Robust aperiodic scheduling under dynamic priority systems

When hard periodic and firm aperiodic tasks are jointly scheduled in the same system, the processor workload can vary according to the arrival times of aperiodic requests. In order to guarantee the schedulability of the periodic task set, in overload conditions some aperiodic tasks must be rejected. In this paper we propose a technique that, in overload conditions, adds robustness to the joint scheduling of periodic and aperiodic tasks in systems with dynamic priorities. Our technique is based on an aperiodic server, called total bandwidth server, already proven effective in a previous work. Here the algorithm is first extended to efficiently handle firm aperiodic tasks and then integrated with a robust guarantee mechanism that allows to achieve graceful degradation in case of transient overloads. Extensive simulations show that the proposed new algorithm is effective in all workload conditions.