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15thJANUARY2017

READING

I. Goodfellow, Y. Bengio, A. Courville. Deep Learning. Chapter 8, MIT Press, 2016.

In chapter 8, Goodfellow et al. discuss optimization for deep neural networks. In particular, they focus on three different aspects:

  1. How learning differs from classical optimization;
  2. Why learning deep neural networks is difficult;
  3. And concrete optimization techniques (both "fixed" learning rate and adaptive learning rate) as well as parameter initialization schemes.

How learning differs from classical optimization. One of the most important difference between learning and optimization is that learning tries to indirectly optimize a (usually) intractable measure while for optimization the goal is to directly optimize the objective at hand. In learning, the overall goal is to minimize the expected generalization error, i.e. the risk. However, as the generating distribution is usually unknown (and/or intractable), the empirical risk on a given training set is minimized instead. Furthermore, in learning, the empirical risk is usually not minimized directly, for example if the loss is not differentiable. Instead, a surrogate loss is minimized and optimization is usually stopped early to prevent overfitting and improve generalization. This is in contrast to classical optimization where the objective is usually not replaced by a surrogate objective. Lastly, in learning the optimization problem can usually be decomposed as a sum over the training samples.

Due to computational limits and considerations regarding generalization and implementation, researchers have early argued about so-called stochastic (mini-batch) optimization schemes. This discussion is usually specific to the task of learning and cannot be generalized to classical optimization. Some of the arguments made by Goodfellow et al. are summarized in the following points:

  1. The standard error of a mean estimator is $\frac{\sigma}{\sqrt{n}}$ where $\sigma$ is the true standard deviation and $n$ the number of samples. Thus, there is less than linear returns in using more samples for estimating statistics (e.g. the gradient).
  2. Small batches can offer regularization effects due to the introduced noise. However, this usually requires a smaller learning rate and induces slow learning.

Why learning deep neural networks is difficult. Goodfellow et al. discuss several well-known problems when training deep neural networks. However, they also give valuable insights of how these problems are related and approached in practice. The obvious argument is that used optimization techniques assume access to the true required statistics, such as the true gradient. However, in practice, the gradient is noisy and usually estimated based on stochastic mini-batches. Furthermore, the problem can be ill-conditioned, i.e. the corresponding Hessian matrix may be ill-conditioned. Goodfellow et al. provide an intuitive explanation based on a second-order Taylor expansion of the gradient descent update. Then, it can easily be shown that a gradient descent update of $-\epsilon g$, with $g$ begin the gradient, adds $\frac{1}{2} \epsilon^2 g^T H g - \epsilon g^Tg$ to the cost. However, this may get positive. In particular, Goodfellow et al. describe the case that $g^T g$ stays mostly constant during training while $g^T Hg$ may increase by an order of magnitude. They also describe that monitoring both values during training might be beneficial (the former is also useful to detect whether learning has problems with local minima).

Another important discussion provided by Goodfellow et al. is concerned with the importance of local minima and saddle points. The main insight is that in high-dimensional spaces, local minima become rare and saddle points become much more frequent. This can be explained by the corresponding Hessian matrix. For local minima, all eigenvalues need to be positive, while for saddle points, both positive and eigenvalues are present. Obviously, the latter case becomes more frequent in high-dimensional spaces. Furthermore, local minima are much more likely to have low cost. This, too, can be explained by the Hessian being more likely to have only positive eigenvalues in low cost areas. Therefore, in high-dimensional spaces, saddle-points are the more serious problem in learning. Still, both cases induce a difficulty and require optimization methods to be tuned to the specific learning task.

Lastly, Goodfellow et al. discuss the case of cliffs in the energy landscape corresponding to the learning problem. In particular, cliffs refer to extremely steep regions (which may occur suddenly in more or less flat regions). These cliffs usually cause extremely high gradient and may result in large jumps made by the gradient descent update, potentially increasing the cost. However, they also provide a simple counter-measure (see Chapter 11): gradient clipping. Gradient clipping is usually done by either clipping the individual entries of the gradient vector at a maximum value, or clipping the gradient vector as whole at a specific norm. Both approaches seem to work well in practice.

Basic optimization techniques. As basis for the remaining chapters, Goodfellow et al. discuss stochastic gradient descent with momentum as well as Nesterov's accelerated gradient method. Details can be found in the chapter. Especially, the detailed explanation of the momentum term can be recommended.

Some interesting arguments made by Goodfellow et al. concern the convergence rate. It is well known that the convergence rate of stochastic gradient descent in the strictly convex case is $\mathcal{O}\left(\frac{1}{k}\right)$. However, the generalization error cannot be reduced faster than $\mathcal{O}\left(\frac{1}{k}\right)$ such that, from the machine learning perspective, it might not be beneficial to consider algorithms offering faster convergence (as this may correspond to overfitting).

Weight initialization schemes. Goodfellow et al. discuss several weight initialization schemes without going into too much details. Instead of looking at the individual schemes, value can be found in the discussed heuristics - especially as they explicitly state that initialization (and optimization) is not well understood yet. The following presents an unordered list detailing some of the discussed heuristics:

  • An important aspect of weight initialization is to break symmetry, i.e. units with the same or similar input should have different initial weights as otherwise they would develop very similarly during training. This motivates random initialization using a high-entropy distribution - usually Gaussian or uniform.
  • Biases are usually initialized to constant values. In many cases, 0 might be sufficient, however for saturating activation functions or output activation function non-zero initialization should be considered.
  • The right balance between large initial weights and not-too-large initial weights is important. While too large weights may cause gradient explosion (if no clipping is used), large weights ensure that activations and errors (in forward and backward pass, respectively) are still numerically significant (i.e. distinct from 0).
  • Independent of the initialization scheme used, it is recommended to monitor the activations and gradients of all layers on individual mini-batches. If activations or gradients in specific layers vanish, the scale or range of initialization may need to be adapted.

Adaptive learning rate techniques. Goodfellow et al. discuss several techniques including AdaGrad, Adam and RMSProp (with and without momentum). However, they are not able to answer the question which of the techniques is to be preferred.

Meta algorithms. Finally, Goodfellow et al. discuss a set of meta algorithms to aid optimization. The most interesting part is concerned with batch normalization. In particular, they are able to provide an extremely intuitive motivation. Using any gradient descent based optimization technique, the computed updates for a particular layer assume that preceeding layers do not change - which of course is wrong. They also discuss that batch normalization - through normalizing the first and second moments - implicitly reduces the expressive power of the network, especially when using linear activation functions. This results in the model being easier to train. To avoid the restrictions in expressive power, batch normalization applies a reparameterization to allow non-zero mean and non-unit variance.

What is your opinion on the summarized work? Or do you know related work that is of interest? Let me know your thoughts in the comments below or get in touch with me: