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L-SR1 Adaptive Regularization by Cubics for Deep Learning

Stochastic gradient descent and other first-order variants, such as Adam and AdaGrad, are commonly used in the field of deep learning due to their computational efficiency and low-storage memory requirements. However, these methods do not exploit curvature information. Consequently, iterates can converge to saddle points and poor local minima. To avoid these points, directions of negative curvature can be utilized, which requires computing the second-derivative matrix. In Deep Neural Networks (DNNs), the number of variables ($n$) can be of the order of tens of millions, making the Hessian impractical to store ($\mathcal{O}(n^2)$) and to invert ($\mathcal{O}(n^3)$). Alternatively, quasi-Newton methods compute Hessian approximations that do not have the same computational requirements. Quasi-Newton methods re-use previously computed iterates and gradients to compute a low-rank structured update. The most widely used quasi-Newton update is the L-BFGS, which guarantees a positive semi-definite Hessian approximation, making it suitable in a line search setting. However, the loss function in DNNs are non-convex, where the Hessian is potentially non-positive definite. In this paper, we propose using a Limited-Memory Symmetric Rank-1 quasi-Newton approach which allows for indefinite Hessian approximations, enabling directions of negative curvature to be exploited. Furthermore, we use a modified Adaptive Regularized Cubics approach, which generates a sequence of cubic subproblems that have closed-form solutions. We investigate the performance of our proposed method on autoencoders and feed-forward neural network models and compare our approach to state-of-the-art first-order adaptive stochastic methods as well as L-BFGS.

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