Performance Evaluation of a Threshold Policy for Scheduling Readers and Writers

This paper is concerned with the analysis of a threshold policy for scheduling “readers” and “writers” in a multiserver system and a comparison of its performance with the FCFS policy. A writer must be processed singly, thus using all servers in the system, while readers can be processed concurrently, each using one server. Higher throughput and better response time can be achieved by increasing the degree of reader concurrency; this improvement is more significant when the reader processing time is much longer than the writer processing time. A high degree of concurrency cannot be sustained with the FCFS policy, due to the service order imposed on arrivals to the system, in addition to the inefficiency resulting from the need to frequently empty the system from readers before processing a writer. On the other hand, high levels of reader concurrency can be achieved with the Threshold Fastest Emptying (TFE) policy, which empties the system from readers (if any) to serve writers only when a threshold li on the number of writers in the system is reached. If there are no readers, the system starts processing writers (if any), even if the threshold is not reached. The TFE policy is analyzed, in a system with writer arrivals and an infinite backlog of readers (reader-saturated system), using a Markovian model as well as a vacationing server model to yield closed form expressions for the mean writer response time and the reader throughput. The maximum throughput achieved by the TFE policy is shown to be an increasing function of I<, which even at I< = 1 exceeds the maximum throughput achieved by FCFS. In a system with reader and writer arrivals (nonsaturated system), simulation is used to study and compare the mean response time for the FCFS and the TFE policies.