An EDF-based scheduling algorithm for multiprocessor soft real-time systems

Abstract

We consider the use of the earliest-deadline-first (EDF) scheduling algorithm in soft real-time multiproces-sor systems. In hard real-time systems, a significant disparity exists between EDF-based schemes and Pfair scheduling (which is the only known way of optimally scheduling recurrent real-time tasks on multiprocessors): on M processors, all known EDF variants have utilization-based schedulability bounds of approximately M/2, while Pfair algorithms can fully utilize all processors. This is unfortunate because EDF-based algorithms entail lower scheduling and task-migration overheads. In work on hard real-time systems, it has been shown that this disparity in schedulability can be lessened by placing caps on per-task utilizations. In this paper, we show that it can also be lessened by easing the requirement that all deadlines be met. Our main contribution is a new EDF-based scheme that ensures bounded deadline tardiness. In this scheme, per-task utilizations must be capped, but overall utilization need not be restricted. The required cap is quite liberal. Hence, our scheme should enable a wide range of soft real-time applications to be scheduled with no constraints on total utilization. We also propose techniques and heuristics that can be used to reduce tardiness. To the best of our knowledge, this paper is the first to examine multiprocessor EDF scheduling in the context of soft real-time systems