argument goes something like this: by composing multiple layers of (limited capacity) nonlinearities, the overall architecture is able to explore an interesting subset of highly flexible models, relative to shallow models of similar leading order flexibility. Interesting here means that the models have sufficient flexibility to model the target concept but are sufficiently constrained to be learnable with only moderate data requirements. This is really a claim about the kinds of target concepts we are trying to model (e.g., in Artificial Intelligence tasks). Another way to say this is (paraphrasing) “looking for regularizers which are more constraining than smoothness assumptions, but which are still broadly applicable across tasks of interest.”<br /><br />So is it true?<br /><br />As a purely mathematical statement it is definitely true that composing nonlinearities through bottlenecks leads to a subset of larger model space. For example, composing order $d$ polynomial units in a deep architecture with $m$ levels results in something whose leading order terms are monomials of order $m d$; but many of the terms in a full $m d$ polynomial expansion (aka “shallow architecture”) are missing. Thus, leading order flexibility, but a constrained model space. However, does this matter? <br /><br />For me the best evidence comes from that old chestnut MNIST. For many years the Gaussian kernel yielded better results than deep learning on MNIST among solutions that did not exploit spatial structure. Since the discovery of dropout this is no longer true and one can see a gap between the Gaussian kernel (at circa 1.2% test error) and, e.g., maxout networks (at 0.9% test error). The Gaussian kernel essentially works by penalizing all function derivatives, i.e., enforcing smoothness. Now it seems something more powerful is happening with deep architectures and dropout. You might say, “hey 1.2% vs. 0.9%, aren't we splitting hairs?” but I don't think so. I suspect something extra is happening here, but that's just a guess, and I certainly don't understand it.<br /><br />The counterargument is that, to date, the major performance gains in deep learning happen when the composition by depth is combined with a decomposition of the feature space (e.g., spatial or temporal). In speech the Gaussian kernel (in the highly scalable form of random fourier features) is able to <a href="http://www.ifp.illinois.edu/~huang146/papers/Kernel_DNN_ICASSP2014.pdf">approach the performance of deep learning on TIMIT</a>, if the deep net cannot exploit temporal structure, i.e., RFF is competitive with non-convolutional DNNs on this task, but is surpassed by convolutional DNNs. (Of course, from a computational standpoint, a deep network starts to look downright parsimonious compared to hundreds of thousands of random fourier features, but we're talking statistics here.)<br /><br /><h4>The Dangers of Long-Distance Relationships</h4>So for general problems it's not clear that ``regularization via depth'' is obviously better than general smoothness regularizers (although I suspect it is). However for problems in computer vision it is intuitive that deep composition of representations is beneficial. This is because the spatial domain comes with a natural concept of neighborhoods which can be used to beneficially limit model complexity.<br /><br />For a task such as natural scene understanding, various objects of limited spatial extent will be placed in different relative positions on top of a multitude of backgrounds. In this case some key aspects for discrimination will be determined by local statistics, and others by distal statistics. However, given a training set consisting of 256x256 pixel images, each example in the training set provides one realization of a pair of pixels which are offset by 256 pixels down and to the right (i.e., the top-left bottom-right pixel). In contrast each example provides $252^2$ realizations of a pair of pixels which are offset by 4 pixels down and to the right. Although these realizations are not independent, for images of natural scenes at normal photographic scales, there is much more data about local dependencies than distal dependencies per training example. This indicates that, statistically speaking, it is safer to attempt to estimate highly complex relationships between nearby pixels, but that long-range dependencies must be more strongly regularized. Deep hierarchical architectures are a way to achieve these dual objectives.<br /><br />One way to appreciate the power of this prior is to note that it applies to model classes not normally associated with deep learning. On the venerated MNIST data set, a Gaussian kernel least squares achieves 1.2% test error (with no training error). Dividing each example into 4 quadrants, computing a Gaussian kernel on each quadrant, and then computing Gaussian kernel least squares on the resulting 4-vectors achieves 0.96% test error (with no training error). The difference between the Gaussian kernel and the “deep” Gaussian kernel is that the ability to model distal pixel interactions is constrained. Although I haven't tried it, I'm confident that a decision tree ensemble could be similarly improved, by constraining every path from a root to a leaf to involve splits over pixels which are spatially adjacent.<br /><br /><h4>It's a Beautiful Day in the Neighborhood</h4>The outstanding success of hard-wiring hierarchical spatial structure into a deep architecture for computer vision has motivated the search for similar concepts of local neighborhoods for other tasks such as speech recognition and natural language processing. For temporal data time provides a natural concept of locality, but for text data the situation is more opaque. Lexical distance in a sentence is only a moderate indicator of semantic distance, which is why much of NLP is about uncovering latent structure (e.g., topic modeling, parsing). One line of active research synthesizes NLP techniques with deep architectures hierarchically defined given a traditional NLP decomposition of the input. <br /><br />Another response to the relative difficulty of articulating a neighborhood for text is to ask “can I learn the neighborhood structure instead, just using a general deep architecture?” There is a natural appeal of learning from scratch, especially when intuition is exhausted; however in vision it is currently necessary to hard-wire spatial structure into the model to get anywhere near state of the art performance (given current data and computational resources).<br /><br />Therefore it is an open question to what extent a good solution to, e.g., machine translation, will involve hand-specified prior knowledge versus knowledge derived from data. This sounds like the old “nature vs. nuture” debates from cognitive science, but I suspect more progress will be made on this question, as now the debate is informed by actual attempts to engineer systems that perform the tasks in question.<br />Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-46790582306539151182014-06-30T09:55:00.000-07:002014-10-21T08:42:35.526-07:00ICML 2014 ReviewICML 2014 went well, kudos to the organizers. The location (Beijing) and overlap with CVPR definitely impacted the distribution of attendees, so the conference felt different than last year. (I also learned that my blog is blocked from China, collateral damage from some kind of spat between Google and the Chinese government).<br /><br />Deep learning was by far the most popular conference track, to the extent that the conference room for this track was overwhelmed and beyond standing room only. I missed several talks I wanted to attend because there was no physical possibility of entrance. This is despite the fact that many deep learning luminaries and their grad students were at CVPR. Fortunately Yoshua Bengio chose ICML and via several talks provided enough insight into deep learning to merit another blog post. Overall the theme is: having conquered computer vision, deep learning researchers are now turning their attention to natural language text, with some notable early successes, e.g., <a href="http://cs.stanford.edu/~quocle/paragraph_vector.pdf ">paragraph vector</a>. And of course the brand is riding high, which explains some of the paper title choices, e.g., “<a href="http://jmlr.org/proceedings/papers/v32/cortesb14.pdf">deep boosting</a>”. There was also a conference track titled “Neural Theory and Spectral Methods” ... interesting bedfellows!<br /><br />ADMM suddenly became popular (about 18 months ago given the latency between idea, conference submission, and presentation). By this I don't mean using ADMM for distributed optimization, although there was a bit of that. Rather there were several papers using ADMM to solve constrained optimization problems that would otherwise be vexing. The take-home lesson is: before coming up with a customized solver for whatever constrained optimization problem which confronts you, try ADMM.<br /><br />Now for the laundry list of papers (also note the papers described above):<br /><ol><li> <a href="http://arxiv.org/abs/1402.0929">Input Warping for Bayesian Optimization of Non-stationary Functions</a>. If you want to get the community's attention, you have to hit the numbers, so don't bring a knife to a gunfight.</li><li> <a href="http://www.cs.utexas.edu/~cjhsieh/nuclear_icml_cameraready.pdf">Nuclear Norm Minimization via Active Subspace Selection</a>. The inimitable Cho-Jui Hsieh has done it again, this time applying ideas from active variable methods to nuclear norm regularization.</li><li> <a href="http://arxiv.org/abs/1402.0555">Taming the Monster: A Fast and Simple Algorithm for Contextual Bandits</a>. A significant improvement in the computational complexity required for agnostic contextual bandits.</li><li> <a href="http://arxiv.org/abs/1406.1837">Efficient programmable learning to search</a>. Additional improvements in imperative programming since NIPS. If you are doing structured prediction, especially in industrial settings where you need to put things into production, you'll want to investigate this methodology. First, it eases the burden of specifying a complicated structured prediction task. Second, it reduces the difference between training and evaluation, which not only means faster deployment, but also less defects introduced between experiments and the production system.</li><li> <a href="http://jmlr.org/proceedings/papers/v32/yangb14.pdf">Quasi-Monte Carlo Feature Maps for Shift-Invariant Kernels</a>. It is good to confirm quasi-random numbers can work better for randomized feature maps.</li><li> <a href="http://jmlr.org/proceedings/papers/v32/yib14.pdf">A Single-Pass Algorithm for Efﬁciently Recovering Sparse Cluster Centers of High-dimensional Data</a>. I'll need to spend some quality time with this paper.</li><li> <a href="http://people.cs.uchicago.edu/~risi/papers/KondorTenevaGargMMF.pdf">Multiresolution Matrix Factorization</a>. Nikos and I have had good luck learning discriminative representations using classical matrix decompositions. I'm hoping this new decomposition technique can be analogously adapted.</li><li> <a href="http://jmlr.org/proceedings/papers/v32/bachman14.pdf">Sample-based Approximate Regularization</a>. I find data-dependent regularization promising (e.g., dropout on least-squares is equivalent to a scale-free L2 regularizer), so this paper caught my attention.</li><li> <a href="http://cs.stanford.edu/~pliang/papers/eg-icml2014.pdf">Adaptivity and Optimism: An Improved Exponentiated Gradient Algorithm</a>. No experiments in the paper, so maybe this is a ``pure theory win'', but it looks interesting.</li></ol>Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-25620566042875471102014-06-16T10:21:00.000-07:002014-10-21T08:41:27.041-07:00Microsoft starts an ML blog and an ML productMy employer, Microsoft, has started a <a href="http://blogs.technet.com/b/machinelearning/">new blog around ML</a> and also announced a <a href="http://blogs.technet.com/b/microsoft_blog/archive/2014/06/16/microsoft-azure-machine-learning-combines-power-of-comprehensive-machine-learning-with-benefits-of-cloud.aspx">new product for ML</a>.<br /><br />The blog is exciting, as there are multiple ML luminaries at Microsoft who will hopefully contribute. <a href="https://www.linkedin.com/pub/joseph-sirosh/1/3b/398">Joseph Sirosh</a> is also involved so there will presumably be a healthy mix of application oriented content as well.<br /><br />The product is also exciting. However if you are an ML expert already comfortable with a particular toolchain, you might wonder why the world needs this product. Those who work at large companies like Microsoft, Google, Facebook, or Yahoo are presumably aware that there is an army of engineers who maintain and improve the systems infrastructure underlying the data science (e.g., data collection, ingest and organization; automated model retraining and deployment; monitoring and quality assurance; production experimentation). However if you've never worked at a startup then you aren't really aware of how much work all those people are doing to enable data science. If those functions become available as part of a service offering, than an individual data scientist with a hot idea has a chance of competing with the big guys. More realistically, given my experience at startups, the individual data scientist will have a chance to determine that their hot idea is not so hot before having to invest large amount of capital developing infrastructure :)<br /><br />Of course there is a lot more that has to happen for “Machine Learning as a Service” to be fully mature, but this product announcement is a nice first step.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-75037083505602923072014-05-03T15:10:00.000-07:002014-10-21T08:40:16.632-07:00The Most Causal Observer<a href="https://twitter.com/davidchungpark">David K. Park</a> recently had a <a href="http://andrewgelman.com/2014/04/27/big-data-big-deal-maybe-used-caution/">guest post</a> on Gelman's blog. You should read it. The <a href="http://www.urbandictionary.com/define.php?term=tl%3Bdr">tl;dr</a> is ``Big Data is a Big Deal, but causality is important and not the same as prediction.''<br /><br />I agree with the basic message: <i>causality is important</i>. As a bit of career advice, if you are just starting your career, focusing on causality would be a good idea. Almost never does one put together a predictive model for predictive purposes; rather, the point is to <i>suggest an intervention</i>. For example, why predict the fraud risk of a credit card transaction? Presumably the goal is to decline some transactions. When you do this, things change. Most simply, if you decline a transaction you do not learn about the counterfactual of what would have happened had you approved the transaction. Additional issues arise because of the adversarial nature of the problem, i.e., fraudsters will react to your model. Not paying attention to these effects will cause unintended consequences.<br /><br />However I have reservations with the idea that ``creative humans who need to think very hard about a problem and the underlying mechanisms that drive those processes'' are necessarily required to ``fulfill the promise of Big Data''. When I read those words, I translate it as ``strong structural prior knowledge will have to be brought to bear to model causal relationships, despite the presence of large volumes of data.'' That statement appears to leave on the table the idea that Big Data, gathered by Big Experimentation systems, will be able to discover casual relationships in an agnostic fashion. Here ``agnostic'' basically means ``weak structural assumptions which are amenable to automation.'' Of course there are always assumptions, e.g., when doing Vapnik-style ERM, one makes an iid assumption about the data generating process. The question is whether humans and creativity will be required.<br /><br />Perhaps a better statement would be ``creative humans will be required to fulfill the promise of Big Observational Data.'' I think this is true, and the social sciences have been working with observational data for a while, so they have relevant experience, insights, and training to which we should pay more attention. Furthermore another reasonable claim is that ``Big Data will be observational for the near future.'' Certainly it's easy to monitor a Twitter firehouse, whereas it is completely unclear to me how an experimentation platform would manipulate Twitter to determine causal relationships. Nonetheless I think that automated experimental design at a massive scale has enormous disruptive potential.