Improving fixed priority schedulability with dynamic priority as secondary criterion

Abstract

In this paper, we focus on a non-preemptive scheduling, combining fixed priorities and dynamic priorities on a uniprocessor. This scheduling is called FP/DP. Results presented in this paper can be applied indifferently to tasks and flows. With any flow are associated a fixed priority denoting the importance degree of the flow and a temporal parameter, used to compute the dynamic priority. A packet can be transmitted only if there is no packet having a higher fixed priority and if this packet has the highest dynamic priority among all packets having its fixed priority. We are interested in the worst case response time of a sporadic flow set scheduled FP/DP and establish new results improving the classical FP analysis. Two examples of FP/DP scheduling are considered: FP/FIFO and FP/EDF. We show that FP/EDF dominates FP/FIFO when packets sharing the same fixed priority have the same processing time.