Sharp utilization thresholds for some realtime scheduling problems

Abstract

Scheduling policies for real-time systems exhibit threshold behavior that is related to the utilization of the task set they schedule, and in some cases this threshold is sharp. A task set is considered schedulable if it can be scheduled to meet all associated deadlines. A schedulability test for a chosen policy is a test of feasibility: given a task set, can all deadlines be met? For the rate monotonic scheduling policy, we show that periodic workload with utilization less than a threshold U RM can be scheduled almost surely and that all workload with utilization greater than U RM is almost surely not schedulable. We study such sharp threshold behavior in the context of processor scheduling using static task priorities, not only for periodic real-time tasks but for aperiodic real-time tasks as well. The notion of a utilization threshold provides a simple schedulability test for most real-time applications. These results improve our understanding of scheduling policies and provide an interesting characterization of the typical behavior of policies. The threshold is sharp (small deviations around the threshold cause schedulability, as a property, to appear or disappear) for most policies; this is a happy consequence that can be used to address the limitations of existing utilization-based tests for schedulability. We demonstrate the use of such an approach for balancing power consumption with the need to meet deadlines in web servers.