<br /><br />The main difference I'm positing is that Machine Learning will increasingly move from working with a pile of data generated by another process to driving the process that gathers the data. For computational advertising this is already the case: advertisements are placed by balancing exploitation (making money) and exploration (learning about what ads will do well under what conditions). Contextual bandit technology is already mature and Big Experimentation is not a myth, it happens every day. One could argue that advertising is a particular application vertical of such extreme economic importance that creative humans have worked out a structural model that allows for causal reasoning, c.f., <a href="http://arxiv.org/abs/1209.2355">Bottou et. al.</a> I would say this is correct, but perhaps just an initial first step. For prediction we no longer have to do parametric modeling where the parameters are meaningful: nowadays we have lots of models with essentially nuisance parameters. Once we have systems that are gathering data and well as modeling it, will it be required to have strong structural models with meaningful parameters, or will there be some agnostic way of capturing a large class of casual relationships with enough data <i>and</i> experimentation?<br /><br /><br /><br />Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-54359854600908675172014-04-25T08:36:00.000-07:002014-10-21T08:40:59.539-07:00A Discriminative Representation Learning TechniqueNikos and I have developed a technique for learning discriminative features using numerical linear algebra techniques <a href="http://arxiv.org/abs/1310.1934">which gives good results</a> for some problems. The basic idea is as follows. Suppose you have a multiclass problem, i.e., training data of the form $S = \{ (x, y) | x \in \mathbb{R}^d, y \in \{ 1, \ldots, k \} \}$. Here $x$ is the original representation (features) and you want to learn new features that help your classifier. In deep learning this problem is tackled by defining a multi-level parametric nonlinearity of $x$ and optimizing the parameters. Deep learning is awesome but the resulting optimization problems are challenging, especially in the distributed setting, so we were looking for something more computationally felicitous. <br /><br />First consider the two class case. Imagine looking for features of the form $\phi (w^\top x)$, where $w \in \mathbb{R}^d$ is a “weight vector” and $\phi$ is some nonlinearity. What is a simple criterion for defining a good feature? One idea is for the feature to have small average value on one class and large average value on another. Assuming $\phi$ is non-negative, that suggests maximizing the ratio \[<br />w^* = \arg \max_w \frac{\mathbb{E}[\phi (w^\top x) | y = 1]}{\mathbb{E}[\phi (w^\top x) | y = 0]}.<br />\] For the specific choice of $\phi (z) = z^2$ this is tractable, as it results in a <a href="http://en.wikipedia.org/wiki/Rayleigh_quotient#Generalization">Rayleigh quotient</a> between two class-conditional second moments, \[<br />w^* = \arg \max_w \frac{w^\top \mathbb{E}[x x^\top | y = 1] w}{w^\top \mathbb{E}[x x^\top | y = 0] w},<br />\] which can be solved via generalized eigenvalue decomposition. Generalized eigenvalue problems have been extensively studied in machine learning and elsewhere, and the above idea looks very similar to many other proposals (e.g., <a href="http://en.wikipedia.org/wiki/Linear_discriminant_analysis#Fisher.27s_linear_discriminant">Fisher LDA</a>), but it is different and more empirically effective. I'll refer you to the paper for a more thorough discussion, but I will mention that after the paper was accepted someone pointed out the similarity to <a href="http://en.wikipedia.org/wiki/Common_spatial_pattern">CSP</a>, which is a technique from time-series analysis (c.f., <a href="http://www.bible.ca/ef/expository-ecclesiastes-1-4-11.htm">Ecclesiastes 1:4-11</a>).<br /><br />The features that result from this procedure pass the smell test. For example, starting from a raw pixel representation on <a href="http://yann.lecun.com/exdb/mnist/">mnist</a>, the weight vectors can be visualized as images; the first weight vector for discriminating 3 vs. 2 looks like<br /><div class="separator" style="clear: both; text-align: center;"><a href="http://3.bp.blogspot.com/-XBTJyt1KH04/U1qAlFbWJ_I/AAAAAAAAAV0/4GofoRD7-rg/s1600/1steig3vs2.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://3.bp.blogspot.com/-XBTJyt1KH04/U1qAlFbWJ_I/AAAAAAAAAV0/4GofoRD7-rg/s400/1steig3vs2.png" /></a></div>which looks like a pen stroke, c.f., figure 1D of <a href="http://yann.lecun.com/exdb/publis/pdf/ranzato-nips-07.pdf">Ranzato et. al.</a><br /><br />We make several additional observations in the paper. The first is that multiple isolated minima of the Rayleigh quotient are useful if the associated generalized eigenvalues are large, i.e., one can extract multiple features from a Rayleigh quotient. The second is that, for moderate $k$, we can extract features for each class pair independently and use all the resulting features to get good results. The third is that the resulting directions have additional structure which is not completely captured by a squaring non-linearity, which motivates a (univariate) basis function expansion. The fourth is that, once the original representation has been augmented with additional features, the procedure can be repeated, which sometimes yields additional improvements. Finally, we can compose this with randomized feature maps to approximate the corresponding operations in a RKHS, which sometimes yields additional improvements. We also made a throw-away comment in the paper that computing class-conditional second moment matrices is easily done in a map-reduce style distributed framework, but this was actually a major motivation for us to explore in this direction, it just didn't fit well into the exposition of the paper so we de-emphasized it.<br /><br />Combining the above ideas, along with Nikos' preconditioned gradient learning for multiclass described in a <a href="/2014/02/the-machine-learning-doghouse.html">previous post</a>, leads to the following Matlab script, which gets 91 test errors on (permutation invariant) mnist. Note: you'll need to download <a href="http://cs.nyu.edu/~roweis/data/">mnist_all.mat</a> from Sam Roweis' site to run this.<br /><pre class="brush:matlabkey">function calgevsquared<br /><br />more off;<br />clear all;<br />close all;<br /><br />start=tic;<br />load('mnist_all.mat');<br />xxt=[train0; train1; train2; train3; train4; train5; ...<br /> train6; train7; train8; train9];<br />xxs=[test0; test1; test2; test3; test4; test5; test6; test7; test8; test9];<br />kt=single(xxt)/255;<br />ks=single(xxs)/255;<br />st=[size(train0,1); size(train1,1); size(train2,1); size(train3,1); ...<br /> size(train4,1); size(train5,1); size(train6,1); size(train7,1); ...<br /> size(train8,1); size(train9,1)];<br />ss=[size(test0,1); size(test1,1); size(test2,1); size(test3,1); ... <br /> size(test4,1); size(test5,1); size(test6,1); size(test7,1); ...<br /> size(test8,1); size(test9,1)];<br />paren = @(x, varargin) x(varargin{:});<br />yt=zeros(60000,10);<br />ys=zeros(10000,10);<br />I10=eye(10);<br />lst=1;<br />for i=1:10; yt(lst:lst+st(i)-1,:)=repmat(I10(i,:),st(i),1); lst=lst+st(i); end<br />lst=1;<br />for i=1:10; ys(lst:lst+ss(i)-1,:)=repmat(I10(i,:),ss(i),1); lst=lst+ss(i); end<br /><br />clear i st ss lst<br />clear xxt xxs<br />clear train0 train1 train2 train3 train4 train5 train6 train7 train8 train9<br />clear test0 test1 test2 test3 test4 test5 test6 test7 test8 test9<br /><br />[n,k]=size(yt);<br />[m,d]=size(ks);<br /><br />gamma=0.1;<br />top=20;<br />for i=1:k<br /> ind=find(yt(:,i)==1);<br /> kind=kt(ind,:);<br /> ni=length(ind);<br /> covs(:,:,i)=double(kind'*kind)/ni;<br /> clear ind kind;<br />end<br />filters=zeros(d,top*k*(k-1),'single');<br />last=0;<br />threshold=0;<br />for j=1:k<br /> covj=squeeze(covs(:,:,j)); l=chol(covj+gamma*eye(d))';<br /> for i=1:k<br /> if j~=i<br /> covi=squeeze(covs(:,:,i));<br /> C=l\covi/l'; CS=0.5*(C+C'); [v,L]=eigs(CS,top); V=l'\v;<br /> take=find(diag(L)>=threshold);<br /> batch=length(take);<br /> fprintf('%u,%u,%u ', i, j, batch);<br /> filters(:,last+1:last+batch)=V(:,take);<br /> last=last+batch;<br /> end<br /> end<br /> fprintf('\n');<br />end<br /><br />clear covi covj covs C CS V v L<br /><br />% NB: augmenting kt/ks with .^2 terms is very slow and doesn't help<br /><br />filters=filters(:,1:last);<br />ft=kt*filters;<br />clear kt;<br />kt=[ones(n,1,'single') sqrt(1+max(ft,0))-1 sqrt(1+max(-ft,0))-1];<br />clear ft;<br />fs=ks*filters;<br />clear ks filters;<br />ks=[ones(m,1,'single') sqrt(1+max(fs,0))-1 sqrt(1+max(-fs,0))-1];<br />clear fs;<br /><br />[n,k]=size(yt);<br />[m,d]=size(ks);<br /><br />for i=1:k<br /> ind=find(yt(:,i)==1);<br /> kind=kt(ind,:);<br /> ni=length(ind);<br /> covs(:,:,i)=double(kind'*kind)/ni;<br /> clear ind kind;<br />end<br /><br />filters=zeros(d,top*k*(k-1),'single');<br />last=0;<br />threshold=7.5;<br />for j=1:k<br /> covj=squeeze(covs(:,:,j)); l=chol(covj+gamma*eye(d))';<br /> for i=1:k<br /> if j~=i<br /> covi=squeeze(covs(:,:,i));<br /> C=l\covi/l'; CS=0.5*(C+C'); [v,L]=eigs(CS,top); V=l'\v;<br /> take=find(diag(L)>=threshold);<br /> batch=length(take);<br /> fprintf('%u,%u,%u ', i, j, batch);<br /> filters(:,last+1:last+batch)=V(:,take);<br /> last=last+batch;<br /> end<br /> end<br /> fprintf('\n');<br />end<br />fprintf('gamma=%g,top=%u,threshold=%g\n',gamma,top,threshold);<br />fprintf('last=%u filtered=%u\n', last, size(filters,2) - last);<br /><br />clear covi covj covs C CS V v L<br /><br />filters=filters(:,1:last);<br />ft=kt*filters;<br />clear kt;<br />kt=[sqrt(1+max(ft,0))-1 sqrt(1+max(-ft,0))-1];<br />clear ft;<br />fs=ks*filters;<br />clear ks filters;<br />ks=[sqrt(1+max(fs,0))-1 sqrt(1+max(-fs,0))-1];<br />clear fs;<br /><br />trainx=[ones(n,1,'single') kt kt.^2];<br />clear kt;<br />testx=[ones(m,1,'single') ks ks.^2];<br />clear ks;<br /><br />C=chol(0.5*(trainx'*trainx)+sqrt(n)*eye(size(trainx,2)),'lower');<br />w=C'\(C\(trainx'*yt));<br />pt=trainx*w;<br />ps=testx*w;<br /><br />[~,trainy]=max(yt,[],2);<br />[~,testy]=max(ys,[],2);<br /><br />for i=1:5<br /> xn=[pt pt.^2/2 pt.^3/6 pt.^4/24];<br /> xm=[ps ps.^2/2 ps.^3/6 ps.^4/24];<br /> c=chol(xn'*xn+sqrt(n)*eye(size(xn,2)),'lower');<br /> ww=c'\(c\(xn'*yt));<br /> ppt=SimplexProj(xn*ww);<br /> pps=SimplexProj(xm*ww);<br /> w=C'\(C\(trainx'*(yt-ppt)));<br /> pt=ppt+trainx*w;<br /> ps=pps+testx*w;<br /><br /> [~,yhatt]=max(pt,[],2);<br /> [~,yhats]=max(ps,[],2);<br /> errort=sum(yhatt~=trainy)/n;<br /> errors=sum(yhats~=testy)/m;<br /> fprintf('%u,%g,%g\n',i,errort,errors)<br />end<br />fprintf('%4s\t', 'pred');<br />for true=1:k<br /> fprintf('%5u', true-1);<br />end<br />fprintf('%5s\n%4s\n', '!=', 'true');<br />for true=1:k<br /> fprintf('%4u\t', true-1);<br /> trueidx=find(testy==true);<br /> for predicted=1:k<br /> predidx=find(yhats(trueidx)==predicted);<br /> fprintf('%5u', sum(predidx>0));<br /> end<br /> predidx=find(yhats(trueidx)~=true);<br /> fprintf('%5u\n', sum(predidx>0));<br />end<br /><br />toc(start)<br /><br />end<br /><br />% http://arxiv.org/pdf/1309.1541v1.pdf<br />function X = SimplexProj(Y)<br /> [N,D] = size(Y);<br /> X = sort(Y,2,'descend');<br /> Xtmp = bsxfun(@times,cumsum(X,2)-1,(1./(1:D)));<br /> X = max(bsxfun(@minus,Y,Xtmp(sub2ind([N,D],(1:N)',sum(X>Xtmp,2)))),0);<br />end<br /></pre>When I run this on my desktop machine it yields<br /><pre class="brush:matlabkey">>> calgevsquared<br />2,1,20 3,1,20 4,1,20 5,1,20 6,1,20 7,1,20 8,1,20 9,1,20 10,1,20 <br />1,2,20 3,2,20 4,2,20 5,2,20 6,2,20 7,2,20 8,2,20 9,2,20 10,2,20 <br />1,3,20 2,3,20 4,3,20 5,3,20 6,3,20 7,3,20 8,3,20 9,3,20 10,3,20 <br />1,4,20 2,4,20 3,4,20 5,4,20 6,4,20 7,4,20 8,4,20 9,4,20 10,4,20 <br />1,5,20 2,5,20 3,5,20 4,5,20 6,5,20 7,5,20 8,5,20 9,5,20 10,5,20 <br />1,6,20 2,6,20 3,6,20 4,6,20 5,6,20 7,6,20 8,6,20 9,6,20 10,6,20 <br />1,7,20 2,7,20 3,7,20 4,7,20 5,7,20 6,7,20 8,7,20 9,7,20 10,7,20 <br />1,8,20 2,8,20 3,8,20 4,8,20 5,8,20 6,8,20 7,8,20 9,8,20 10,8,20 <br />1,9,20 2,9,20 3,9,20 4,9,20 5,9,20 6,9,20 7,9,20 8,9,20 10,9,20 <br />1,10,20 2,10,20 3,10,20 4,10,20 5,10,20 6,10,20 7,10,20 8,10,20 9,10,20 <br />2,1,15 3,1,20 4,1,20 5,1,20 6,1,20 7,1,20 8,1,20 9,1,20 10,1,20 <br />1,2,20 3,2,20 4,2,20 5,2,20 6,2,20 7,2,20 8,2,20 9,2,20 10,2,20 <br />1,3,20 2,3,11 4,3,17 5,3,20 6,3,20 7,3,19 8,3,18 9,3,18 10,3,19 <br />1,4,20 2,4,12 3,4,20 5,4,20 6,4,12 7,4,20 8,4,19 9,4,15 10,4,20 <br />1,5,20 2,5,12 3,5,20 4,5,20 6,5,20 7,5,20 8,5,16 9,5,20 10,5,9 <br />1,6,18 2,6,13 3,6,20 4,6,12 5,6,20 7,6,18 8,6,20 9,6,13 10,6,18 <br />1,7,20 2,7,14 3,7,20 4,7,20 5,7,20 6,7,20 8,7,20 9,7,20 10,7,20 <br />1,8,20 2,8,14 3,8,20 4,8,20 5,8,20 6,8,20 7,8,20 9,8,20 10,8,12 <br />1,9,20 2,9,9 3,9,20 4,9,15 5,9,18 6,9,11 7,9,20 8,9,17 10,9,16 <br />1,10,20 2,10,14 3,10,20 4,10,20 5,10,14 6,10,20 7,10,20 8,10,12 9,10,20 <br />gamma=0.1,top=20,threshold=7.5<br />last=1630 filtered=170<br />1,0.0035,0.0097<br />2,0.00263333,0.0096<br />3,0.00191667,0.0092<br />4,0.00156667,0.0093<br />5,0.00141667,0.0091<br />pred 0 1 2 3 4 5 6 7 8 9 !=<br />true<br /> 0 977 0 1 0 0 1 0 1 0 0 3<br /> 1 0 1129 2 1 0 0 1 1 1 0 6<br /> 2 1 1 1020 0 1 0 0 6 3 0 12<br /> 3 0 0 1 1004 0 1 0 2 1 1 6<br /> 4 0 0 0 0 972 0 4 0 2 4 10<br /> 5 1 0 0 5 0 883 2 1 0 0 9<br /> 6 4 2 0 0 2 2 947 0 1 0 11<br /> 7 0 2 5 0 0 0 0 1018 1 2 10<br /> 8 1 0 1 1 1 1 0 1 966 2 8<br /> 9 1 1 0 2 5 2 0 4 1 993 16<br />Elapsed time is 186.147659 seconds.<br /></pre>That's a pretty good confusion matrix, comparable to state-of-the-art deep learning results on (permutation invariant) mnist. In the paper we report a slightly worse number (96 test errors) because for a paper we have to choose hyperparameters via cross-validation on the training set rather than cherry-pick them as for a blog post.<br /><br />The technique as stated here is really only useful for tall-thin design matrices (i.e., lots of examples but not too many features): if the original feature dimensionality is too large (e.g., $> 10^4$) than naive use of standard generalized eigensolvers becomes slow or infeasible, and other tricks are required. Furthermore, if the number of classes is too large than solving $O (k^2)$ generalized eigenvalue problems is also not reasonable. We're working on remedying these issues, and we're also excited about extending this strategy to structured prediction. Hopefully we'll have more to say about it at the next few conferences.<br />Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-58573775088746825502014-03-09T11:53:00.000-07:002014-10-21T08:41:54.728-07:00Failing FastI spent Christmas break working on some matrix factorization related ideas. There were two things I was wondering about: first, whether dropout is a good regularizer for discriminative low-rank quadratics; and second, how to do the analog of <a href="http://arxiv.org/abs/1310.1934">GeV representation learning</a> for discriminative low-rank quadratics. For the latter, I had an idea I was sure would work, but I wasn't able to make it work. There's a saying: “your baby is not as beautiful as you think it is”. Most ideas are not good ideas despite our prior beliefs, so it's important to eliminate ideas as quickly as possible.<br /><br />Startups have popularized the idea of <a href="http://www.feld.com/wp/archives/2009/04/the-best-entrepreneurs-know-how-to-fail-fast.html">failing fast</a>, since most business ideas are also not good ideas. The idea of the <a href="http://en.wikipedia.org/wiki/Minimum_viable_product">minimum viable product</a> has become central dogma in the startup community. Analogously, when testing machine learning ideas, it's best to start with the “minimum viable algorithm”. Such an algorithm is written in as high level a language as possible (e.g., Matlab, NumPy), using as many existing libraries and packages as possible (e.g., <a href="http://cvxr.com/cvx/">CVX</a>), and not taking any computational shortcuts for efficiency.<br /><br />I started playing around with dropout regularization for matrix factorization in Matlab, and when I saw that it was working on <a href="http://grouplens.org/datasets/movielens/">movielens</a>, then I spent the time to implement it as a reduction in vw. The fact that I <span style="font-style: italic;">knew</span> it should work allowed me to power through the multiple defects I introduced while implementing. Short story even shorter, the result is in the main branch and you can <a href="https://github.com/JohnLangford/vowpal_wabbit/tree/master/demo/movielens">check out the demo</a>.