Online Learning with Local Permutations and Delayed Feedback

英文摘要：

We propose an Online Learning with Local Permutations (OLLP) setting, in which the learner is allowed to slightly permute the \emph{order} of the loss functions generated by an adversary. On one hand, this models natural situations where the exact order of the learner's responses is not crucial, and on the other hand, might allow better learning and regret performance, by mitigating highly adversarial loss sequences. Also, with random permutations, this can be seen as a setting interpolating between adversarial and stochastic losses. In this paper, we consider the applicability of this setting to convex online learning with delayed feedback, in which the feedback on the prediction made in round $t$ arrives with some delay $τ$. With such delayed feedback, the best possible regret bound is well-known to be $O(\sqrt{τT})$. We prove that by being able to permute losses by a distance of at most $M$ (for $M\geq τ$), the regret can be improved to $O(\sqrt{T}(1+\sqrt{τ^2/M}))$, using a Mirror-Descent based algorithm which can be applied for both Euclidean and non-Euclidean geometries. We also prove a lower bound, showing that for $M<τ/3$, it is impossible to improve the standard $O(\sqrt{τT})$ regret bound by more than constant factors. Finally, we provide some experiments validating the performance of our algorithm.