Increasing Fixed-Priority Schedulability using Non-Periodic Load Shapers

Many real-time systems use fixed-priority scheduling (FPS) for three main reasons: (i) it is easy to understand, configure and analyse, (ii) it has low runtime overheads, and (iii) it is widely implemented in commercial operating systems. Additionally, FPS is enforced by some industrial safety related standards such as Autosar. However, FPS is not optimal from a schedulability viewpoint as it may prioritise the execution of non-urgent high-priority tasks over tasks with urgent deadlines. As an attempt to increase FPS schedulability, variations of the FPS scheduling scheme such as Preemption Threshold Scheduling (PTS) [1] and Dual-Priroity (DP) scheduling [2] have been introduced. PTS raises the priority of a task (possibly several times) based on its remaining execution time, while DP promotes the task’s priority after a given time following its release. It was shown that PTS is not optimal [3] and designing a strategy for correctly assigning priorities and promotion times such that all deadlines are met is still an open problem for DP [4]. In this work, we introduce load-shaping (LS); a technique that shapes the workload of tasks to limit their impact on the system’s schedulability while improving the system’s safety. LS is implemented using a reservation server for each task. Each server has a budget that replenishes itself following a given pattern.