Approximation methods for networks of queues with priorities

Abstract

Queuing network models are commonly used to analyze the performance of computer system. Unfortunately, the class of queuing network models which can be exactly analyzed excludes CPU priority scheduling disciplines, conspicuously present in most computer systems. A popular approximation technique which we denote the reduced occupancy approximation, is often used to analyze such priority service disciplines because of its simplicity and intuitive appeal. However, despite its widespread use, questions about its accuracy and applicability have received very little attention. Further compounding this situation, is the existence of proprietary software packages which purport to analyze such priority disciplines, but which in fact exhibit behavior remarkably similar to the roa. In this paper we show where, and more importantly why, the roa fails. This understanding leads to a significantly improved approximation technique which sacrifices neither simplicity nor applicability. Although our primary focus is on a two class preemptive priority closed network structure, the basic idea is quite general and extensions to multiclass and nonpreemptive priority structures are indicated.