Gradient Perturbation is Underrated for Differentially Private Convex Optimization
Authors: Da Yu, Huishuai Zhang, Wei Chen, Tie-yan Liu, Jian Yin
Keywords: minimum curvature, gradient perturbation, DP-GD, DP-SGD
TL;DR: We establish a new and tighter utility guarantee for DP-GD and DP-SGD, and justify the advantage of gradient perturbation theoretically over output/objective pertubation.
Abstract: Gradient perturbation, widely used for differentially private optimization, injects noise at every iterative update to guarantee differential privacy. Previous work first determines the noise level that  can satisfy the privacy requirement and then analyzes the utility  of noisy gradient updates as in non-private case.  In this paper, we explore how the privacy noise affects the optimization property. We show that for differentially private convex optimization, the utility guarantee of both DP-GD and  DP-SGD is determined by an \emph{expected curvature} rather than the minimum curvature. The \emph{expected curvature} represents the average curvature over the optimization path, which is usually much larger than the minimum curvature and hence can help us achieve a significantly improved utility guarantee. By using the \emph{expected curvature}, our theory justifies the advantage of gradient perturbation over other perturbation methods and closes the gap between theory and practice. Extensive experiments on real world datasets corroborate our theoretical findings.