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Scalable Kernel Methods via Doubly Stochastic Gradients

Neural Information Processing Systems

The general perception is that kernel methods are not scalable, so neural nets become the choice for large-scale nonlinear learning problems. Have we tried hard enough for kernel methods? In this paper, we propose an approach that scales up kernel methods using a novel concept called ``doubly stochastic functional gradients''. Based on the fact that many kernel methods can be expressed as convex optimization problems, our approach solves the optimization problems by making two unbiased stochastic approximations to the functional gradient---one using random training points and another using random features associated with the kernel---and performing descent steps with this noisy functional gradient. Our algorithm is simple, need no commit to a preset number of random features, and allows the flexibility of the function class to grow as we see more incoming data in the streaming setting. We demonstrate that a function learned by this procedure after t iterations converges to the optimal function in the reproducing kernel Hilbert space in rate O(1/t), and achieves a generalization bound of O(1/\sqrt{t}). Our approach can readily scale kernel methods up to the regimes which are dominated by neural nets. We show competitive performances of our approach as compared to neural nets in datasets such as 2.3 million energy materials from MolecularSpace, 8 million handwritten digits from MNIST, and 1 million photos from ImageNet using convolution features.


Topmoumoute Online Natural Gradient Algorithm

Neural Information Processing Systems

Guided by the goal of obtaining an optimization algorithm that is both fast and yielding good generalization, we study the descent direction maximizing the decrease in generalization error or the probability of not increasing generalization error. The surprising result is that from both the Bayesian and frequentist perspectives this can yield the natural gradient direction. Although that direction can be very expensive to compute we develop an efficient, general, online approximation to the natural gradient descent which is suited to large scale problems. We report experimental results showing much faster convergence in computation time and in number of iterations with TONGA (Topmoumoute Online natural Gradient Algorithm) than with stochastic gradient descent, even on very large datasets.


SGD Implicitly Regularizes Generalization Error

arXiv.org Machine Learning

We derive a simple and model-independent formula for the change in the generalization gap due to a gradient descent update. We then compare the change in the test error for stochastic gradient descent to the change in test error from an equivalent number of gradient descent updates and show explicitly that stochastic gradient descent acts to regularize generalization error by decorrelating nearby updates. These calculations depends on the details of the model only through the mean and covariance of the gradient distribution, which may be readily measured for particular models of interest. We discuss further improvements to these calculations and comment on possible implications for stochastic optimization. Note added: this paper appeared in the "Workshop on Integration of Deep Learning Theories" at NeurIPS in 2018 [1]. Given the current interest in this topic (see e.g.


Stable Robbins-Monro approximations through stochastic proximal updates

arXiv.org Machine Learning

The need for parameter estimation with massive data has reinvigorated interest in iterative estimation procedures. Stochastic approximations, such as stochastic gradient descent, are at the forefront of this recent development because they yield simple, generic, and extremely fast iterative estimation procedures. Such stochastic approximations, however, are often numerically unstable. As a consequence, current practice has turned to proximal operators, which can induce stable parameter updates within iterations. While the majority of classical iterative estimation procedures are subsumed by the framework of Robbins and Monro (1951), there is no such generalization for stochastic approximations with proximal updates. In this paper, we conceptualize a general stochastic approximation method with proximal updates. This method can be applied even in situations where the analytical form of the objective is not known, and so it generalizes many stochastic gradient procedures with proximal operators currently in use. Our theoretical analysis indicates that the proposed method has important stability benefits over the classical stochastic approximation method. Exact instantiations of the proposed method are challenging, but we show that approximate instantiations lead to procedures that are easy to implement, and still dominate classical procedures by achieving numerical stability without tradeoffs. This last advantage is akin to that seen in deterministic proximal optimization, where the framework is typically impossible to instantiate exactly, but where approximate instantiations lead to new optimization procedures that dominate classical ones.


Understanding Deep Learning (Still) Requires Rethinking Generalization

Communications of the ACM

Despite their massive size, successful deep artificial neural networks can exhibit a remarkably small gap between training and test performance. Conventional wisdom attributes small generalization error either to properties of the model family or to the regularization techniques used during training. Through extensive systematic experiments, we show how these traditional approaches fail to explain why large neural networks generalize well in practice. Specifically, our experiments establish that state-of-the-art convolutional networks for image classification trained with stochastic gradient methods easily fit a random labeling of the training data. This phenomenon is qualitatively unaffected by explicit regularization and occurs even if we replace the true images by completely unstructured random noise. We corroborate these experimental findings with a theoretical construction showing that simple depth two neural networks already have perfect finite sample expressivity as soon as the number of parameters exceeds the number of data points as it usually does in practice. We interpret our experimental findings by comparison with traditional models. We supplement this republication with a new section at the end summarizing recent progresses in the field since the original version of this paper. For centuries, scientists, policy makers, actuaries, and salesmen alike have exploited the empirical fact that unknown outcomes, be they future or unobserved, often trace regularities found in past observations. We call this idea generalization: finding rules consistent with available data that apply to instances we have yet to encounter. Supervised machine learning builds on statistical tradition in how it formalizes the idea of generalization. We assume observations come from a fixed data generating process, such as samples drawn from a fixed distribution. In a first optimization step, called training, we fit a model to a set of data.