Sustainable Multiprocessor Scheduling of Sporadic Task Systems

A scheduling policy or a schedulability test is defined to be sustainable with respect to a particular workload model if any task system represented in that model that is determined to be schedulable remains so if it behaves "better" than mandated by its specifications. We investigate the sustainability properties of global scheduling algorithms when applied to systems represented using the sporadic task model. We show that Fixed-Priority (FP) scheduling of sporadic task sets is sustainable under a variety of scheduling parameter relaxations, including decreased execution requirements, later arrivals, and deadline relaxations. It follows that all sufficient tests of global FP schedulability are sustainable for sporadic task systems. We show that the Earliest Deadline First (EDF) and Earliest-Deadline with Zero Laxity scheduling policies are sustainable with respect to decreased execution requirements and later arrivals. We also introduce a notion of self-sustainability, and show that many widely-used EDF schedulability tests are not self-sustainable but one is.