<br /><br />The next idea I tried was to pose learning low-rank quadratic features as a alternating linear-fractional optimization problem. The analogy to alternating least squares was so strong that asthetically I was sure it was a winner. For a multi-class prediction task (e.g., movielens without side information) over binary dyadic examples $S = \{ \{ ( l, r ), y \} | l \in \{0, 1\}^m, r \in \{ 0,1 \}^n, y \in \{ 0, 1, \ldots, k \} \}$, a predictor with a single latent MF-style feature looks like \[<br />f (( l, r ); w, p, q) = w^\top (l, r) + (l^\top p) (r^\top q),<br />\] ignoring the constant feature for simplicity. Here $p \in \mathbb{R}_+^m$ and $q \in \mathbb{R}_+^n$ are the single latent feature. On movielens without side information $l$ and $r$ are indicator variables of the user id and movie id respectively, so $p$ and $q$ are indexed by these identifiers and each produces a scalar whose product is added to the predictor.<br /><br />The idea was to choose the latent feature to be highly active on class $i$ and highly inactive on another class $j$, \[<br />\max_{p \in \mathbb{R}_+^m, q \in \mathbb{R}_+^n} \frac{p^\top \mathbb{E}[l r^\top | y = i] q}{\alpha + p^\top \mathbb{E}[l r^\top| y = j] q}.<br />\] subject to $p \preceq 1$ and $q \preceq 1$ (otherwise it can diverge). $\alpha> 0$ is a hyperparameter which regularizes the denominator. Of course in practice expectations are converted to averages over the training set.<br /><br />For fixed $p$ this is a linear-fractional program in $q$ and vice versa, so starting from a random point I was able to quickly alternate into features that looked good visually (high product energy on high rating user-movie pairs, low product energy on low rating user-movie pairs). However, the predictive lift on the test set from these features, compared to a linear model without interactions, was almost nonexistent. Then I tried a boosting variant, where first I fit a linear model without interactions and then tried to discriminate between positive and negative residual examples. This was more interesting: the features end up mostly being zero except for a small percentage of the data, suggesting that although the original features look good visually they are mostly providing information redundant with a linear model.<br /><br />I was able to crank out these negative results in just a few days using Matlab and CVX (it helps that there are no meetings at work during the holidays). Is it possible I screwed this up and actually the idea is a good one? Yes, but working at such a high level eliminates concerns about the optimizer, which makes it more likely that it is actually the strategy at fault. In any event, I have a portfolio of ideas, and I need to invest my time in those ideas that are most likely to yield something interesting. Although not definitive, these quick experiments suggested that I should spend my time somewhere else.<br /><br />Think of it as <a href="http://en.wikipedia.org/wiki/Bayesian_search_theory">Bayesian search theory</a> over the space of ideas.<br /><br /><br /><br /><br /><br /><br /><br />Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-43426381047494622112014-02-21T21:08:00.000-08:002014-10-21T08:42:20.440-07:00Stranger in a Strange LandI attended the <a href="http://www.siam.org/meetings/pp14/">SIAM PP 2014</a> conference this week, because I'm developing an interest in MPI-style parallel algorithms (also, it was close by). My plan was to observe the HPC community, try to get a feel how their worldview differs from my internet-centric “Big Data” mindset, and broaden my horizons. Intriguingly, the HPC guys are actually busy doing the opposite. They're aware of what we're up to, but they talk about Hadoop like it's some giant livin' in the hillside, comin down to visit the townspeople. Listening to them mapping what we're up to into their conceptual landscape was very enlightening, and helped me understand them better.<br /><br /><h2>The Data Must Flow</h2>One of the first things I heard at the conference was that “map-reduce ignores data locality”. The speaker, Steve Plimpton, clearly understood map-reduce, having implemented <a href="http://mapreduce.sandia.gov/">MapReduce for MPI</a>. This was a big clue that they mean something very different by data locality (i.e., they do not mean “move the code to the data”).<br /><br />A typical MPI job consists of loading a moderate amount of initial state into main memory, then doing an extreme amount of iterative computation on that state, e.g., simulating biology, the weather, or nuclear explosions. Data locality in this context means rearranging the data such that synchronization requirements between compute nodes is mitigated.<br /><br />Internet companies, on the other hand, generally have a large amount of data which parameterizes the computation, to which they want to apply a moderate amount of computation (e.g., you only need at most 30 passes over the data to get an excellent logistic regression fit). While we do some iterative computations the data-to-computation ratio is such that <a href="http://en.wikipedia.org/wiki/Dataflow_programming">dataflow programming</a>, moderately distorted, is a good match for what we desire. This difference is why the CTO of Cray felt compelled to point out that Hadoop “does I/O all the time”.<br /><br /><h2>Failure Is Not An Option</h2>The HPC community has a schizophrenic attitude towards fault-tolerance. In one sense they are far more aware and worried about it, and in another sense they are oblivious.<br /><br />Let's start with obliviousness. The dominant programming model for HPC today provides the abstraction of a reliable machine, i.e., a machine that does not make errors. Current production HPC systems deliver on this promise via error detection combined with global checkpoint-restart. The hardware vendors do this in an application-agnostic fashion: periodically they persist the entire state of every node to durable storage, and when they detect an error <a href="http://www.youtube.com/watch?v=LuN6gs0AJls">they restore the most recent checkpoint</a>.<br /><br />There are a couple problems which threaten this approach. The first is fundamental: as systems become more parallel, mean time between failure decreases, but checkpoint times do not (more nodes means more I/O capacity but also more state to persist). Thanks to constant factor improvements in durable storage due to SSDs and NVRAM, the global checkpoint-restart model has gained two or three years of runway, but it looks like a different strategy will soon be required.<br /><br />The second is that error detection is itself error prone. <a href="http://en.wikipedia.org/wiki/ECC_memory">ECC</a> only guards against the most probable types of errors, so if a highly improbable type of error occurs it is not detected; and other hardware (and software) componentry can introduce additional undetected errors. These are called <a href="http://en.wikipedia.org/wiki/Data_corruption#Silent_data_corruption">silent corruption</a> in the HPC community, and due to their nature the frequency at which they occur is not well known, but it is going to increase as parallelism increases.<br /><br />Ultimately, what sounds like a programmer's paradise (“I don't have to worry about failures, I just program my application using the abstraction of a reliable machine”) becomes a programmer's nightmare (“there is no way to inform the system about inherent fault-tolerance of my computation, or to write software to mitigate the need for expensive general-purpose reliability mechanisms which don't even always work.”). Paraphrasing one panelist, “... if an ECC module detects a double-bit error then my process is toast, even if the next operation on that memory cell is a write.”<br /><br /><h2>Silent But Not Deadly</h2>Despite the dominant programming model, application developers in the community are highly aware of failure possibilities, including all of the above but also issues such as numerical rounding. In fact they think about failure far more than I ever have: the most I've ever concerned myself with is, “oops I lost an entire machine from the cluster.” Meanwhile I'm not only not checking for silent corruption, I'm doing things like buying cheap RAM, using half-precision floating point numbers, and ignoring suddenly unavailable batches of data. How does anything ever work? <br /><br />One answer, of course, is that typical total number core-hours of a machine learning compute task is so small that extremely unlikely things generally do not occur. While it takes a lot of computers to <a href="http://research.google.com/archive/large_deep_networks_nips2012.html">recognize a cat</a>, the total core-hours is still less than 10<sup>6</sup>. Meanwhile <a href="http://en.wikipedia.org/wiki/IBM_Sequoia">the Sequoia</a> at LLNL has 100K compute nodes (1.6M cores) so a simulation which takes a week will have somewhere between 10<sup>2</sup>-10<sup>4</sup> more core-hours of exposure. Nonetheless the ambition in the machine learning community is to scale up, which begs the question: should we be worried about data corruption? I think the answer is: probably not to the same level as the HPC community.<br /><br />I saw a presentation on self-stabilizing applications, which was about designing algorithms such that randomly injected incorrect calculations were fixed by later computation. The third slide indicated “some applications are inherently self-stabilizing without further modification. For instance, convergent fixed point methods, such as Newton's method.” Haha! Most of machine learning is “the easy case” (as is, e.g., PageRank). Not that surprising, I guess, given that stochastic gradient descent algorithms <a href="http://leon.bottou.org/projects/sgd">appear to somehow work despite bugs</a>.<br /><br />Remember the butterfly effect? That was inspired by observed choatic dynamics in weather simulation. Predicting the weather is not like machine learning! One question is whether there is anything in machine learning or data analytics akin to weather simulation. Model state errors during training are corrected by contractive dynamics, and errors in single inputs or intermediate states at evaluation time only affect one decision, so their impact is bounded. However, model state errors at evaluation time affect <i>many</i> decisions, so it's worth being more careful. For example, one could ship a validation set of examples with each model to a production system, and when a model is loaded the output on the validation set is computed: if it doesn't match desired results, the new model should be rejected. Mostly however machine learning can afford to be cavalier, because there are statistical limits to the information content of the input data and we want to generalize to novel situations. Furthermore, the stakes are lower: a mistargeted advertisement is less dangerous than a mistargeted nuclear weapon.<br /><br /><h2>Anything To Declare?</h2>There appeared to be at least two distinct subcamps in the HPC community. In one camp were those who wanted to mostly preserve the abstraction of a reliable machine, possibly moving failure handling up the stack a bit into the systems software but still mostly keeping the application programmer out of it. As I heard during a panel discussion, this camp wants “a coherent architecture and strategy, not a bag of tricks.” In the other camp were those who wanted more application-level control over reliability strategies, in order to exploit specific aspects of their problem and avoiding the large penalty of global checkpoint restore. For example, maybe you have a way to check the results of a computation in software, and redo some work if it doesn't pass (aka <a href="http://lph.ece.utexas.edu/public/CDs/ContainmentDomains">Containment Domains</a>). You would like to say “please don't do an expensive restore, I'll handle this one”. Current generation HPC systems do not support that.<br /><br />At the application level being declarative appears key. The current HPC abstraction is designed to make an arbitrary computation reliable, and is therefore expensive. By declaring computational intent, simpler models of reliability can be employed. For instance, map-reduce is a declarative framework: the computation is said to have a particular structure (data-parallel map followed by associative reduce) which admits localized fault handling (when a node fails, only the map output associated with that node need be recomputed, and this can be done speculatively). These simpler models of reliability aren't just cheaper they are also faster (less redundant work when an error occurs). However, they do not work for general purpose computations. <br /><br />Putting together a collection of special purpose computation frameworks with associated reliability strategies either sounds great or horrible depending upon which camp you are in. I'm sure some in the HPC community look at the collection of projects in the Apache Foundation with fear and loathing. Others, however, are saying that in fact a small number of computation patterns capture the majority of work (e.g., numerical linear algebra, stencil/grid computations, and Monte Carlo), so that a collection of bespoke strategies could be viable.<br /><br /><h2>Cathedral vs. Bazaar</h2>In the internet sector, the above level of disagreement about the way forward would be considered healthy. Multiple different open source projects would emerge, eventually the best ideas would rise to the top, and the next generation of innovation would leverage the lessons and repeat the cycle. Meanwhile in the HPC world, the MPI spec has yet to adopt any of the competing proposals for fault-tolerance. Originally there was hope for 3.0, then 3.1, and now it looks like <a href="http://meetings.mpi-forum.org/mpi3.0_ft.php">4.0 is the earliest possibility</a>.<br /><br />Compared to the Apache Foundation, the <a href="http://en.wikipedia.org/wiki/The_Cathedral_and_the_Bazaar">cathedral vs. bazaar</a> analogy is apt. However the standards committee is a bit more conservative than the community as a whole, which is racing ahead with prototype designs and implementations that relax the abstraction of a reliable machine, e.g., <a href="http://www.redmpi.com/">redundant MPI</a> and <a href="http://fault-tolerance.org/">fault-tolerant MPI</a>. There is also a large body of computation specific strategies under the rubric of “Algorithm Based Fault Tolerance”.<br /><br /><h2>Takeaways</h2>There are some lessons to be learned from this community. <br /><br />The first is that <a href="http://en.wikipedia.org/wiki/Declarative_programming">declarative programming</a> is going to win, at least with respect to the distributed control flow (non-distributed portions will still be dominated by imperative specifications, but for example learning algorithms specified via linear algebra can be declarative all the way down). Furthermore, distributed declarative expressive power will not be general purpose. The HPC community has been trying to support general purpose computation with a fault-free abstraction, and this is proving expensive. Some in the HPC community are now calling for restricted expressiveness declarative models that admit less expensive fault-tolerance strategies (in the cloud we have to further contend with multi-tenancy and elasticity). Meanwhile the open source community has been embracing more expressive but still restricted models of computation, e.g., <a href="https://giraph.apache.org/">Giraph</a> and <a href="http://en.wikipedia.org/wiki/GraphLab">GraphLab</a>. More declarative frameworks with different but limited expressiveness will arise in the near-term, and creating an easy way to run them all in one unified cluster, and to specify a task that spans all of them, will be a necessity.<br /><br />The second is that, if you wait long enough, extremely unlikely things are guaranteed to happen. Mostly we ignore this in the machine learning community right now, because our computations are short: but we will have to worry about this given our need and ambition to scale up. Generic strategies such as <a href="http://lph.ece.utexas.edu/public/CDs/ContainmentDomains">containment domains</a> and <a href="http://arxiv.org/abs/1402.3809">skeptical programming</a> are therefore worth understanding.