  Existing performance measures for bandit algorithms such as regret, PAC
bounds, or uniform-PAC (Dann et al., 2017), typically evaluate the cumulative
performance, while allowing the play of an arbitrarily bad arm at any finite
time t. Such a behavior can be highly detrimental in high-stakes applications.
This paper introduces a stronger performance measure, the uniform last-iterate
(ULI) guarantee, capturing both cumulative and instantaneous performance of
bandit algorithms. Specifically, ULI characterizes the instantaneous
performance since it ensures that the per-round regret of the played arm is
bounded by a function, monotonically decreasing w.r.t. (large) round t,
preventing revisits to bad arms when sufficient samples are available. We
demonstrate that a near-optimal ULI guarantee directly implies near-optimal
cumulative performance across aforementioned performance measures. To examine
the achievability of ULI in the finite arm setting, we first provide two
positive results that some elimination-based algorithms and high-probability
adversarial algorithms with stronger analysis or additional designs, can attain
near-optimal ULI guarantees. Then, we also provide a negative result,
indicating that optimistic algorithms cannot achieve a near-optimal ULI
guarantee. Finally, we propose an efficient algorithm for linear bandits with
infinitely many arms, which achieves the ULI guarantee, given access to an
optimization oracle.
