A New Approach for Limited Preemptive Scheduling in Systems with Preemption Overhead

摘要

This paper considers the problem of reducing the number of preemptions in a system with periodic tasks and preemption overhead. The proposed solution is based on the key observation that for periodic task sets, the task with the smallest period plays an important role in determining the maximum interval of time during which a lower priority task can be executed without being preempted. We use this property to build a new limited preemptive scheduling algorithm, named RS-LP, based on fixed-priority scheduling. In RS-LP, the length of each task's non-preemptive region is varying during the system execution so as to keep the preemptions aligned with the releases of the highest priority task. This simple mechanism allows us to reduce the overall number of preemptions. The proposed algorithm, decides whether or not to preempt the currently executing task based on the maximum blocking tolerance of the higher priority tasks. In any case, the preemptions are authorized only at release instants of the task with the smallest period, thereby limiting the maximum number of preemptions to the number of releases of the highest priority task. Moreover, in this paper, we provide two different preemption overhead aware schedulability tests for periodic and loose-harmonic task sets (i.e., where each period is an integer multiple of the smallest period), together with a lower bound on the maximum number of preemptions. To conclude, extensive experiments comparing RS-LP with the state of the art limited preemptive scheduling algorithms are finally presented.