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# Delayed Feedback in Kernel Bandits

*Proceedings of the 40th International Conference on Machine Learning*, PMLR 202:34779-34792, 2023.

#### Abstract

Black box optimisation of an unknown function from expensive and noisy evaluations is a ubiquitous problem in machine learning, academic research and industrial production. An abstraction of the problem can be formulated as a kernel based bandit problem (also known as Bayesian optimisation), where a learner aims at optimising a kernelized function through sequential noisy observations. The existing work predominantly assumes feedback is immediately available; an assumption which fails in many real world situations, including recommendation systems, clinical trials and hyperparameter tuning. We consider a kernel bandit problem under stochastically delayed feedback, and propose an algorithm with $\tilde{\mathcal{O}}\left(\sqrt{\Gamma_k(T) T}+\mathbb{E}[\tau]\right)$ regret, where $T$ is the number of time steps, $\Gamma_k(T)$ is the maximum information gain of the kernel with $T$ observations, and $\tau$ is the delay random variable. This represents a significant improvement over the state of the art regret bound of $\tilde{\mathcal{O}}\left(\Gamma_k(T)\sqrt{ T}+\mathbb{E}[\tau]\Gamma_k(T)\right)$ reported in (Verma et al., 2022). In particular, for very non-smooth kernels, the information gain grows almost linearly in time, trivializing the existing results. We also validate our theoretical results with simulations.