Models for evaluating throughput

Analysts are interested in two categories of performance metrics: those concerned with time (response or wait time), and those concerned with rates (throughput or utilization, which reflect productivity and how well resources are used). In principle, these two categories are independent of each other, and both should be evaluated. But a common mistake is to "measure" the throughput in open-system evaluations, where the throughput is actually dictated directly by the workload. In order to evaluate throughput, the system model must include a feedback loop which modulates the workload being processed. The common solution is to create a pure closed system with a fixed number of users, who submit jobs in a loop. However, such behavior is often unrealistic. We review and analyze two alternative models that provide the required feedback by combining open and closed components: the mixed model which includes two such job classes, and the re-open model in which open user arrivals are combined with performance-dependent closed repetition of jobs by these users. These models allow evaluations of the trade-off between response time and throughput, including the throughput as it is observed by each user.