Control and recovery from rare congestion events in a large multi-server system

We develop deterministic uid approximations to describe the recovery from rare congestion events in a large multi-server system in which customer holding times have a general distribution. There are two cases, depending on whether or not we exploit the age distribution (the distribution of elapsed holding times of customers in service). If we do not exploit the age distribution, then the rare congestion event is a large number of customers present. If we do exploit the age distribution, then the rare event is an unusual age distribution, possibly accompanied by a large number of customers present. As an approximation, we represent the large multi-server system as an M/G/1 model. We prove that, under regularity conditions, the uid approximations are asymptotically correct as the arrival rate increases. The uid approximations show the impact upon the recovery time of the holding-time distribution beyond its mean. The recovery time may or not be aected by the holding-time distribution having a long tail, depending on the precise denition of recovery. The uid approximations can be used to analyze various overload control schemes, such as reducing the arrival rate or interrupting services in progress. We also establish large deviations principles to show that the two kinds of rare events have the same exponentially small order. We give numerical examples showing the eect of the holding-time distribution and the age distribution, focusing especially on the consequences of long-tail distributions