Pollaczek–Khinchine formula

In queueing theory, a discipline within the mathematical theory of probability, the Pollaczek–Khinchine formula states a relationship between the queue length and service time distribution Laplace transforms for an M/G/1 queue (where jobs arrive according to a Poisson process and have general service time distribution). The term is also used to refer to the relationships between the mean queue length and mean waiting/service time in such a model.

The formula was first published by Felix Pollaczek in 1930 and recast in probabilistic terms by Aleksandr Khinchin two years later. In ruin theory the formula can be used to compute the probability of ultimate ruin (probability of an insurance company going bankrupt).

The formula states that the mean queue length L is given by

where

For the mean queue length to be finite it is necessary that ${\displaystyle \rho <1}$ as otherwise jobs arrive faster than they leave the queue. "Traffic intensity," ranges between 0 and 1, and is the mean fraction of time that the server is busy. If the arrival rate ${\displaystyle \lambda _{a}}$ is greater than or equal to the service rate ${\displaystyle \lambda _{s}}$, the queuing delay becomes infinite. The variance term enters the expression due to Feller's paradox.

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