Optimal Priority Assignment for Aperiodic Tasks with Firm Deadlines in Fixed Priority Pre-Emptive Systems

An optimal priority assignment policy is presented for independent aperiodic tasks with arbitrary ready times and firm deadlines, scheduled on a uniprocessor along with a set of hard periodic/sporadic tasks. The latter tasks are assumed to have been assigned unique fixed priorities according to some arbitrary policy and guaranteed, via off-line feasibility analysis, to meet their deadlines. In contrast, priority assignment and acceptance testing of aperiodic tasks must be carried out on-line. The priority assignment policy introduced is shown to be optimal both in terms of maximising the computation time which can be made available before the aperiodic deadline and w.r.t. guaranteeing subsequent aperiodic arrivals.