Hard Real-time Scheduling: the Deadline-monotonic Approach1

The scheduling of processes to meet deadlines is a difficult problem often simplified by placing severe restrictions upon the timing characteristics of individual processes. One restriction often introduced is that processes must have deadline equal to period. This paper investigates schedulability tests for sets of periodic processes whose deadlines are permitted to be less than their period. Such a relaxation enables sporadic processes to be directly incorporated without alteration to the process model. Following an introduction outlining the constraints associated with existing scheduling approaches and associated schedulability tests, the deadline-monotonic approach is introduced. New schedulability tests are derived which vary in computational complexity. The tests are shown to be directly applicable to the scheduling of sporadic processes.