<br /><br />The third is that <a href="http://en.wikipedia.org/wiki/Bulk_synchronous_parallel">Bulk Synchronous Parallel</a> has a lot of headroom. There's a lot of excitment in the machine learning community around parameter servers, which is related to <a href="http://en.wikipedia.org/wiki/Partitioned_global_address_space">async PGAS</a> in HPC (and also analogous to relaxations of BSP, e.g., <a href="http://www.cs.cmu.edu/~seunghak/hotOS-13-cipar.pdf">stale synchronous parallel</a>). However BSP works at petascale today, and is easy to reason about and program (e.g., BSP is what <a href="http://hunch.net/~vw/">Vowpal Wabbit</a> does when it cajoles Hadoop into doing a distributed logistic regression). With an <a href="http://grids.ucs.indiana.edu/ptliupages/publications/MammothDataintheCloudClusteringSocialImages.pdf">optimized pipelined implementation of allreduce</a>, BSP algorithms look attractive, especially if they can declare semantics about how to make progress given partial responses (e.g., due to faults or multi-tenancy issues) and how to leverage newly available additional resources (e.g., due to multi-tenancy).<br /><br />I could have sworn there was a fourth takeaway but unfortunately I have forgotten it, perhaps due to an <a href="http://en.wikipedia.org/wiki/Soft_error">aberrant thermal neutron</a>.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com3tag:blogger.com,1999:blog-4446292666398344382.post-26384206550653882082014-02-17T23:21:00.002-08:002014-10-21T09:22:57.823-07:00The Machine Learning DoghouseThis is a follow-up on the <a href="/2013/08/cosplay.html">cosplay</a> post. When I did that post I had to use a sub-optimal optimization strategy because <a href="http://lowrank.net/nikos/index.html">Nikos</a> was still refining the publication of a <a href="http://arxiv.org/abs/1310.1949">superior strategy</a>. Now he has agreed to do a guest post detailing much better techniques.<br /><br /><h2>The Machine Learning Doghouse</h2>About a year ago, <a href="http://research.microsoft.com/en-us/um/people/skakade/">Sham Kakade</a> was visiting us here in Redmond. He came to give a talk about his cool work on using the method of moments, instead of maximum likelihood, for estimating models such as mixtures of Gaussians and Latent Dirichlet Allocation. Sham has a penchant for simple and robust algorithms. The method of moments is one such example: you don't need to worry about local minima, initialization, and such. Today I'm going to talk about some work that came out of my collaboration with Sham (and <a href="http://research.microsoft.com/en-us/um/people/alekha/">Alekh</a> and <a href="http://www.cc.gatech.edu/~lsong/">Le</a> and <a href="http://theory.stanford.edu/~valiant/">Greg</a>).<br /><br />When Sham visited, I was fresh out of grad school, and had mostly dealt with problems in which the examples are representated as high dimensional sparse vectors. At that time, I did not fully appreciate his insistence on what he called “dealing with correlations in the data”. You see, Sham had started exploring a very different set of problems. Data coming from images, audio and video, are dense, and not as high dimensional. Even if the data is nominally high dimensional, the eigenvalues of the data matrix are rapidly decaying, and we can reduce the dimension (say, with randomized SVD/PCA) without hurting the performance. This is simply not true for text problems.<br /><br />What are the implications of this for learning algorithms? First, theory suggests that for these ill-conditioned problems (online) first order optimizers are going to converge slowly. In practice, things are even worse. These methods do not just require many passes, they simply never get to the test accuracy one can get with second order optimization methods. I did not believe it until I tried it. But second order optimization can be slow, so in this post I'll describe two algorithms that are fast, robust, and have no (optimization related) tuning parameters. I will also touch upon a way to scale up to high dimensional problems. Both algorithms take $O(d^2k)$ per update and their convergence does not depend on the condition number $\kappa$. This is considerably cheaper than the $O(d^3k^3)$ time per update needed for standard second order algorithms. First order algorithms on the other hand, take $O(dk)$ per update but their convergence depends on $\kappa$, so the methods below are preferable when the condition number is large.<br /><br />We will be concerned with mutliclass (and multilabel) classification as these kinds of problems have special structure we will take advantage of. As a first recipe, suppose we want to fit a multinomial logistic model which posits \[<br />\mathbb{E}[y|x]=g(x^\top W^*),<br />\]<br />where $y$ is an indicator vector for one of the $k$ classes, $x \in \mathbb{R}^d$ is our input vector, $W^*$ is a $d\times k$ matrix of parameters to be estimated and $g:\mathbb{R}^k \to \Delta^k$ is the softmax link function mapping a vector of reals to the probability simplex: \[<br />g(v) = \left[\begin{array}{c}<br />\frac{\exp(v_1)}{\sum_{j=1}^k\exp(v_j)}\\<br />\vdots\\<br />\frac{\exp(v_k)}{\sum_{j=1}^k\exp(v_j)}\\<br />\end{array} \right].<br />\] The basic idea behind the first algorithm is to come up with a nice proxy for the Hessian of the multinomial logistic loss. This bad boy is $dk \times dk$ and depends the current parameters. Instead, we will use a matrix that does not depend on the parameters and is computationally easy to work with. The bottom line is for multinomial logistic regression we can get away with a block diagonal proxy with $k$ identical blocks on the diagonal each of size $d\times d$. Selecting the blocks to be $\frac{1}{2} X^\top X$ ensures that our updates will never diverge while at the same time avoiding line searches and messing with step sizes. With this matrix as a preconditioner we can go ahead and basically run preconditioned (batch) gradient descent. The script <a href="https://github.com/fest/secondorderdemos/blob/master/mls.m">mls.m</a> does this with two (principled) modifications that speed things up a lot. First, we compute the preconditioner on a large enough subsample. The script includes in comments the code for the full preconditioner. The second modification is that we use <a href="http://en.wikipedia.org/wiki/Gradient_descent#Extensions">accelerated gradient descent</a> instead of gradient descent.<br /><br />Plugging this optimizer in the <a href="/2013/08/cosplay.html">cosplay</a> script from a few months ago gives a test accuracy of 0.9844 in 9.7 seconds on my machine, which is about 20 times faster and much more accurate than LBFGS.<br /><br />The second algorithm is even faster and is applicable to multiclass as well as multilabel problems. There is also a downside in that you won't get very accurate probability estimates in the tails: this method is not optimizing cross entropy. The basic idea is we are going to learn the link function, sometimes known as calibration.<br /><br />For binary classification, the <a href="http://en.wikipedia.org/wiki/Isotonic_regression">PAV algorithm</a> can learn a link function that minimizes squared error among all monotone functions. Interestingly, the <a href="http://academic.research.microsoft.com/Publication/4908670/the-isotron-algorithm-high-dimensional-isotonic-regression">Isotron paper</a> showed that iterating between PAV and least squares learning of the parameters of a linear classifier, leads to the global minimum of this nonconvex problem.<br /><br />The script <a href="https://github.com/fest/secondorderdemos/blob/master/cls.m">cls.m</a> extends these ideas to multiclass classification in the sense that we alternate between fitting the targets and calibration. The notion of calibration used in the implementation is somewhat weak and equivalent to assuming the inverse of the unknown link function can be expressed as a low degree polynomial of the raw predictions. For simplicity's sake, we cut two corners: First we do not force the link to be monotone (though monotonicity is <a href="http://en.wikipedia.org/wiki/Monotonic_function#Monotonicity_in_functional_analysis">well defined for high dimensions</a>). Second we assume having access to the unlabeled test data at training time (aka the transductive setting). An implementation that does not assume this is more complicated without any additional insights.<br /><br />Plugging this optimizer in the aforementioned cosplay script I get a test accuracy of 0.9844 in 9.4 seconds on my machine. Again, we are more than 20 times faster than LBFGS, and more accurate. Interestingly, extending this algorithm to work in the multilabel setting is very simple: instead of projecting onto the simplex, we project onto the unit hypercube.<br /><br />What about high dimensional data? This is the main reason why second order methods are in the doghouse of the machine learning community. A simple and practical solution is to adapt ideas from boosting and coordinate descent methods. We take a batch of features and optimize over them as above with either recipe. Then we take another batch of features and fit the residual. Typically batch sizes can range between 300 and 2000 depending on the problem. Smaller sizes offer the most potential for speed and larger ones offer the most potential for accuracy. The batch size that offers the best running time/accuracy tradeoff is problem dependent. Script <a href="https://github.com/fest/secondorderdemos/blob/master/mlsinner.m">mlsinner.m</a> deals with the inner loop of this procedure. It takes two additional parameters that will be provided by the outer loop. It only performs a few iterations trying to find how to extend our initial predictions using a new batch of features so that we approximate the labels better. We also pass in a vector of weights which tell us on which examples should the preconditioner focus on. The outer loop <a href="https://github.com/fest/secondorderdemos/blob/master/stagewisemls.m">stagewisemls.m</a> simply generates new batches of features, keeps track of the predictions, and updates the importance weights for the preconditioner.<br /><br />Plugging this optimizer in the cosplay script gives a test accuracy of 0.986 in 67 seconds.<br /><br />Finally, <a href="https://github.com/fest/secondorderdemos/blob/master/cosplaydriver.m">cosplaydriver.m</a> runs all of the above algorithms on the mnist dataset. Here's how to replicate with octave. (The timings I report above are with MATLAB.)<br /><pre class="brush:bash">git clone https://github.com/fest/secondorderdemos.git<br />cd secondorderdemos<br />octave -q cosplaydriver.m<br /></pre>Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-44526455530955715522014-01-12T11:37:00.000-08:002014-10-21T08:44:35.744-07:00Cluster F***kedIt's a safe bet that, for the near future, data will continue to accumulate in clustered file systems such as HDFS, powered by commodity multicore CPUs with ethernet interconnect. Such clusters are relatively inexpensive, fault-tolerant, scalable, and have an army of systems researchers working on them. <br /><br />A few years ago, it was a safe bet that the iterative processing workloads of machine learning would increasingly migrate to run on the same hardware the data was accumulating on, after all, we want to “move code to the data”. Now this is looking less clear. The first serious challenge to this worldview arose when deep learning catapulted to the front of several benchmark datasets by leveraging the GPU. <a href="http://research.google.com/archive/large_deep_networks_nips2012.html">Dean et. al.</a> set out to replicate and surpass these results using large-scale multicore CPU clusters with ethernet interconnect, and while they were successful the amount of hardware required was surprising. Then <a href="http://www.stanford.edu/~acoates/papers/CoatesHuvalWangWuNgCatanzaro_icml2013.pdf">Coates et. al.</a> achieved comparable results using far fewer machines by paying very close attention to the communication costs (by laying out the model in a communication-friendly format, abstracting communication primitives, and leveraging Infiniband interconnect).<br /><br />Is the Coates et. al. result a bespoke solution for deep learning? Interestingly, <a href="http://www.cs.berkeley.edu/~jfc/papers/13/BD.pdf">Canny and Zhao</a> come to a similar conclusion in their “squaring the cloud” paper, and they don't mention neural networks explicitly at all. Here's a key quote from the paper:<br /><blockquote>“Fast-mixing algorithms (SGD and MCMC) in particular suffer from communication overhead. The speedup is typically a sublinear function $f(n)$ of $n$, since network capacity decreases at larger scales (typical approximations are $f(n) = n^\alpha$ for some $\alpha < 1$). This means that the cost of the computation in the cloud increases by a factor of $n/f(n)$ since the total work has increased by that factor. Energy use similarly increases by the same factor. By contrast, a single-node speedup by a factor of $k$ implies a simple $k$-fold saving in both cost and power.”<br /></blockquote>In other words, for some algorithms that we really care about, by treating communication costs as dominant you can do equivalent work with far fewer machines resulting in lower total costs.<br /><br />So here is the current state of affairs as I see it. There are still lots of algorithms that will run most efficiently on the same hardware that runs the distributed file-systems, e.g., <a href="http://www.stanford.edu/~boyd/papers/admm_distr_stats.html">the ADMM family</a>, which includes tasty morsels like L1-regularized logistic regression. However there will also be algorithms of high economic interest that do not map onto such hardware felicitously. Therefore we should expect to see data centers deploying “HPC islands” consisting of relatively small numbers of machines packed full of vectorized processors, high-bandwidth (to the vectorized processor) memory, and fast interconnect. These types of clusters are popular with certain communities such as high-energy physics researchers, but now consumer-facing internet companies will be widely adopting this technology. <br /><br />These HPC islands do not need to stage all the data they are working on before they start doing useful work, e.g., SGD algorithms can start as soon as they receive their first mini-batch. <a href="http://caffe.berkeleyvision.org/imagenet.html">Caffe</a> and a single K20 can train on Imagenet at 7ms per image amortized, which works out to roughly 40 megabytes per second of image data that needs to be streamed to the training node. That's not difficult to arrange if the HPC island is collocated with the HDFS cluster, and difficult otherwise, so the prediction is near the HDFS cluster is where the HPC islands will be. Of course the HPC island should have a smart caching policy so that not everything has to be pulled from HDFS storage all the time. A <span style="font-style: italic;">really</span> smart caching policy would be task aware, e.g., leveraging <a href="http://arxiv.org/abs/1306.1840">para-active learning</a> to maximize the information transfer between HDFS and the HPC island.<br /><br />Programming such a heterogenous system is going to be very challenging, which will provide lots of opportunity for individuals suitably situated.<br />Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-83967282814904228902013-12-12T20:49:00.000-08:002014-10-21T08:44:50.170-07:00NIPSplosion 2013<a href="http://nips.cc/Conferences/2013/">NIPS</a> was fabulous this year, kudos to all the organizers, area chairs, reviewers, and volunteers. Between the record number of attendees, multitude of corporate sponsors, and the <a href="http://techcrunch.com/2013/12/09/facebook-artificial-intelligence-lab-lecun/">Mark Zuckerburg show</a>, this year's conference is most notable for sheer magnitude. It's past the point that one person can summarize it effectively, but here's my retrospective, naturally heavily biased towards my interests.<br /><br />The keynote talks were all excellent, consistent with the integrative “big picture” heritage of the conference. My favorite was by Daphne Koller, who talked about the “other online learning”, i.e., pedagogy via telecommunications. Analogous to how moving conversations online allows us to precisely characterize the popularity of <a href="http://en.wikipedia.org/wiki/Snooki">Snooki</a>, moving instruction online facilitates the use of machine learning to improve human learning. Based upon the general internet arc from early infovore dominance to mature limbic-stimulating pablum, it's clear the ultimate application of the <a href="http://en.wikipedia.org/wiki/Coursera">Coursera</a> platform will be around <a href="http://xkcd.com/1027/">courtship techniques</a>, but in the interim a great number of people will experience more substantial benefits.