Limited Pre-emptive Global Fixed Task Priority

Abstract-In this paper a limited pre-emptive global fixed task priority scheduling policy for multiprocessors is presented. This scheduling policy is a generalization of global fully pre-emptive and non-pre-emptive fixed task priority policies for platforms with at least two homogeneous processors. The scheduling protocol devised is such that a job can only be blocked at most once by a body of lower priority non-pre-emptive workload. The presented policy dominates both fully pre-emptive and fully non-pre-emptive with respect to schedulability. A sufficient schedulability test is presented for this policy. Several approaches to estimate the blocking generated by lower priority non-pre-emptive regions are presented. As a last contribution it is experimentally shown that, on the average case, the number of pre-emptions observed in a schedule are drastically reduced in comparison to global fully pre-emptive scheduling

Comparative evaluation of limited preemptive methods

Schedulability analysis of real-time systems requires the knowledge of the worst-case execution time (WCET) of each computational activity. A precise estimation of such a task parameter is quite difficult to achieve, because execution times depend on many factors, including the task structure, the system architecture details, operating system features and so on. While some of these features are not under our control, selecting a proper scheduling algorithm can reduce the runtime overhead and make the WCETs smaller and more predictable. In particular, since task execution times can be significantly affected by preemptions, a number of scheduling methods have been proposed in the real-time literature to limit preemption during task execution. In this paper, we provide a comprehensive overview of the possible scheduling approaches that can be used to contain preemptions and present a comparative study aimed at evaluating their impact on task execution times