Bounding Preemptions under EDF and RM Schedulers Technical Report: MS-CIS-06-07

Assurance of timing requirements from a real-time system schedule can be obtained using schedulability analysis. Existing analysis techniques ignore the preemption overhead incurred by the system, when the tasks are scheduled using preemptive schedulers. Preemptive schedulers can preempt the execution of a real-time task when a higher priority task is released by the system. Every job preemption will then induce an execution overhead because of the need to swap task contexts. In this paper we derive bounds on the number of preemptions incurred by a real-time task system scheduled using RM or EDF schedulers. We only consider task systems which consist of periodic task models where the tasks can be released asynchronously. Job response time equations developed in this paper, for a given task arrival pattern, are used in bounding the preemptions. We then modify schedulability conditions for RM and EDF schedulers to account for preemption overheads, using these preemption bounds. Simulation results show that the bounds developed in this paper are tighter than previously known preemption bounds by a factor of upto 90%. We also provide an estimate for the number of preemptions and show using simulations that this estimate is very close to the actual number of preemptions.