Computer Science Department
School of Computer Science, Carnegie Mellon University


Analysis of SRPT Scheduling: Investigating Unfairness

Nikhil Bansal, Mor Harchol-Balter

July 2000

Keywords: M/G/1, processor sharing, shortest, processing remaining time, time-sharing, priortiy, unfiarness, starvation, slowdown, heavy-tailed behavior, high variances, single server, job scheduling

The Shortest-Remaining-Processing-Time (SRPT) scheduling policy has long been known to be optimal for minimizing mean response time. Despite this fact, SRPT scheduling is rarely used in practice. It is believed that the performance improvements of SRPT over other scheduling policies with respect to mean response time stem from the fact that SRPT unfairly penalizes the large jobs in order to help the small jobs. This belief has caused people to instead adopt "fair" scheduling policies such as Processor-Sharing (PS), which produces the same expected slowdown for jobs of all sizes.

This paper investigates formally via analysis the problem of unfairness in SRPT. The analysis assumes an M/G/1 model, but emphasizes heavy tailed job size distributions, as are characteristic of empirical workloads. We end with a trace-driven simulation experiment which agrees with the analysis, even though arrivals are no longer Poisson.

25 pages

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