Fixed-Priority Scheduling of Mixed Soft and Hard Real-Time Tasks on Multiprocessors

This paper answers several open questions of practical concerns to schedule soft real-time (SRT) tasks, to guarantee their bounded tardiness, under fixed-priority scheduling in homogeneous multiprocessor systems. We consider both cases with only SRT tasks and with mixed sets of SRT and hard real-time (HRT) tasks. For the case in which the system has only SRT tasks, we show that any fixed priority assignment policy yields a capacity augmentation factor of 2 - 1/M where M is the number of processors. We prove the optimality of the utilization-monotonic (UM) priority assignment (i.e., assigning higher priorities to high-utilization tasks) under our sufficient test for guaranteeing bounded tardiness. We show that UM priority assignment can yield a utilization bound of M + 1/2 M, which is shown asymptotically the best possible bound. For the case in which the system has mixed SRT and HRT tasks, we present two new fixed-priority assignment algorithms and their associated schedulability tests. One is a clustering-based greedy priority assignment policy and another is based on Audsley's optimal priority assignment (OPA) approach. We show that the utilization bounds, augmentation factors, and speedup factors are still maintained by the hard real-time cases. Therefore, introducing soft real-time tasks does not create additional problems (at least in those metrics) for scheduling if the priority assignments are properly done. As demonstrated by extensive experiments, these two policies yield reasonably good performance overall and much better performance than the deadline-monotonic priority assignment.