<br /><br />As far as overall themes, I didn't detect any emergent technologies, unlike previous years where things like deep learning, randomized methods, and spectral learning experienced a surge. Intellectually the conference felt like a consolidation phase, as if the breakthroughs of previous years were still being digested. However, output representation learning and extreme classification (large cardinality multiclass or multilabel learning) represent interesting new frontiers and hopefully next year there will be further progress in these areas.<br /><br />There were several papers about improving the convergence of stochastic gradient descent which appeared broadly similar from a theoretical standpoint (<a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3769">Johnson and Zhang</a>; <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3754">Wang et. al.</a>; <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3844">Zhang et. al.</a>). I like the <a href="http://en.wikipedia.org/wiki/Control_variates">control variate</a> interpretation of Wang et. al. the best for generating an intuition, but if you want to implement something than Figure 1 of Johnson and Zhang has intelligible pseudocode. <br /><br />Covariance matrices were hot, and not just for PCA. The BIG & QUIC algorithm of <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=4086">Hseih et. al.</a> for estimating large sparse inverse covariance matrices was technically very impressive and should prove useful for causal modeling of biological and neurological systems (presumably some hedge funds will also take interest). <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3942">Bartz and Müller</a> had some interesting ideas regarding <a href="http://en.wikipedia.org/wiki/Estimation_of_covariance_matrices#Shrinkage_estimation">shrinkage estimators</a>, including the “orthogonal complement” idea that the top eigenspace should <span style="font-style: italic;">not</span> be shrunk since the sample estimate is actually quite good. <br /><br />An interesting work in randomized methods was from <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3783">McWilliams et. al.</a>, in which two random feature maps are then aligned with CCA over unlabeled data to extract the “useful” random features. This is a straightforward and computationally inexpensive way to leverage unlabeled data in a semi-supervised setup, and it is consistent with theoretical results from CCA regression. I'm looking forward to trying it out.<br /><br />The workshops were great, although as usual there are so many interesting things going on simultaneously that it made for difficult choices. I bounced between <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3707">extreme classification</a>, <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3700">randomized methods</a>, and <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3698">big learning</a> the first day. Michael Jordan's talk in big learning was excellent, particularly the part juxtaposing decreasing computational complexity of various optimization relaxations with increasing statistical risk (both effects due to the expansion of the feasible set). This is starting to get at the tradeoff between data and computation resources. Extreme classification (large cardinality multiclass or multilabel learning) is an exciting open area which is important (e.g., for structured prediction problems that arise in NLP) and appears tractable in the near-term. Two relevant conference papers were <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3970">Frome et. al.</a> (which leveraged <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=4080">word2vec</a> to reduce extreme classification to regression with nearest-neighbor decode) and <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3940">Cisse et. al.</a> (which exploits the near-disconnected nature of the label graph often encountered in practice with large-scale multi-label problems).<br /><br />The second day I mostly hung out in <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3722">spectral learning</a> but I saw Blei's talk in <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3727">topic modeling</a>. Spectral learning had a fun discussion session. The three interesting questions were<br /><ol><li> Why aren't spectral techniques more widely used?</li><li> How can spectral methods be made more broadly easily applicable, analogous to variational Bayes or MCMC for posterior inference?</li><li> What are the consequences of model mis-specification, and how can spectral methods be made more robust to model mis-specification?</li></ol>With respect to the first issue, I think what's missing is rock solid software that can easily found, installed, and experimented with. Casual practitioners do not care about theoretical benefits of algorithms, in fact they tend to view “theoretical” as a synonym for “putative”. Progress on the second issue would be great, c.f., <a href="http://probabilistic-programming.org/wiki/Home">probabilistic programming</a>. Given where <a href="http://en.wikipedia.org/wiki/Intel_MIC">hardware is going</a>, the future belongs to the most declarative. The third issue is a perennial Bayesian issue, but perhaps has special structure for spectral methods that might suggest, e.g., robust optimization criterion. Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-17785172143510879422013-12-10T14:12:00.000-08:002014-10-21T08:45:03.233-07:00The Flipped WorkshopThis year at NIPS one of the great keynotes was by <a href="http://nips.cc/Conferences/2013/Program/event.php?ID=3692">Daphne Koller</a> about Coursera and the <a href="http://en.wikipedia.org/wiki/Flip_teaching">flipped classroom</a>. On another day I was at lunch with Chetan Bhole from Amazon, who pointed out that all of us go to conferences to hear each other's lectures: since the flipped classroom is great, we should apply the concept to the conference. <br /><br />I love this idea.<br /><br />It's impractical to consider moving an entire conference over to this format (at least until the idea gains credibility), but the workshops provide an excellent experimental testbed, since the organizers are plenary. Here's how it would work: for some brave workshop, accepted submissions to the workshop (and invited speakers!) would have accompanying videos, which workshop participants would be expected to watch before the workshop. (We could even use Coursera's platform perhaps, to get extra things like mastery questions and forums.) During the workshop, speakers only spend 2 minutes or so reminding the audience who they are and what was the content of their video. Then, it becomes entirely interactive Q-and-A, presumably heavily whiteboard or smartboard driven.<br /><br />Feel free to steal this idea. Otherwise, maybe I'll try to organize a workshop just to try this idea out.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com1tag:blogger.com,1999:blog-4446292666398344382.post-42355736417570031102013-11-09T22:23:00.001-08:002014-10-21T09:30:37.414-07:00We can hash thatThe lab is located in the Pacific Northwest, so it's natural to ask what machine learning primitives are as ubiquitously useful as <a href="http://www.youtube.com/watch?v=yYey8ntlK_E">pickling</a>. There are two leading candidates at the moment: <a href="/2013/08/cosplay.html">randomized feature maps</a> and the <a href="http://en.wikipedia.org/wiki/Feature_hashing">hashing trick</a>. The latter, it turns out, can be beneficially employed for randomized PCA.<br /><br />Randomized PCA algorithms, as I've <a href="/2013/10/another-random-technique.html">discussed recently</a>, are awesome. Empirically, two (or more) pass algorithms seem necessary to get really good results. Ideally, one could just do one pass over the data with a (structured) randomness down to some computationally suitable dimension, and then use exact techniques to finish it off. In practice this doesn't work very well, although the computational benefits (single pass over the data and low memory usage) sometimes justifies it. Two pass algorithms use the first pass to construct an orthogonal basis, and then use that basis for the second pass. In addition to that extra data pass, two pass algorithms require storage for the basis, and an orthogonalization step. If the original feature dimensionality is $p$ and the number of desired components is $k$ than the storage requirements are $O (p k)$ and the orthogonalization step has time complexity $O (p k)$. If $O (p k)$ fits in main memory, this is not a problem, but otherwise, it can be a bother as essentially a distributed QR decomposition is required.<br /><br />The hashing trick (more generally, structured randomness) can provide a bridge between the two extremes. The idea is to use structured randomness to reduce the feature dimensionality from $p$ to $d$, such that $O (d k)$ fits in main memory, and then use a two pass randomized algorithm. This can be seen as an interpolation between a one pass algorithm leveraging structured randomness and a traditional two-pass algorithm. Practically speaking, we're just trying to use the available space resources to get a good answer. We've found hashing to be a good structured randomness for sparse domains such as text or graph data, while other structured randomness (e.g., subsampled <a href="http://en.wikipedia.org/wiki/Hartley_transform">Hartley transforms</a>) are better for dense data. When using hashing, other conveniences of the hashing trick, such as not needing to know the feature cardinality of the input data apriori, are inherited by the approach.<br /><br />These randomized methods should not intimidate: once you understand them, they are very simple. Here is some Matlab to do randomized PCA with hashing:<br /><pre class="brush:matlabkey">function H=makehash(d,p)<br /> i = linspace(1,d,d);<br /> j = zeros(1,d);<br /> s = 2*randi(2,1,d)-3;<br /><br /> perm = randperm(d);<br /> j=1+mod(perm(1:d),p);<br /> H = sparse(i,j,s);<br />end<br /></pre><pre class="brush:matlabkey">function [V,L]=hashpca(X,k,H)<br /> [~,p] = size(H);<br /> Omega = randn(p,k+5);<br /> [n,~] = size(X);<br /> Z = (X*H)'*((X*H)*Omega)/n;<br /> Q = orth(Z);<br /> Z = (X*H)'*((X*H)*Q)/n;<br /> [V,Lm,~] = svd(Z,'econ');<br /> V = V(:,1:k);<br /> L = diag(Lm(1:k,1:k));<br />end<br /></pre>which you can invoke with something like <br /><pre class="brush:matlabkey">>> H=makehash(1000000,100000); [V,L]=hashpca(sprandn(4000000,1000000,1e-5),5,H); L'<br /><br />ans =<br /><br /> 1.0e-03 *<br /><br /> 0.1083 0.1082 0.1081 0.1080 0.1079<br /></pre>So as usual one benefit is the shock-and-awe of allowing you to achieve some computation on your commodity laptop that brings other implementations to their knees. Here's a picture that results from PCA-ing a <a href="http://datahub.io/dataset/twitter-social-graph-www2010">publicly available Twitter social graph</a> on my laptop using about 800 megabytes of memory. The space savings from hashing is only about a factor of 20, so if you had a machine with 16 gigabytes of memory you could have done this with <a href="https://code.google.com/p/redsvd/">redsvd</a> without difficulty, but of course with larger data sets eventually memory gets expensive.<br /><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody><tr><td style="text-align: center;"><a href="http://4.bp.blogspot.com/-Hmqsi6YWoNY/Un8n4zi5MdI/AAAAAAAAAVY/4q06K0r9Lo0/s1600/twitterpca.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://4.bp.blogspot.com/-Hmqsi6YWoNY/Un8n4zi5MdI/AAAAAAAAAVY/4q06K0r9Lo0/s640/twitterpca.png" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">This image can be hard to read, but if you click on it it gets bigger, and then if you open the bigger version in a new tab and zoom in you can get more detail.</td></tr></tbody></table><br />If you like this sort of thing, you can check out the <a href="http://arxiv.org/abs/1310.6304">arxiv paper</a>, or you can visit the <a href="http://www.randomizedmethods.org/">NIPS Randomized Methods for Machine Learning</a> workshop where Nikos will be talking about it. <a href="http://pages.cs.wisc.edu/~arun/">Arun Kumar</a>, who interned at CISL this summer, also has a poster at <a href="http://biglearn.org/index.php/">Biglearn</a> regarding a distributed variant implemented on <a href="http://strataconf.com/stratany2013/public/schedule/detail/30895">REEF</a>.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-46444880968769873132013-10-15T18:10:00.000-07:002014-10-21T09:08:03.047-07:00Another Random TechniqueIn a <a href="/2013/08/cosplay.html">previous post</a> I discussed randomized feature maps, which can combine the power of kernels and the speed of primal linear methods. There is another randomized technique I've been using lately for great justice, <a href="http://arxiv.org/abs/0909.4061">randomized SVD</a>. This is a great primitive with many applications, e.g., you can mash up with randomized feature maps to get a fast kernel PCA, use as a fast initializer to bilinear latent factor models aka matrix factorization, or leverage to compute a <a href="http://nr.com/whp/notes/CanonCorrBySVD.pdf">giant CCA</a>. <br /><br />The basic idea is to probe a matrix with random vectors to discover the low-dimensional top range of the matrix, and then perform cheaper computations in this space. For square matrices, this is intuitive: the eigenvectors form a basis in which the action of the matrix is merely scaling, and the top eigenvectors have larger scale factors associated with them, so a random vector scaled by the matrix will get proportionally much bigger in the top eigendirections. This intuition suggests that if the eigenspectrum is nearly flat, it will be really difficult to capture the top eigenspace with random probes. Generally this is true, and these randomized approaches do well when there is a “large spectral gap”, i.e., when there is a large difference between successive eigenvalues. However even this is a bit pessimistic, because in machine learning sometimes you don't care if you get the subspace “wrong”, e.g., if you are trying to minimize squared reconstruction error then a nearly equal eigenvalue has low regret.<br /><br />Here's an example of a randomized PCA on mnist: you'll need to <a href="http://cs.nyu.edu/~roweis/data/mnist_all.mat">download mnist in matlab format</a> to run this.<br /><pre class="brush:bash">rand('seed',867);<br />randn('seed',5309);<br /><br />tic<br />fprintf('loading mnist');<br /><br />% get mnist from http://cs.nyu.edu/~roweis/data/mnist_all.mat<br />load('mnist_all.mat');<br /><br />trainx=single([train0; train1; train2; train3; train4; train5; train6; train7; train8; train9])/255.0;<br />testx=single([test0; test1; test2; test3; test4; test5; test6; test7; test8; test9])/255.0;<br />st=[size(train0,1); size(train1,1); size(train2,1); size(train3,1); size(train4,1); size(train5,1); size(train6,1); size(train7,1); size(train8,1); size(train9,1)];<br />ss=[size(test0,1); size(test1,1); size(test2,1); size(test3,1); size(test4,1); size(test5,1); size(test6,1); size(test7,1); size(test8,1); size(test9,1)];<br />paren = @(x, varargin) x(varargin{:});<br />yt=[]; for i=1:10; yt=[yt; repmat(paren(eye(10),i,:),st(i),1)]; end<br />ys=[]; for i=1:10; ys=[ys; repmat(paren(eye(10),i,:),ss(i),1)]; end<br /><br />clear i st ss<br />clear train0 train1 train2 train3 train4 train5 train6 train7 train8 train9<br />clear test0 test1 test2 test3 test4 test5 test6 test7 test8 test9<br /><br />fprintf(' finished: ');<br />toc<br /><br />[n,k]=size(yt);<br />[m,p]=size(trainx);<br /><br />tic<br />fprintf('estimating top 50 eigenspace of (1/n) X”X using randomized technique');<br /><br />d=50;<br />r=randn(p,d+5); % NB: we add an extra 5 dimensions here<br />firstpass=trainx'*(trainx*r); % this can be done streaming in O(p d) space<br />q=orth(firstpass);<br />secondpass=trainx'*(trainx*q); % this can be done streaming in O(p d) space<br />secondpass=secondpass/n;<br />z=secondpass'*secondpass; % note: this is small, i.e., O(d^2) space<br />[v,s]=eig(z);<br />pcas=sqrt(s);<br />pcav=secondpass*v*pinv(pcas);<br />pcav=pcav(:,end:-1:6); % NB: and we remove the extra 5 dimensions here<br />pcas=pcas(end:-1:6,end:-1:6); % NB: the extra dimensions make the randomized<br /> % NB: algorithm more accurate.