The Marginal Value of Adaptive Gradient Methods in Machine Learning
Adaptive optimization methods, which perform local optimization with a metric constructed from the history of iterates, are becoming increasingly popular for training deep neural networks. Examples include AdaGrad, RMSProp, and Adam. We show that for simple overparameterized problems, adaptive methods often find drastically different solutions than gradient descent (GD) or stochastic gradient descent (SGD). We construct an illustrative binary classification problem where the data is linearly separable, GD and SGD achieve zero test error, and AdaGrad, Adam, and RMSProp attain test errors arbitrarily close to half. We additionally study the empirical generalization capability of adaptive methods on several stateof-the-art deep learning models. We observe that the solutions found by adaptive methods generalize worse (often significantly worse) than SGD, even when these solutions have better training performance. These results suggest that practitioners should reconsider the use of adaptive methods to train neural networks.
// Summary:
In an optimization and a learning context, the authors compare recently introduced adaptive gradient methods and more traditional gradient descent methods (with potential momentum). Adaptive methods are based on metrics which evolve along the optimization process. Contrary to what happens for gradient descent, Nesterov's method or the heavy ball method, this may result in estimates which are outside of the linear span of past visited points and estimated gradients. These methods became very popular recently in a deep learning context. The main question adressed by the authors is to compare both categories of method. First the authors construct an easy classification example for which they prove that adaptive methods behave very badly while non adaptive methods achieve perfect accuracy. Second the authors report extensive numerical comparisons of the different classes of algorithms showing consistent superiority of non adaptive methods.
// Main comments:
I do not consider myself knowledgable enough to question the relevance and correctness of the numerical section.
This paper asks a very relevant question about adaptive methods. Giving a definitive answer is a complicated task and the authors provide elements illustrating that adaptive methods do not necessarily have a systematic advantage over non adaptive ones and that non adaptive optimization methods could be considered as serious challengers. The content of the paper is simple and really acessible, hilighting the simplicity of the question. Although simple, this remains extremely relevant and goes beyond the optimization litterature, questioning the reasons why certain methods reach very high popularity in certain communities. The elements given illustrate the message of the authors very clearly.
The main points I would like to raise:
- The artificial example given by the authors is only relevant for illustrative purposes. Comparison of algorithmic performances should be based on the ability to solve classes of problems rather than individual instances. Therefore, it is not really possible to draw any deep conclusion from this. For example, a bad choice of the starting point for non adaptive methods would make them equally bad as adaptive methods on this specific problem. I am not sure that the authors would consider this being a good argument against methods such as gradient descent.
- Popularity of adaptive methods should be related to some practical advantage from a numerical point of view. I guess that positive results and comparison with non adaptive methods have already been reported in the litterature. Do the authors have a comment on this?