Efficient exact schedulability tests for fixed priority real-time systems

Efficient exact schedulability tests are required both for online admission of applications to dynamic systems and as an integral part of design tools for complex distributed real- time systems. This paper addresses performance issues with exact response time analysis (RTA) for fixed priority preemptive systems. Initial values are introduced that improve the efficiency of the standard RTA algorithm 1) when exact response times are required and 2) when only exact schedulability need be determined. This paper also explores modifications to the standard RTA algorithm, including the use of a response time upper bound to determine when exact analysis is needed, incremental computation aimed at faster convergence, and checking tasks in reverse priority order to identify unschedulable tasksets early. The various initial values and algorithm implementations are compared by means of experiments on a PC recording the number of iterations required and execution time measurements on a real- time embedded microprocessor. Recommendations are provided for engineers tasked with the problem of implementing exact schedulability tests as part of online acceptance tests and spare capacity allocation algorithms or as part of offline system design tools.