Unifying fixed- and dynamic-priority scheduling based on priority promotion and an improved ready queue management technique

This paper proposes a new preemptive scheduling algorithm, called Fixed-Priority with Priority Promotion (FPP), for scheduling sporadic tasks on uni- and multiprocessor platform. In FPP scheduling, tasks are executed similar to traditional fixed-priority (FP) scheduling but the priority of some tasks may be promoted at fixed time interval (called, promotion point) relative to the release time of each job. A policy called Increase Priority at Deadline Difference (IPDD) to compute the promotion points and promoted priorities for each task is proposed. It is shown that when all tasks' priorities are governed under IPDD policy, then FPP scheduling essentially prioritizes jobs according to Earliest-Deadline-First (EDF) priority. It is known that inserting and removing jobs to and from the ready queue of traditional EDF scheduler is more complex and has higher overhead than that of FP scheduler. To avoid such problem in FPP scheduling, a simple data structure and efficient operations to insert and remove jobs to and from the ready queue are proposed. Finally, an effective scheme to reduce overhead due to priority promotion is proposed: if a task set is not schedulable using traditional FP scheduling, then promotion points are assigned only to those tasks that need them to meet the deadlines; otherwise, tasks are assigned fixed priorities without any priority promotion and executed same as traditional FP scheduling. Empirical investigation shows the effectiveness of the proposed scheme in reducing overhead on uniprocessor and in accepting larger number of task sets in comparison to that of using state-of-the-art global schedulability tests for multiprocessors.