On-line scheduling of hard deadline aperiodic tasks in fixed-priority systems

This paper presents a new algorithm for the on-line scheduling of hard deadline aperiodic tasks in a real-time system in which hard deadline periodic tasks are scheduled using a fixed priority algorithm. The algorithm for jointly scheduling the periodic and aperiodic tasks extends the slack stealing algorithm introduced by Lehoczky and Ramos-Thuel (1992), and thus provides the largest amount of time for aperiodic task processing subject to meeting all periodic task deadlines. The analysis permits the periodic tasks to consist of a set of serially executed subtasks each of which has its own fixed execution priority as defined by (Gonzalez Harbour et al., (1991)) Thus the model permits the direct inclusion of operating system functions as well as certain types of synchronization protocols directly into the schedulability analysis. The methods provide a unified framework for dealing with several related problems including: reclaiming unused periodic and aperiodic execution time, load shedding, balancing hard and soft aperiodic execution time and coping with transient overloads