Combining EDF and FP Scheduling: Analysis and Implementation in Ada 2005

Earliest Deadline First (EDF) and Fixed Priority (FP) scheduling represent the two main dispatching policies within the research domain of real-time systems engineering. Both dispatching policies are now supported by Ada. In this paper the two approaches are combined to maximize the advantages of both schemes. From EDF comes efficiency, from FP predictability. A system model is presented in which a relatively small number of high-integrity tasks are scheduled by FP, with the rest of the tasks being handled via an EDF domain of lower priority. Two aspects of integration are covered in this paper. Firstly, ResponseTime Analysis (for FP) and Processor-Demand Analysis (for EDF) are brought together to provide a single analysis framework. Secondly, the programming of systems which combine FP and EDF is addressed within the facilities provided by Ada 2005. Both partitioned and dynamic schemes are covered.