Keywords: implicit bias, implicit regularization, sharpness, sharpness aware minimization
TL;DR: We prove the implicit bias of Sharpness-Aware Minimization (SAM) is minimizing the top eigenvalue of Hessian in the full-batch setting or minimizing the trace of Hessian when batch size is 1.
Abstract: Sharpness-Aware Minimization (SAM) is a highly effective regularization technique for improving the generalization of deep neural networks for various settings. However, the underlying working of SAM remains elusive because of various intriguing approximations in the theoretical characterizations. SAM intends to penalize a notion of sharpness of the model but implements a computationally efficient variant; moreover, a third notion of sharpness was used for proving generalization guarantees. The subtle differences in these notions of sharpness can indeed lead to significantly different empirical results. This paper rigorously nails down the exact sharpness notion that SAM regularizes and clarifies the underlying mechanism. We also show that the two steps of approximations in the original motivation of SAM individually lead to inaccurate local conclusions, but their combination accidentally reveals the correct effect, when full-batch gradients are applied. Furthermore, we also prove that the stochastic version of SAM in fact regularizes another notion of sharpness, which is most likely to be the preferred notion for practical performance. The key mechanism behind this intriguing phenomenon is the implicit alignment between the gradient and the top eigenvector of Hessian when running SAM.