FIRM APERIODIC TASK SCHEDULING IN HARD REAL-TIME MULTIPROCESSOR SYSTEMS 1 Sergio S´ aez * Joan Vila * Alfons Crespo * * Dpto. de Inform´ atica de Sistemas y Computadores, Universidad Polit´ ecnica de Valencia, Camino de Vera, s/n, 46022, Valencia, Spain E-mail: {ssaez,jvila,alfons}@disca.upv.es Phone: +34 6 387 9577 Fax: +34 6 387 7579 Abstract: The problem of extending monoprocessor methods to multiprocessors for scheduling soft aperiodic tasks has been addressed and evaluated in previous works, where the most usual goal for aperiodic tasks is minimising response times. This paper introduces a new problem where the goal for aperiodic tasks is guaranteeing their deadlines. Guaranteeing aperiodic deadlines is done using an acceptance test based on extending the slack stealing algorithm for EDF schedulers in a way such that it can guarantee a given response time for an aperiodic task with the assumption that it can migrate from one processor to another. Proposed solutions are evaluated through extensive simulations in terms of acceptance ratios for different periodic workload conditions. Results show that allowing task migration among processors can improve the acceptance ratio by an almost a 50%. Other interesting parameters like the number of required processor switches are also evaluated. Copyright c  2003 IFAC Keywords: Dynamic systems, real-time tasks, multiprocessor systems 1. INTRODUCTION The problem of jointly scheduling periodic tasks with hard deadlines and aperiodic tasks with soft or firm deadlines has deserved a lot of attention under both the Rate Monotonic (RM) and Earli- est Deadline First (EDF) scheduling approaches. In the field of monoprocessor scheduling, there are well known solutions to this problem that include, on one hand, several aperiodic servers with different performance/cost ratios (Lehoczky et al., 1987; Sprunt et al., 1989) and, on the other, slack stealing algorithms (Lehoczky and Ramos- 1 This work was supported by the Spanish Government Research Office (CICYT) under grant TIC2002-04123- C03-03 Thuel, 1992; Davis et al., 1993; Tia et al., 1996). Slack stealer algorithms have been proved to be optimal under a given set of conditions. These algorithms have been also extended to handle aperiodic tasks with firm deadlines (Ramos-Thuel and Lehoczky, 1993). Some algorithms proposed for scheduling aperiodic tasks under the RM approach have been later extended to dynamic systems under the EDF approach. This refers both to aperiodic servers (Homayoun and Ra- manathan, 1994; Ghazalie and Baker, 1995; Spuri and Buttazzo, 1996), and to slack stealer al- gorithms (Spuri and Buttazzo, 1996; Ripoll et al., 1997) too. However, the problem of jointly scheduling critical periodic and aperiodic tasks on a multiprocessor