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Session Based Admission Control: A Mechanism for Improving the Performance of an Overloaded Web Server

Cherkasova, Ludmila; Phaal, Peter

HPL-98-119

Keyword(s): web servers; session-based workload; performance; simulation; overloaded server; admission control mechanism; web quality of service

Abstract: In this paper, we introduce a new, session-based workload for measuring a web server performance. We define a session as a sequence of client's individual requests. We then measure server throughput as a number of successfully completed sessions. Using a simulation model, we show that an overloaded web server can experience a severe loss of throughput measured as a number of completed sessions comparing against the server throughput measured in requests per second. Moreover, statistical analysis of completed sessions reveals that the overloaded web server discriminates against longer sessions. For e-commerce retail sites, longer sessions are typically the ones that would result in purchases, so they are precisely the ones for which the companies want to guarantee the completion. We introduce a session based admission control (SBAC) to prevent a web server from becoming overloaded and to ensure that longer sessions can be completed. If a server is functioning near its capacity a new session will be rejected (or redirected to another server if one is available). If there is enough capacity, the admission control mechanism will admit a new session and process all future requests related to it. We introduce a simple implementation of session based admission control based on server CPU utilization and analyze its properties and performance characteristics. We show that a web server augmented with the admission control mechanism is able to provide a fair guarantee of completion, for any accepted session, independent of a session length. This provides a predictable and controllable platform for web applications, and is a critical requirement for any e-business.

40 Pages

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