An Evaluation of the Graceful Degradation Properties of Real-Time Schedulers

Abstract

Real-time, fault-tolerant systems require schedulers that provide graceful degradation during transient overloads resulting from fault recovery workloads or other system uncertainties. We initially hypothesize that a scheduler ideally suited to this environment should dispatch tasks using only response time criterion as long as all deadlines can be met, and that in the presence of overload the best a scheduler can do is semantic-driven load shedding. By temporarily eliminating less important tasks, the more intelligent scheduling algorithms were expected to service the more important tasks well without harming the non-critical tasks unnecessarily. Experimentally we then show this hypothesis to be false. On-line tracking of system load does not provide enough information about future load to effectively trigger semantic-driven load shedding. Simpler algorithms can insure predictable behavior, but sacrifice fault-free schedulability to do so. We show that a compromise can be reached by d..