Performance Analysis of EDF Scheduling in a Multi-Priority Preemptive M/G/1 Queue

摘要

This paper presents a queueing theoretic performance model for a multipriority preemptive $M/G/1/./EDF$ system. Existing models on EDF scheduling consider them to be $M/M/1$ queues or nonpreemptive $M/G/1$ queues. The proposed model approximates the mean waiting time for a given class based on the higher and lower priority tasks receiving service prior to the target and the mean residual service time experienced. Additional time caused by preemptions is estimated as part of mean request completion time for a given class and as part of the mean delay experienced due to jobs in execution, on an arrival. The model is evaluated analytically and by simulation. Results confirm its accuracy, with the difference being a factor of two on average in high loads. Comparisons with other algorithms (such as First-Come-First-Served, Round-Robin and Nonpreemptive Priority Ordered) reveal that EDF achieves a better balance among priority classes where high priority requests are favored while preventing lower priority requests from overstarvation. EDF achieves best waiting times for higher priorities in lower to moderate loads (0.2-0.6) and while only being 6.5 times more than static priority algorithms in high loads (0.9). However, for the lowest priority classes, it achieves comparable waiting times to Round-Robin and First-Come-First-Served in low to moderate loads and achieves waiting times only twice the amount of Round-Robin in high system loads.