Solaris System Resource Manager: All I Ever Wanted Was My Unfair Advantage (And Why You Can't Have It!)

Abstract

Traditional UNIX time-share schedulers attempt to be fair to all users by employing a round-robin style algorithm for allocating CPU time. Unfortunately, a loophole exists whereby the scheduler can be biased in favor of a greedy user running many short CPU-time processes. This loophole is not a defect but an intrinsic property of the round-robin scheduler that ensures responsiveness to the short CPU demands associated with multiple interactive users. A new generation of UNIX system resource management software constrains the scheduler to be equitable to all users regardless of the number of processes each may be running. This "fair-share" scheduling draws on the concept of pro rating resource "shares" across users and groups and then dynamically adjusting CPU usage to meet those share proportions. The simple notion of statically allocating these shares, however, belies the potential consequences for performance as measured by user response time and service level targets. We demonstrate this point by modeling several simple share allocation scenarios and analyzing the corresponding performance effects. A brief comparison of commercial system resource management implementations from HP, IBM, and SUN is also given.