The concurrent consideration of uncertainty in WCETs and processor speeds in mixed-criticality systems

Abstract

Most prior work on mixed-criticality (MC) scheduling has focused on a model in which multiple WCET parameters are specified for each job, the interpretation being that the larger values represent "safer" estimates of the job's true WCET. More recently, a different MC model has been studied in which it is assumed that the precise speed of the processor upon which the system is implemented varies in an a priori unknown manner during runtime, and estimates must be made about how low the actual speed may fall. The research reported in this paper seeks to integrate the varying-speed MC model and the multi-WCET one into a unified framework. A general model is proposed in which each job may have multiple WCETs specified, and the precise speed of the processor upon which the system is implemented may vary during run-time. We reinterpreted the key idea behind the table-driven MC scheduling scheme proposed in one of our recent work, and provide a more efficient algorithm named LE-EDF. This algorithm strictly generalizes algorithms that were previously separately proposed for MC scheduling of systems with multiple WCETs as well as for MC scheduling on variable-speed processors. It is shown that LE-EDF outperforms (via simulation) and/or dominates existing algorithms (under theoretical proof). LE-EDF is also compared with optimal clairvoyant algorithm using the metric of speedup factor.