Exact Worst-case Performance of First-order Methods for Composite Convex Optimization

TL;DR

This paper introduces a framework to compute exact worst-case performance of a broad class of first-order methods for composite convex optimization, providing tight guarantees and explicit problem instances.

Contribution

It generalizes performance estimation to a wide range of algorithms and improves existing worst-case bounds for several methods, including proximal point and accelerated gradient algorithms.

Findings

Tighter worst-case guarantee for proximal point algorithm.

Improved worst-case bounds for conditional gradient method.

Extension of optimized gradient method with faster convergence.

Abstract

We provide a framework for computing the exact worst-case performance of any algorithm belonging to a broad class of oracle-based first-order methods for composite convex optimization, including those performing explicit, projected, proximal, conditional and inexact (sub)gradient steps. We simultaneously obtain tight worst-case guarantees and explicit instances of optimization problems on which the algorithm reaches this worst-case. We achieve this by reducing the computation of the worst-case to solving a convex semidefinite program, generalizing previous works on performance estimation by Drori and Teboulle [13] and the authors [43]. We use these developments to obtain a tighter analysis of the proximal point algorithm and of several variants of fast proximal gradient, conditional gradient, subgradient and alternating projection methods. In particular, we present a new analytical worst-case guarantee for the proximal point algorithm that is twice better than previously known, and improve the standard worst-case guarantee for the conditional gradient method by more than a factor of two. We also show how the optimized gradient method proposed by Kim and Fessler in [22] can be extended by incorporating a projection or a proximal operator, which leads to an algorithm that converges in the worst-case twice as fast as the standard accelerated proximal gradient method [2].