Improved feasibility of fixed-priority scheduling with deferred preemptions using preemption thresholds for preemption points

This paper aims at advancing the relative strength of limited-preemptive schedulers by improving the feasibility of a task set and simultaneously limiting, or even precluding, arbitrary preemptions. In particular, we present a refinement of existing limited-preemptive fixed-priority scheduling (FPS) schemes with preemption thresholds for preemption points next to preemption thresholds for sub-jobs, termed fixed-priority scheduling with varying preemption thresholds (FPVS). We derive exact schedulability analysis for FPVS and we develop algorithms to maximize the schedulability of a set of sporadic tasks for given priorities. Since FPVS generalizes existing FPS schemes, we apply our algorithms to those schemes to compare the ratio of schedulable systems. Our experiments show that FPVS can achieve the same schedulability ratio with limited-preemptive sub-jobs as with entirely non-preemptive sub-jobs