<br /><br />fprintf(' finished: ');<br />toc<br /><br />tic<br />fprintf('estimating top 50 eigenspace of (1/n) X”X using eigs');<br /><br />opts.isreal = true; <br />[fromeigsv,fromeigss]=eigs(double(trainx'*trainx)/n,50,'LM',opts);<br /><br />fprintf(' finished: ');<br />toc<br /><br /><br />% relative accuracy of eigenvalues<br />%<br />% plot((diag(pcas)-diag(fromeigss))./diag(fromeigss))<br /><br />% largest angle between subspaces spanned by top eigenvectors<br />% note: can't be larger than pi/2 ~ 1.57<br />%<br />% plot(arrayfun(@(x) subspace(pcav(:,1:x),fromeigsv(:,1:x)),linspace(1,50,50))); xlabel('number of eigenvectors'); ylabel('largest principal angle'); set(gca,'YTick',linspace(0,pi/2,5)); <br /></pre>When I run this on my laptop I get<br /><pre class="brush:bash">>> randsvd<br />loading mnist finished: Elapsed time is 6.931381 seconds.<br />estimating top 50 eigenspace of (1/n) X'X using randomized technique finished: Elapsed time is 0.505763 seconds.<br />estimating top 50 eigenspace of (1/n) X'X using eigs finished: Elapsed time is 1.051971 seconds.<br /></pre>That difference in run times is not very dramatic, but on larger matrices the difference can be a few minutes versus “longer than you can wait”. Ok, but how good is it? Even assuming <span style="font-family: monospace;">eigs</span> is ground truth, there are several ways to answer that question, but suppose we want to get the same eigenspace from the randomized technique as from <span style="font-family: monospace;">eigs</span> (again, this is often too stringent a requirement in machine learning). In that case we can measure the largest <a href="http://en.wikipedia.org/wiki/Principal_angles">principle angle</a> between the top-$k$ subspaces discovered by <span style="font-family: monospace;">eigs</span> and by the randomized PCA, as a function of $k$. Values near zero indicate that the two subspaces are very nearly identical, whereas values near $\pi / 2$ indicate one subspace contains vectors which are orthogonal to vectors in the other subspace.<br /><br /><div class="separator" style="clear: both; text-align: center;"><a href="http://4.bp.blogspot.com/-olwDCiy9KPw/Ul3nP6hu4CI/AAAAAAAAAUQ/UQLBZsZLtyA/s1600/randsvd.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://4.bp.blogspot.com/-olwDCiy9KPw/Ul3nP6hu4CI/AAAAAAAAAUQ/UQLBZsZLtyA/s640/randsvd.png" /></a></div>In general we see the top 6 or so extracted eigenvectors are spot on, and then it gets worse, better, and worse again. Note it is not monotonic, because if two eigenvectors are reordered, once we have both of them the subspaces will have a small largest principle angle. Roughly speaking anywhere there is a large spectral gap we can expect to get the subspace up to the gap correct, i.e., if there is a flat plateau of eigenvalues followed by a drop than at the end of the plateau the largest principle angle should decrease.<br /><br /><a href="https://code.google.com/p/redsvd/">Redsvd</a> provides an open-source implementation of a two-pass randomized SVD and PCA.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com2tag:blogger.com,1999:blog-4446292666398344382.post-71304340669436763892013-10-02T18:35:00.000-07:002014-10-21T09:08:56.216-07:00Lack of SupervisionFor computational advertising and internet dating, the standard statistical learning theory playbook worked pretty well for me. Yes, there were nonstationary environments, explore-exploit dilemmas, and other covariate shifts; but mostly the intuition from the textbook was valuable. Now, in a potential instance of the <a href="http://en.wikipedia.org/wiki/Peter_Principle ">Peter Principle</a>, I'm mostly encountering problems in operational telemetry and security that seem very different, for which the textbook is less helpful. In a nod to Gartner, I have summarized my intimidation into a 4 quadrant mnemonic. <br /><table align="center" cellpadding="10"><tr><th colspan="2"></th><th colspan="2">Environment</th></tr><tr><th colspan="2"></th><th>Oblivious</th><th>Adversarial</th></tr><tr><th rowspan="2">Labels</th><th>Abundant</th><td style="border: solid 1px black;">Textbook Machine Learning</td><td style="border: solid 1px black;">Malware Detection</td></tr><tr><th>Rare</th><td style="border: solid 1px black;">Service Monitoring and Alerting</td><td style="border: solid 1px black;">Intrusion Detection</td></tr></table><br />The first dimension is the environment: is it oblivious or adversarial? Oblivious means that, while the environment might be changing, it is doing so independent of any decisions my system makes. Adversarial means that the environment is changing based upon the decisions I make in a manner to make my decisions worse. (Adversarial is not the opposite of oblivious, of course: the environment could be beneficial.) The second dimension is the prevalence of label information, which I mean in the broadest sense as the ability to define model quality via data. For each combination I give an example problem.<br /><br />In the top corner is textbook supervised learning, in which the environment is oblivious and labels are abundant. My current employer has plenty of problems like this, but also has lots of people to work on them, and plenty of cool tools to get them done. In the bottom corner is intrusion detection, a domain in which everybody would like to do a better job, but which is extremely challenging. Here's where the quadrant starts to help, by suggesting relaxations of the difficulties of intrusion detection that I can use as a warm-up. In malware detection, the environment is highly adversarial, but labels are abundant. That may sound surprising given that <a href="http://en.wikipedia.org/wiki/Stuxnet">Stuxnet</a> stayed hidden for so long, but actually all the major anti-virus vendors employ legions of humans whose daily activities provide abundant label information, albeit admittedly incomplete. In service monitoring and alerting, certain labels are relatively rare (because severe outages are thankfully infrequent), but the engineers are not injecting defects in a manner designed to explicitly evade detection (although it can feel like that sometimes).<br /><br />I suspect the key to victory when label information is rare is to decrease the cost of label acquisition. That almost sounds tautological, but it does suggest ideas from active learning,crowdsourcing, exploratory data analysis, search, and implicit label imputation; so it's not completely vacuous. In other words, I'm looking for a system that interrogates domain experts judiciously, asks a question that can be reliably answered and whose answer has high information content, presents the information they need to answer the question in an efficient format, allows the domain export to direct the learning, and can be bootstrapped from existing unlabeled data. Easy peasy!<br /><br />For adversarial setups I think online learning is an important piece of the puzzle, but only a piece. In particular I'm sympathetic to the notion that <a href="https://www.cs.drexel.edu/~sa499/papers/aisec08-kantchelian.pdf">in adversarial settings intelligible models have an advantage</a> because they work better with the humans who need to maintain them, understand their vulnerabilities, and harden them against attacks both proactively and reactively. I grudgingly concede this because I feel a big advantage of machine learning to date is the ability to use unintelligible models: intelligibility is a severe constraint! However intelligibility is not a fixed concept, and given the right (model and data) visualization tools a wider class of machine learning techniques become intelligible.<br /><br />Interestingly both for rare labels and for adversarial problems user interface issues seem important, because both require efficient interaction with a human (for different purposes).Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-75201399149220488152013-09-21T00:04:00.000-07:002014-10-21T09:09:42.735-07:00What Lies Between R and DAt Microsoft I'm part of an applied research team, technically part of MSR but forward deployed near a product team. Microsoft is experimenting with this kind of structure because, like many organizations, they would like to lower the impedance mismatch between research and production. After a year situated as such, I'm starting to appreciate the difficulty.<br /><br />Consider the following scenario: a production team has a particular problem which has been vexing them lately and they are flummoxed. They schedule a meeting with some authorities in the research department, there's some discussion back and forth, but then no follow-up. What happened? (There's also the converse scenario: a researcher develops or hears about a new technique that they feel is applicable to some product, so they schedule a meeting with a product group, there's some discussion back and forth, but then no follow-up. I won't be discussing that today.)<br /><br />I think about why nothing resulted from such a meeting in terms of incentives and motivations. In other words, there is some reason why the researchers felt there were better uses for their time. This leads to the question of what are the desires and goals of someone who would devote their lives to research (remember philosophy means “love of knowledge”). Once they achieve a minimum level of financial support, intellectuals have other motivations that kick in. A big one is the desire for prestige or <a href="http://en.wikipedia.org/wiki/Egoboo">egoboo</a> (the same force that drives blogging and open-source software). The popular culture academic caricature of the anti-social misanthrope in the corner seems highly inaccurate: the successful researchers I've known are highly social and collaborative people who identify with a research community and seek the respect and attention of (and influence over) that community. Ultimately such prestige is redeemable for opportunities to join institutions (e.g., universities or industrial research departments), and hanging out with other smart people is another major motivation for intellectuals, as many of them recognize the nonlinear power of agglomeration effects. In other words, it is widely recognized that hanging out with smart people makes you smarter and gives you better ideas than you would have in isolation.<br /><br />Cognizant of the previous, I'm trying to understand how a researcher would go about allocating their own time. First let me say I'm not trying to be normative, or give the impression researchers are obsessively self-centered. To some extent everybody in a company is self-centered and getting activity aligned with group goals is imho mostly related to incentives (including social norms). One thing that should be clear from the preceding paragraph is that money will not be an effective incentive for most intellectuals, unless you are talking about so much money that they can essentially build their own research institute à la Stephan Wolfram. Just like in the VISA commercial, there are some things money can't buy, and it turns out intellectuals want those things. Those things are roughly: overcoming intellectual challenges, working with other smart people, and influencing entire research communities.<br /><br />So back to that problem the product team brought to the researchers. Is it that the problem is not sufficiently challenging? From what I've seen that is not the issue: if there is a straightforward solution, the researchers will provide some pointers and consultation and everybody will be happy. More typically, the problem is too challenging, sometimes fundamentally, but often due more to idiosyncratic aspects. <br /><br />Fundamentally challenging problems are like the problem Hal Duame <a href="http://nlpers.blogspot.com/2013/09/active-learning-for-positive-examples.html">recently blogged about</a>, and the best part of his blog post was the line “Ok I'll admit: I really don't know how to do this.” I think the response from researchers is often silence because it takes a very confident person to say something like that, especially when they are supposed to be the expert. For the researcher deciding how to allocate their time, fundamentally challenging problems are risky, because it is difficult to obtain prestige from lack of progress. Therefore I think it is reasonable for researchers to only devote a portion of their problem portfolio on the fundamentally difficult.[<a href="#footnote1">1</a>] (By the way, there is an art to knowing where the frontier is: that portion of the limitless unknown which is challenging but potentially within reach and therefore worthy of attention.) It is sometimes possible to make partial progress on fundamental problems via heuristic approaches (aka hacks), but it is difficult to get community recognition for this kind of activity.<br /><br />In contrast to fundamental challenges, challenges due to idiosyncratic constraints are pervasive. After all, the product team is often somewhat familiar with the possibilities of the state of art in a field, which is what motivated the meeting to begin with. However there is some reason why the straightforward solution cannot be applied, e.g., too expensive, too strategically implausible, too complicated to implement reliably, too incompatible with legacy infrastructure, etc. Whether or not such problems get addressed has to do with whether the community will find the constraints interesting (or, with a really senior thought leader, whether or not the community can be convinced that the constraints are interesting). Interesting is often a function of generality, and idiosyncratic problem aspects are inherently problem specific. Possibly after addressing many different idiosyncratic problem presentations, a researcher might be able to generalize across the experiences and abstract a new class of problems with a common solution, but it is again a risky strategy to allocate time to idiosyncratic problems with the hope that a generalization will emerge, because without such a generalization obtaining community recognition will be difficult.<br /><br />Sometimes problems present a multi-objective optimization scenario which goes beyond conceptual complexity into ambiguity. In other words, it's not clear what's better. Under those conditions the community can focus on an objective which is well-defined but irrelevant. At <a href="http://amazonml.com/event/uai2013-workshop/">UAI this year</a> Carlos Uribe stated that more accurate prediction of the star rating of a Netflix movie has, as far as they can tell, no impact on the customer experience. He had to say something like this because for several years it was possible to get a best paper by doing better on the Netflix data set, and he'd like to see us focused on something else.<br /><br />So what should an organization with a multi-billion dollar research department do to lower the impedance mismatch between research and production? I don't know! I think part of the answer is to change what is considered prestigious. I could almost see an institution taking the position of «no publications», not because they are afraid of informing the competition, and not because they fail to see the value of collecting one's thoughts presentably and subjecting them to peer review; but rather because the external communities that manage publications allocate prestige and therefore effectively control the compensation of the research department. However I don't think this is tenable. So instead one has to create and foster venues where the idiosyncratic is embraced, where partial solutions are recognized, and where mere accounts of practical challenges and experiences (i.e., confusion) is considered a contribution.<br /><br />For me personally, it's clear I need to get out more. I like going to the big ML conferences like NIPS, ICML, and UAI, but I've never been to KDD. KDD papers like <a href="http://www.exp-platform.com/documents/puzzlingoutcomesincontrolledexperiments.pdf">Trustworthy Online Controlled Experiments: Five Puzzling Outcomes Explained</a> suggest I'm missing out.<br /><br /><a name="footnote1"><h4>1</h4></a><br />You might be asking, ``don't researchers devote themselves exclusively to the fundamentally difficult?'' Video game designers will tell you people like problems that are hard but not too hard; but even this perspective assumes researchers have plenary discretion in problem selection. In practice there are career considerations. Additionally, researchers invest and develop a certain proficiency in a certain area over time, and there are switching costs. The result is a large portion of activity is incremental progress. They're called Grand Challenge Problems for a reason!Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-32101812284265380032013-08-27T10:02:00.000-07:002014-10-21T09:11:11.797-07:00CosplayIf you're a fan of primal linear methods, then you've probably spent a lot of time thinking about feature engineering. If you've used kernel learning, then you've probably spent a lot of time thinking about kernels that are appropriate to your problem, which is another way of thinking about feature engineering. It turns out there is a way to leverage the work of the kernel community while solving a primal convex optimization problem: random feature maps. This idea has been around for a while: the <a href="http://pages.cs.wisc.edu/~brecht/papers/07.rah.rec.nips.pdf">paper by Rahimi and Recht</a> that really kicked things off is from 2007, Alex has <a href="http://blog.smola.org/post/10572672684/the-neal-kernel-and-random-kitchen-sinks">blogged about it</a> and <a href="http://cs.stanford.edu/~quocle/LeSarlosSmola_ICML13.pdf">refined the technique</a>, and Veeramachaneni has a <a href="http://mlstat.wordpress.com/2010/10/04/random-fourier-features-for-kernel-density-estimation/">nice blog post</a> which graphically explores the technique. One general strategy is to find an integral representation of a kernel, and then approximate the integral representation via Monte Carlo. At the end of the day, this looks like a randomized feature map $\phi$ for which dot products $\phi (x)^\top \phi(y)$ in the primal are converging to the value of kernel function $k (x, y)$. When using the Fourier basis for the integral representation of the kernel, the random feature map consists of cosines, so we call it ``cosplay'' over in here in CISL. <br /><br />The technique deserves to be more well known than it is, because it gives good learning performance (when the associated kernel is a good choice for the problem), it is straightforward to implement, and it is very fast. I'm hoping that I can increase awareness by providing a simple implementation on a well-known dataset, and in that spirit here is a Matlab script which applies the technique to mnist. Before running this you need to download <a href="http://cs.nyu.edu/~roweis/data/mnist_all.mat">mnist in matlab format</a>, and download <a href="http://www.cs.grinnell.edu/~weinman/code/">maxent and lbfgs for matlab</a>.<br /><br /><pre class="brush:bash">rand('seed',867);<br />randn('seed',5309);<br /><br />tic<br />fprintf('loading mnist');<br /><br />% get mnist from http://cs.nyu.edu/~roweis/data/mnist_all.mat<br />load('mnist_all.mat');<br /><br />trainx=single([train0; train1; train2; train3; train4; train5; train6; train7; train8; train9])/255.0;<br />testx=single([test0; test1; test2; test3; test4; test5; test6; test7; test8; test9])/255.0;<br />st=[size(train0,1); size(train1,1); size(train2,1); size(train3,1); size(train4,1); size(train5,1); size(train6,1); size(train7,1); size(train8,1); size(train9,1)];<br />ss=[size(test0,1); size(test1,1); size(test2,1); size(test3,1); size(test4,1); size(test5,1); size(test6,1); size(test7,1); size(test8,1); size(test9,1)];<br />paren = @(x, varargin) x(varargin{:});<br />yt=[]; for i=1:10; yt=[yt; repmat(paren(eye(10),i,:),st(i),1)]; end<br />ys=[]; for i=1:10; ys=[ys; repmat(paren(eye(10),i,:),ss(i),1)]; end<br /><br />clear i st ss<br />clear train0 train1 train2 train3 train4 train5 train6 train7 train8 train9<br />clear test0 test1 test2 test3 test4 test5 test6 test7 test8 test9<br /><br />fprintf(' finished: ');<br />toc<br /><br />tic<br />fprintf('computing random feature map');<br /><br />% (uncentered) pca to 50 ... makes subsequent operations faster,<br />% but also makes the random projection more efficient by focusing on<br />% where the data is<br /><br />opts.isreal = true; <br />[v,~]=eigs(double(trainx'*trainx),50,'LM',opts);<br />trainx=trainx*v;<br />testx=testx*v; <br />clear v opts;<br /><br />% estimate kernel bandwidth using the "median trick"<br />% this is a standard Gaussian kernel technique<br /><br />[n,k]=size(yt);<br />[m,p]=size(testx);<br />sz=3000;<br />perm=randperm(n);<br />sample=trainx(perm(1:sz),:);<br />norms=sum(sample.^2,2);<br />dist=norms*ones(1,sz)+ones(sz,1)*norms'-2*sample*sample';<br />scale=1/sqrt(median(dist(:)));<br /><br />clear sz perm sample norms dist;<br /><br />% here is the actual feature map:<br />% Gaussian random matrix, uniform phase, and cosine<br /><br />d=4000;<br />r=randn(p,d);<br />b=2.0*pi*rand(1,d);<br />trainx=cos(bsxfun(@plus,scale*trainx*r,b));<br />testx=cos(bsxfun(@plus,scale*testx*r,b));<br /><br />fprintf(' finished: ');<br />toc<br /><br />tic<br />fprintf('starting logistic regression (this takes a while)\n');<br /><br />% get @maxent and lbfgs.m from http://www.cs.grinnell.edu/~weinman/code/<br />% if you get an error about randint being undefined, change it to randi<br /><br />addpath recognition;<br />addpath opt;<br />addpath local;<br /><br />C0=maxent(k,d);<br />[~,trainy]=max(yt');<br />options.MaxIter=300; <br />options.Display='off';<br />C1=train(C0,trainy,trainx,'gauss',4.2813,[],[],[],options);<br />% regularizer was chosen by cross-validation as follows<br />%perm=randperm(n);<br />%it=logical(zeros(1,n));<br />%it(perm(1:int32(0.8*n)))=1;<br />%[C1,V]=cvtrain(C0,trainy(perm),trainx(perm,:),'gauss',10.^linspace(-4,4,20), ...<br />% [],0,[],it,[],@accuracy);<br /> <br />fprintf('finished: ');<br />toc<br />fprintf('train accuracy is %g\n',accuracy(C1,trainy,trainx));<br />[~,testy]=max(ys');<br />fprintf('test accuracy is %g\n',accuracy(C1,testy,testx));<br /></pre><br />Here's the result of running the script on my laptop:<br /><pre class="brush:bash">>> clear all; cosplay<br />loading mnist finished: Elapsed time is 2.227499 seconds.<br />computing random feature map finished: Elapsed time is 6.994094 seconds.<br />starting logistic regression (this takes a while)<br />finished: Elapsed time is 219.007670 seconds.<br />train accuracy is 0.99905<br />test accuracy is 0.9822<br /></pre>This approaches the performance of the Gaussian kernel SVM, but with simplicity and speed. By trying different <a href="http://www.maths.lth.se/matematiklth/personal/sminchis/code/randfeat/index.html">random feature maps</a>, you can improve upon this result.<br /><br />If you like this sort of thing, make sure to check out the <a href="https://sites.google.com/site/randomizedmethods/">Randomized Methods for Machine Learning</a> workshop at NIPS 2013.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com1tag:blogger.com,1999:blog-4446292666398344382.post-3830065406936324682013-07-19T20:30:00.000-07:002014-10-21T09:11:55.982-07:00Using Less DataIn a <a href="/2013/06/productivity-is-about-not-waiting.html">previous post</a> I indicated that operating on smaller datasets was one of the best ways to ensure productivity and rapid experimentation when doing data science. At ICML 2013, Nikos and I presented a <a href="http://arxiv.org/abs/1306.1840">general strategy</a> for using less data that applies to a wide variety of problems.<br /><br />The idea was inspired by the <a href="/2010/12/even-more-on-subsampling-zero-reward.html">class-imbalanced subsampling heuristic</a>. This is a well-known trick amongst computational advertising practitioners, and the uncanny effectiveness of this technique always intrigued me. Here's the trick: when a binary classification dataset mostly consists of examples from one class, the more frequent class can be subsampled without affecting the final classifier quality. It turns out we were able to generalize this trick as follows. <br /><br />Suppose you are planning to choose your hypothesis $h^*$ from a set $\mathcal{H}$ by miinimizing a loss function $\mathcal{L}$, i.e., empirical risk minimization (ERM). Also suppose someone hands you a hypothesis $\tilde h \in \mathcal{H}$. You can use $\tilde h$ to subsample your data set prior to performing the ERM, and the excess risk introduced is modest (see the paper for the exact excess risk bound). The amount of data that can be discarded depends upon the empirical loss of $\tilde h$; if $\tilde h$ has low empirical loss, than you can subsample aggressively. The strategy is simple: sample each example $x$ at a rate proportional to the loss of $\tilde h$ on $x$. You have to importance-weight the resulting subsample to remain unbiased and minimize the importance-weighted empirical loss in the final step, but many machine learning algorithms can incorporate importance-weights directly (if not, you can use <a href="http://dl.acm.org/citation.cfm?id=952181">reduce importance-weighted loss to uniformly weighted loss</a> via rejection sampling).<br /><br />In this interpretation, the class-imbalanced subsampling heuristic corresponds to using a $\tilde h$ which is a constant predictor, e.g., $\forall x, \tilde h (x) = 0$ would be a good choice for advertising where positive examples (e.g., clicks) are relatively rare. The generalization of this technique, however, applies more broadly. First, we have a very broad notion of loss which not only includes classification, regression, ranking, and structured prediction; but also some unsupervised settings which optimize a pointwise loss, e.g., reconstruction error or perplexity. Second, we allow the use of any $\tilde h$ which is in the hypothesis set $\mathcal{H}$. In general a good choice for $\tilde h$ is any hypothesis which can be easily estimated at scale but which achieves low loss. For class-imbalanced problems the best constant predictor is quite good and easy to estimate at scale (just count the labels!), but even for problems that are not imbalanced the ability to freely choose $\tilde h$ admits great flexibility.<br /><br />As an example of the power enabled by freely choosing $\tilde h$, consider eHarmony where I used to work. One problem at eHarmony is estimating the probability that two people, if matched, will communicate with each other. eHarmony has been handing out circa 10<sup>6</sup> matches per day for many years, so they have a big dataset. Although eHarmony uses hundreds of features to predict communication, there are a few that are very informative, and a large number that are mildly informative. If you caught Vaclav Petricek's excellent talk during the <a href="http://amazonml.com/event/uai2013-workshop/">recommendation workshop at UAI 2013</a>, you know that height difference, age difference, and physical distance are three features that have high predictive value. A predictor based only upon these three predictors can easily be assembled at scale using only Hive, and while not optimal it will have relatively low loss; therefore, this is a good candidate for $\tilde h$. I haven't tried this on eHarmony's data because I didn't know these things when I worked there, but I talked to Vaclav and he's willing to give it a go.<br /><br />An important special case of this technique is using a linear predictor for $\tilde h$ and either a neural network or a decision tree for the final ERM. This is helpful because linear predictors can be <a href="http://arxiv.org/abs/1110.4198">estimated at scale</a>. Note to meet the conditions of the theorem you have to make sure the linear predictor is in $\mathcal{H}$, which for neural networks implies direct connections from the input to the last layer, and for both techniques implies the nonlinear predictor has access to all the linear features (adding features for the final ERM is ok). As a bonus effect, feature representations that work well for the linear model will also <a href="/2013/02/one-louder.html">tend to help the nonlinear models as well</a>.<br /><br />So now you have a principled way to subsample your giant dataset which will work in more general settings than class imbalance. Go forth and discard some data!Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com7tag:blogger.com,1999:blog-4446292666398344382.post-47244687627601426482013-06-22T11:06:00.000-07:002014-10-21T09:12:28.697-07:00ICML 2013: Sparse, Deep, and Random<a href="http://icml.cc/2013/">ICML 2013</a> was a great conference this year, kudos to the organizers. It's too big for an individual to get a comprehensive view of everything but I did notice three trends.<br /><br />First, sparsity as a structural constraint seemed ubiquitous. Since I know very little about this sub-field, I paid closely attention to the first two minutes of talks where foundational issues are typically (quickly!) discussed, e.g., “why do people care about sparsity at all?”. I heard general motivations such as computational convenience and intelligibility. I also heard somewhat specific motivations, e.g., <a href="http://cseweb.ucsd.edu/~djhsu/papers/dag-icml.pdf">Anandkumar et. al.</a> showed that for particular generative model structures the parameters can be identified via sparse coding techniques; and Ruvolo and Eaton advocated <a href="http://cs.brynmawr.edu/~eeaton/papers/Ruvolo2013Online.pdf">sparse coding of models</a> in multi-task learning to facilitate knowledge transfer between tasks.<br /><br />Second, deep learning continues to enjoy a resurgence. Two talks in particular suggested some important future directions. The first was a talk by Coates about deep learning on the following architecture: <a href="http://www.stanford.edu/~acoates/papers/CoatesHuvalWangWuNgCatanzaro_icml2013.pdf">16 machines with 4 GPUs each wired up via infiniband</a>. I've complained on this blog about how the high communication costs of SGD make it a poor distributed learning algorithm but Coates et. al. are attacking this problem directly with hardware. This is clearly the near future. The bottom line is that we don't really have better training algorithms for neural networks, but the economics of the problems they are solving are so important that to the extent it is possible to “throw hardware at it”, hardware will be thrown. The second talk was <a href="http://arxiv.org/pdf/1211.5063.pdf">On the Difficulty of Training Recurrent Neural Networks</a> by Pascanu et. al., which discussed some improvements to gradient-based learning in the recurrent setting. It's clear that the deep learning guys, having dominated the “natural UI” problems so important in the mobile space (e.g., speech recognition and image tagging), are now looking to dominate sequential prediction tasks (which should increase in importance as autonomous systems become more ubiquitous). They will have strong competition from the kernel guys: Le Song gave an amazing talk on <a href="http://www.stanford.edu/~jhuang11/research/pubs/icml09/icml09.pdf">Hilbert Space Embedding of Conditional Distributions</a> with applications to sequential prediction.<br /><br />Speaking of kernel guys, the third theme was random, and in particular Alex Smola's talk on <a href="http://ai.stanford.edu/~quocle/LeSarlosSmola_ICML13_supp.pdf">fast randomized approximation of kernel learning</a> (“FastFood”) was a real treat. Presumably the combination of randomized computational techniques with Hilbert space representations of conditional distributions will result in strong algorithms for sequential prediction and other latent modeling tasks. Another standout in this area was Mahoney's talk on <a href="http://arxiv.org/pdf/1303.1849v2.pdf">Revisiting the Nyström Method for Improved Large-Scale Machine Learning</a>. <br /><br />Note unlike the first two themes (sparse and deep), I wouldn't say random is a broadly popular theme yet. I happen to be personally very excited by it, and I think the implications for machine learning are large especially in the distributed setting. Basically the numerical linear algebra guys who have advanced these randomized algorithms have been working on “<a href="http://techtalks.tv/talks/54107/">architecture aware computing</a>” for many years now, something that the machine learning community is only starting to appreciate. For a glimpse of what this might mean to you, consider David Gleich's talk on <a href="http://www.slideshare.net/dgleich/whatyoucandowithtsqrfactorization">Tall and Skinny QR Factorizations in Hadoop</a>.<br /><br />Finally, I have to mention John Langford and Hal Daumé gave an awesome talk on imperative learning, which does not fit into any of the above buckets. I couldn't find any online material, which is unfortunate because this is really cool, and if you've ever put a machine learning algorithm into an existing production system you will love this right away. The basic idea is that you, the end user, program “normally” and make calls into a learning algorithm which is implemented as a coroutine. This has two advantages: first, the algorithm naturally experiences the distribution of decisions induced by the program, so the “dataset collection” problem and associated errors is mitigated (this is extra important for sequential prediction); and second, the training and evaluation time code paths are the same so implementation in production is both easier and less error-prone. Note the evaluation time overhead is minimal in this setup so there is no temptation to rewrite the algorithm for production. Testing time overhead is introduced but this can be mitigated by decorating the code with additional annotations that allow the learning algorithm to memoize appropriately. This is so hot that I want to volunteer for some sequential prediction tasks in my neighborhood just to try it out.