Remarkable success of deep neural networks has not been easy to analyze theoretically. It has been particularly hard to disentangle relative significance of architecture and optimization in achieving accurate classification on large datasets. On the flip side, shallow methods have encountered obstacles in scaling to large data, despite excellent performance on smaller datasets, and extensive theoretical analysis. Practical methods, such as variants of gradient descent used so successfully in deep learning, seem to perform below par when applied to kernel methods. This difficulty has sometimes been attributed to the limitations of shallow architecture. In this paper we identify a basic limitation in gradient descent-based optimization in conjunctions with smooth kernels. An analysis demonstrates that only a vanishingly small fraction of the function space is reachable after a fixed number of iterations drastically limiting its power and resulting in severe over-regularization. The issue is purely algorithmic, persisting even in the limit of infinite data. To address this issue, we introduce EigenPro iteration, based on a simple preconditioning scheme using a small number of approximately computed eigenvectors. It turns out that even this small amount of approximate second-order information results in significant improvement of performance for large-scale kernel methods. Using EigenPro in conjunction with stochastic gradient descent we demonstrate scalable state-of-the-art results for kernel methods on a modest computational budget. Finally, these results indicate a need for a broader computational perspective on modern large-scale learning to complement more traditional statistical and convergence analyses. In particular, systematic analysis concentrating on the approximation power of algorithms with a fixed computation budget will lead to progress both in theory and practice.

本文研究了梯度下降算法在光滑内核中的应用限制，提出了基于特征向量预处理的EigenPro迭代优化算法，通过注入小规模二阶信息以改善此限制，从而实现更好的收敛性能。

浅入深出：对大规模浅层学习的计算视角