6.8.2 Queueing Models
On many systems, the processes that are run vary from day to day, so there is no static set of processes (or times) to use for deterministic modeling.
What can be determined, however, is the distribution of CPU and I/O bursts.
These distributions can be measured and then approximated or simply estimated.
The result is a mathematical formula describing the probability of a particular CPU burst.
Commonly, this distribution is exponential and is described by its mean.
Similarly, we can describe the distribution of times when processes arrive in the system (the arrival-time distribution).
From these two distributions, it is