A New Notion of Useful Cache Block to Improve the Bounds of Cache-Related Preemption Delay

In preemptive real-time systems, scheduling analyses are based on the worst-case response time of tasks. This response time includes worst-case execution time (WCET) and context switch costs. In case of preemption, cache memories may suffer interferences between memory accesses of the preempted and of the preempting task. These interferences lead to some additional reloads that are referred to as cache-related preemption delay (CRPD). This CRPD constitutes a large part of the context switch costs. In this article, we focus on the computation of upper bounds on the CRPD using the concept of useful cache blocks (UCB). These are memory blocks that may be in cache before a program point and may be reused after it. When a preemption occurs at that point the number of additional cache-misses is bounded by the number of useful cache blocks. We tighten the CRPD bound by using a modified notion of UCB: Only cache blocks that are definitely cached are considered useful by our approach. As we show in this paper, the computed CRPD based on our notion, when used in combination with the bound on the WCET, delivers a safe bound on the execution time in case of preemption. Furthermore the modified definition simplifies the UCB computation for set-associative LRU and data caches. Experimental results show that our approach provides up to 90% tighter CRPD bounds.