Sharpness, Restart and Acceleration

The {\L}ojasiewicz inequality shows that sharpness bounds on the minimum of convex optimization problems hold almost generically. Sharpness directly controls the performance of restart schemes, as observed by Nemirovsky and Nesterov (1985). The constants quantifying these sharpness bounds are of course unobservable, but we show that optimal restart strategies are robust, in the sense that, in some important cases, finding the best restart scheme only requires a log scale grid search. Overall then, restart schemes generically accelerate accelerated first-order methods.