Optimal Anytime Algorithms for Online Convex Optimization with Adversarial Constraints

TL;DR

This paper introduces an optimal anytime online convex optimization algorithm that adaptively manages adversarial costs and constraints without prior horizon knowledge, using time-varying Lyapunov functions for improved practical performance.

Contribution

It presents a novel anytime algorithm with optimal bounds using time-varying Lyapunov functions, avoiding the standard doubling trick and extending to dynamic regret and adaptive settings.

Findings

Achieves $O( oot{t}]{ ext{regret}})$ and $ ilde{O}( oot{t}]{ ext{constraint violation}}$ bounds.

Extends results to dynamic regret with unknown path length.

Demonstrates practical utility via online shortest path experiments.

Abstract

We propose an anytime online algorithm for the problem of learning a sequence of adversarial convex cost functions while approximately satisfying another sequence of adversarial online convex constraints. A sequential algorithm is called \emph{anytime} if it provides a non-trivial performance guarantee for any intermediate timestep $t$ without requiring prior knowledge of the length of the entire time horizon $T$. Our proposed algorithm achieves optimal performance bounds without resorting to the standard doubling trick, which has poor practical performance due to multiple restarts. Our core technical contribution is the use of time-varying Lyapunov functions to keep track of constraint violations. This must be contrasted with prior works that used a fixed Lyapunov function tuned to the known horizon length $T$. The use of time-varying Lyapunov function poses unique analytical challenges as properties, such as \emph{monotonicity}, on which the prior proofs rest, no longer hold. By introducing a new analytical technique, we show that our algorithm achieves $O(\sqrt{t})$ regret and $\tilde{O}(\sqrt{t})$ cumulative constraint violation bounds for any $t\geq 1$. We extend our results to the dynamic regret setting, achieving bounds that adapt to the path length of the comparator sequence without prior knowledge of its total length. We also present an adaptive algorithm in the optimistic setting, whose performance gracefully scales with the cumulative prediction error. We demonstrate the practical utility of our algorithm through numerical experiments involving the online shortest path problem.