Parallel Prefetching and Caching Is Hard

In this paper we study integrated prefetching and caching in parallel disk systems. This topic has gained a lot of interest in the last years which manifests itself in numerous recent approximation algorithms. This paper provides the first negative result in this area by showing that optimizing the stall time is APX-hard. This also implies that computing the optimal processing time is NP-hard, which settles an open problem posed by Kimbrel and Karlin.