Hierarchical fixed priority pre-emptive scheduling

This paper focuses on the hierarchical scheduling of systems where a number of separate applications reside on a single processor. It addresses the particular case where fixed priority pre-emptive scheduling is used at both global and local levels, with a server associated with each application. Using response time analysis, an exact schedulability test is derived for application tasks. This test improves on previously published work. The analysis is extended to the case of harmonic tasks that can be bound to the release of their server. These tasks exhibit improved schedulability indicating that it is advantageous to choose server periods that enable some tasks to be bound to the release of their server. The use of periodic, sporadic and deferrable servers is considered with the conclusion that the simple periodic server dominates both sporadic and deferrable servers when the metric is application task schedulability.