Open, Closed, and Mixed Networks of Queues with Different Classes of Customers
Journal of the ACM (JACM)
Theory, Volume 1, Queueing Systems
Theory, Volume 1, Queueing Systems
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A number of simple mathematical models were used to predict average response time of a timesharing system. The target system was a very simple trace driven simulation model, but the workloads were trace files obtained from a real system in normal operation. As such, the workloads were characterized by very high coefficients of variation in resource demands and think times. Mathematical models of the system included independent arrival models (M/M/1 and M/G/1, closed network models) admitting product from solutions, and a more general Markov model. Only the final model produced reasonable accuracy.A number of experiments were performed, in an effort to determine what properties of the system being modeled were responsible for the failure of all the simple mathematical models. The large variance in CPU time and the fact that the system was a closed network were found to be critical factors, and appeared to be the major causes for failure of models that do not take them into account.