Limited Preemption EDF Scheduling of Sporadic Task Systems

The optimality of the Earliest Deadline First scheduler for uniprocessor systems is one of the main reasons behind the popularity of this algorithm among real-time systems. The ability of fully utilizing the computational power of a processing unit however requires the possibility of preempting a task before its completion. When preemptions are disabled, the schedulability overhead could be significant, leading to deadline misses even at system utilizations close to zero. On the other hand, each preemption causes an increase in the runtime overhead due to the operations executed during a context switch and the negative cache effects resulting from interleaving tasks´ executions. These factors have been often neglected in previous theoretical works, ignoring the cost of preemption in real applications. A hybrid limited-preemption real-time scheduling algorithm is derived here, that aims to have low runtime overhead while scheduling all systems that can be scheduled by fully preemptive algorithms. This hybrid algorithm permits preemption where necessary for maintaining feasibility, but attempts to avoid unnecessary preemptions during runtime. The positive effects of this approach are not limited to a reduced runtime overhead, but will be extended as well to a simplified handling of shared resources.