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com2tag:blogger.com,1999:blog-4446292666398344382.post-17963471356965744522013-06-06T10:52:00.000-07:002014-10-21T09:12:55.437-07:00Productivity is about not waitingI recently gave a talk about practical tips for applied data science, and I think my best observation is really quite simple: practitioners are subject to long-running data manipulation process, which means there is a lot of waiting going on. If you can wait less, this translates directly to your productivity. There's a cool sci-fi book called <a href="http://en.wikipedia.org/wiki/Altered_Carbon">Altered Carbon</a> in which characters upload themselves to simulators in order to think faster: to the extent that you wait less, you are doing something akin to this.<br /><br />To keep things simple I mentally bucket things into different timescales:<br /><ol><li>Immediate: less than 60 seconds.<br /></li><li>Bathroom break: less than 5 minutes.<br /></li><li>Lunch break: less than 1 hour.<br /></li><li>Overnight: less than 12 hours.<br /></li></ol>There are more than 20 times as many immediate iterations as there are lunch breaks. You only work 20 days a month, so waiting less means you do in a day what somebody else does in a month. Furthermore, there is a superlinear degradation in your productivity as you move from the immediate zone to the bathroom break zone, due to the fact that you (the human) are more prone to attempt to multitask when faced with a longer delay, and people <a href="http://blogs.wsj.com/digits/2009/08/26/why-multitaskers-stink-at-multitasking/">are horrible at multitasking</a>.<br /><br />Here are some tricks I use to avoid waiting.<br /><br /><h3>Use Less Data</h3>This is a simple strategy which people nonetheless fail to exploit. When you are experimenting or debugging you don't know enough about your data or software to justify computing over all of it. As Eddie Izzard says, <a href="http://www.youtube.com/watch?v=jbc9ViqkIm8">``scale it down a bit!''</a> <br /><br /><h3>Sublinear Debugging</h3>The idea here is to output enough intermediate information as a calculation is progressing to determine before it finishes whether you've injected a major defect or a significant improvement. Online learning is especially amenable to this as it makes relatively steady progress and provides instantaneous loss information, but other techniques can be adapted to do something like this. Learning curves do occasionally cross, but sublinear debugging is fabulous for immediately recognizing that you've fat-fingered something. <br /><br />The clever terminology is courtesy of John Langford.<br /><br /><h3>Linear Feature Engineering</h3>I've found that engineering features for a linear model and then switching to a more complicated model on the same representation (e.g., neural network, gradient boosted decision tree) is a fruitful pattern, because the speed of the linear model facilitates rapid experimentation. Something that works well for a linear model will tend to work well for the other models. Of course, it's harder to engineer features that are good for a linear model. In addition you have to keep in mind the final model you want to use, e.g., if you are ultimately using a tree than monotonic transformations of single variables are only helping the linear model. <br /><br /><h3>Move the Code to the Data</h3>This is the raison d'etre of Hadoop, but even in simpler settings making sure that you are doing your work close to where the data is can save you valuable transmission time. If your data is in a cloud storage provider, spin up a VM in the same data center.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com2tag:blogger.com,1999:blog-4446292666398344382.post-90389633770526280532013-05-05T10:08:00.000-07:002014-10-21T09:13:17.822-07:00Data has mass: code accordingly!Machines are getting bigger and faster now: you can rent a machine with 244 gigabytes of RAM and dozens of cores for $3.50 per hour. That means you can take down sizeable datasets without getting into the pain of distributed programming (as noted by Jeff Hodges, ``<a href="http://www.somethingsimilar.com/2013/01/14/notes-on-distributed-systems-for-young-bloods/">problems that are local to one machine are easy</a>.'') However most machine learning software I try is not really up to the task because it fails to respect the fact that <span style="font-style: italic;">data has mass</span>. Here are some nits of mine for people thinking of writing machine learning software and hoping to scale by using a larger single computer.<br /><br />The number thing to remember is that I/O is expensive; in the single machine context in particular moving data to or from disk. It's painful when a piece of software forces you to move data to or from the disk unnecessarily, e.g., forces you to combine multiple files into a single file, forces you to uncompress data, or more generally forces you to materialize any kind of easily computed transformation of the data. Here are some ways to avoid causing end users (including yourself!) pain.<br /><br />First, if you can, stream over the data instead of seeking arbitrarily within it. Why is this important? Because if a program treats the input as a stream than it can be transformed in-memory on demand before being input to the program, which makes the program far more versatile. This alone solves a lot of other problems; for instance if my data is spread over multiple files I can just concatenate them into one stream on the fly, or if my data is compressed in a funny format I can decompress it on the fly. Now you might say ``hey I don't have an online algorithm'', but nonetheless many programs miss an opportunity to access their input as a stream. For instance, the (otherwise truly excellent!) <a href="http://www.csie.ntu.edu.tw/~cjlin/liblinear/">liblinear</a> does exactly two sequential passes over the data (the first determines dimensionality and the second reads the data into core), but between these two passes it calls <span style="font-family: monospace;"><a href="http://www.unix.com/man-page/POSIX/3posix/rewind/">rewind</a></span>. That call to rewind makes things very difficult; if instead liblinear merely closed and reopened the file again, then I could play tricks with named pipes to avoid all sorts of I/O. Even better, if liblinear allowed one to specify two files instead of one (with the second file specification optional and the default being that the second file is the same as the first), then things are even easier since <a href="http://zsh.sourceforge.net/Doc/Release/Expansion.html#Process-Substitution">streaming process redirection</a> tricks can be brought to bear. Alas, it does neither of these things.<br /><br />While religious adherence to streaming patterns of access is the best mode of practice, sometimes it is not possible. In that case, there are still some common patterns that could be supported. <span style="font-style: italic;">Don't make me concatenate files</span>: this is worse than <a href="http://partmaps.org/era/unix/award.html">useless use of cat</a>, it is painfully slow and unnecessary use of cat. If you can take one file argument, you can take more than one file argument and just act like they have been concatenated. <span style="font-style: italic;">Don't make me decompress the data</span>: you should support reading compressed data directly. Not only does this eliminate the need to materialize a decompressed version of the data, it's also faster to read compressed data and decompress it on the fly than to read decompressed data. <span style="font-style: italic;">Do allow me to pretend the input is smaller than it really is</span> and only have your program operate on the first specified units of the input (e.g., first 10000 records); this facilitates experimentation with progressively larger portions of data. (I say data science is like courtship; I don't like to go ``all the way'' with a giant set of data until I've had a few casual encounters of smaller magnitude.) Again, if you treat the input as a stream than I can achieve all of these goals and others without additional assistance from your program, so these admonitions are only for those who for whatever reason need to access their input non-sequentially. By the way, you can tell if you are treating the input as a stream: the only operations you are allowed to do are open, read, and close.<br /><br />There is another level that few machine learning toolkits fully achieve and that is a <a href="http://en.wikipedia.org/wiki/Domain-specific_language">DSL</a> for just-in-time data transformations. This can be emulated without materialization using interprocess communication if the program treats the input data as a stream, and as long as the transformed data is not too large the overhead of <a href="http://en.wikipedia.org/wiki/Inter-process_communication">IPC</a> is acceptable. Here's an example shell script excerpt from the mnist demo in <a href="https://github.com/JohnLangford/vowpal_wabbit">vowpal wabbit</a><br /><pre class="brush:bash">SHUFFLE='BEGIN { srand 69; };<br /> $i = int rand 1000;<br /> print $b[$i] if $b[$i];<br /> $b[$i] = $_; } { print grep { defined $_ } @b;'<br /><br />paste -d' ' \<br /> <(zcat train8m-labels-idx1-ubyte.gz | ./extract-labels) \<br /> <(zcat train8m-images-idx3-ubyte.gz | ./extractpixels) | \<br />perl -ne ${SHUFFLE} | \<br />./roundrobin ./pixelngrams 3 | \<br />time ../../vowpalwabbit/vw --oaa 10 -f mnist8m11png.model \<br /> -b 20 --adaptive --invariant \<br /> --nn 5 --inpass \<br /> -l 0.05<br /></pre>The problems being solved here without intermediate materialization to disk are: 1) the labels and data are in different files (which are not line-oriented) and need to be joined together; 2) it helps to shuffle the data a bit to break up sequential correlation for <a href="http://en.wikipedia.org/wiki/Stochastic_gradient_descent">SGD</a>; and 3) additional pixelngram features need to be computed and this is CPU intensive so 3 cores are used for this (via the helper script roundrobin).<br /><br />So the above looks great, why do we need a DSL to do just-in-time data transformations inside the machine learning software? One big reason is that the intermediates get large and the overhead of IPC and parsing the input becomes substantial; in such cases it is beneficial to defer the transformation until inside the learning core (e.g., <a href="http://www.raetschlab.org/members/sonne/bibliography/SonnenburgFranc2010">COFFIN</a>). The <span style="font-family: monospace;">-q</span>, <span style="font-family: monospace;">--cubic</span>, and <span style="font-family: monospace;">--ngram</span> arguments to vowpal wabbit constitute a simple mini-language with substantial practical utility; even better would be something as expressive as <a href="http://science.nature.nps.gov/im/datamgmt/statistics/r/formulas/">R formulas</a>.Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com0tag:blogger.com,1999:blog-4446292666398344382.post-13606144629052448542013-04-30T11:25:00.000-07:002014-10-21T09:14:13.046-07:00Learning is easier than optimizationThe title of this blog post is a phrase attributed to <a href="http://leon.bottou.org/">Leon Bottou</a> (although you wouldn't know this from a web search: apparently the startup community has a <a href="http://www.startuplessonslearned.com/2010/04/learning-is-better-than-optimization.html">similar aphorism</a>). This is one of the few deep truths in machine learning, and I think it's especially important to keep in mind as we foray into a brave new world where <a href="http://digitaljournal.com/article/308479">the data exponent is overwhelming the computation exponent</a>. The gist is that training error is a proxy for what we really care about, generalization error, and therefore it can be counterproductive to expend computational effort excessively optimizing training error. The counter-productivity arises in (at least) two different ways. The first is that excessive computational effort might cause you to use less data in order to compensate for computation time, and using less data means the proxy is less accurate. The second is that less aggressive optimization can act as a regularizer, which when corrected with a ``better'' optimization routine actually leads to worse generalization error.<br /><br />Two examples of this phenomenon come to mind. First, the dominance of <a href="http://en.wikipedia.org/wiki/Stochastic_gradient_descent">stochastic gradient descent</a> (SGD). When neural networks first came out SGD was the only optimization algorithm, but people quickly started experimenting with fancier quasi-Newton based techniques. Today, neural networks are still trained with algorithms that hug close to SGD, and arguably the main differences we see today are in architectures and regularizers (and architecture is a kind of regularizer). The fact is, SGD is a so-so optimization algorithm, but a great learning algorithm.<br /><br />A second example is the <a href="http://en.wikipedia.org/wiki/Feature_hashing">hashing trick</a>. Features have to be converted into indices for algorithms that operate on real vector representations (i.e., all algorithms afaik), and so one strategy is to build and maintain a dictionary mapping features to indices. For really high cardinality feature spaces this is not just tedious, but can cause an infeasible demand for memory. When utilizing the hashing trick, one applies a hash function to the feature identity in order to determine the feature index. Everybody who first encounters this thinks, "aren't there collisions?". The answer is, "yes there are collisions" and "it doesn't seem to matter much in practice." One can talk about how it preserves dot products in expectation, or that how statistically stable conjunction features can be as informative in the presence of redundancy, but here's what's really neat: I've seen lots of examples where increasing the number of bits in the hash function degrades performance. In other words, less collisions, but worse generalization; this is because the hash collisions are providing a useful constraint on model complexity and <span style="font-style: italic;">learning is easier than optimization</span>.<br /><br />Ok, but so what? Should we all get sloppy in our coding and do the machine-learning equivalent of <a href="http://www.skygod.com/quotes/hitchhikers.html">flying via throwing ourselves at the ground and missing</a>? No, but I think it's worthwhile keeping an open mind. In particular, keeping up with the torrents of modern data requires developing distributed algorithms, and one of the great inefficiencies of parallel programming is synchronization. So I suggest keeping an open mind with respect to synchronization-free learning algorithms.<br /><br />Niu et. al. have been at the forefront of such thinking with their <a href="http://pages.cs.wisc.edu/~brecht/papers/hogwildTR.pdf">Hogwild!</a> proposal of lock-free parallel SGD on a shared representation. When I first heard about this I thought, "that can't possibly work." But, that's what I thought about the hashing trick when I first heard of it. So I haven't tried Hogwild! personally and I can't vouch for it, but I'm not willing to dismiss it either.<br /><br />While Niu et. al. propose doing ``dirty writes'', <a href="http://wan.poly.edu/KDD2012/docs/p177.pdf">Matshushima et. al.</a> propose the more intuitively palatable idea of ``dirty reads''. Among multiple ideas presented is to have a parsing thread maintaining a buffer of examples while a concurrent learning thread trains on an example from the buffer, with minimal synchronization between the two. For many online algorithms parsing is the bottleneck, and therefore the learning algorithm will train on each example several times in expectation before the parsing thread replaces the example. Thus this looks something like a mini-batch algorithm but with the mini-batch changing slowly over time, which sounds like a great idea. I'm playing with their <a href="http://www.r.dl.itc.u-tokyo.ac.jp/~masin/streamsvm.html">StreamSVM</a> software right now, fingers crossed!<br />Paul Mineirohttp://www.blogger.com/profile/05439062526157173163noreply@blogger.com5