Past exam of the mathematics course of the University of Cambridge / 2007 / ia / Paper 5 / 2 / a

The working set $W(t,\Delta )$ is the set of virtual pages referenced by an operating-system process within a recent window of $\Delta$ execution time or memory references. Its size estimates the number of frames needed for the process's current locality of execution. It is a set of recently used pages, not the entirety of the process's address space.

An operating system can estimate it from reference bits or fault history and try to keep those pages resident. Process scheduling and admission control should avoid simultaneously running processes whose combined working sets exceed physical memory; suspending some processes or reducing multiprogramming then allows the rest to make useful progress. If too many active pages compete for too few frames, repeated page faults produce memory thrashing. Working sets connect memory allocation to the choice of processes allowed to run.