Choosing a Service Level Indicator: Why Not Queue Length?

Abstract

Choosing an appropriate service level indicator (SLI) is a fundamental part of managing the performance of a computer installation. Traditionally, measures such as response time and expansion factor have been used. Herein, we propose using queue length as a SLI. Queue length has appeal in. that it eliminates a central difficulty with response time measurements - defming a transaction; in addition, queue length avoids measurement problems associated with expansion factor in that waiting times need not be distinguished from busy times. Another appeal of queue length is that it can be very sensitive to the presence of performance problems. Lastly, because it provides a straight-forward way to calculate the people-cost of a computer system's responsiveness, queue length is in many ways a better measure for answering performance management and capacity planning questions.