Although deep neural networks have given the name to the domain of “deep learning”, it is still an unresolved question of which depth is the best choice for a given task. A large depth brings problems with the convergence of gradient-based learning algorithms through the phenomenon of vanishing gradient. On the other hand, it is a widespread opinion that a small depth has insufficient representational power for many tasks. The discovery of the concept of residual connections—an identity mapping parallel to a conventional layer—has alleviated the convergence problem so that the discussion of optimum depth lost a part of its motivation, resulting in the assumption of “the more the better”. The work presented here shows that a shallow architecture of parallel layers has comparable expressive power as a deep stack of residual layers. This is theoretically justified by expanding the residual layer stack analogical to the Taylor expansion, truncating the higher-order terms into a single broad layer composed of original layers in parallel. This hypothesis has been confirmed by computing experiments with the widespread computer vision benchmark datasets MNIST and CIFAR-10. The 6,912 runs have shown that the shallow and the deep architectures do not substantially differ in performance on both training and validation sets if the total number of parameters is equal. The rough equivalence of the two extreme (deep and shallow) architectures suggests the possibility that an intermediary architecture may be superior. Another series of computing experiments disclosed that the performance does not substantially differ even then. The conclusion is that the performance of an architecture depends more substantially on the total number of parameters than on the sequential or parallel connection of layers.

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Is There an Optimal Depth of Residual Networks?

  • Bernhard Bermeitinger,
  • Tomas Hrycej,
  • Siegfried Handschuh

摘要

Although deep neural networks have given the name to the domain of “deep learning”, it is still an unresolved question of which depth is the best choice for a given task. A large depth brings problems with the convergence of gradient-based learning algorithms through the phenomenon of vanishing gradient. On the other hand, it is a widespread opinion that a small depth has insufficient representational power for many tasks. The discovery of the concept of residual connections—an identity mapping parallel to a conventional layer—has alleviated the convergence problem so that the discussion of optimum depth lost a part of its motivation, resulting in the assumption of “the more the better”. The work presented here shows that a shallow architecture of parallel layers has comparable expressive power as a deep stack of residual layers. This is theoretically justified by expanding the residual layer stack analogical to the Taylor expansion, truncating the higher-order terms into a single broad layer composed of original layers in parallel. This hypothesis has been confirmed by computing experiments with the widespread computer vision benchmark datasets MNIST and CIFAR-10. The 6,912 runs have shown that the shallow and the deep architectures do not substantially differ in performance on both training and validation sets if the total number of parameters is equal. The rough equivalence of the two extreme (deep and shallow) architectures suggests the possibility that an intermediary architecture may be superior. Another series of computing experiments disclosed that the performance does not substantially differ even then. The conclusion is that the performance of an architecture depends more substantially on the total number of parameters than on the sequential or parallel connection of layers.