The group of physical memory pages currently dedicated to a specific process is known as the working set for that process. The number of pages in the working set can grow and shrink, depending on the overall availability of pages on a system-wide basis.
The working set grows as a process page faults. The working set shrinks as fewer and fewer free pages exist. To keep from running out of memory completely, pages must be removed from process's working sets and turned into free pages, available for later use. The operating system shrinks processes' working sets by:
Writing modified pages to a dedicated area on a mass storage device (usually known as swapping or paging space)
Marking unmodified pages as being free (there is no need to write these pages out to disk as they have not changed)
To determine appropriate working sets for all processes, the operating system must track usage information for all pages. In this way, the operating system determines which pages are actively being used (and must remain memory resident) and which pages are not (and therefore, can be removed from memory.) In most cases, some sort of least-recently used algorithm determines which pages are eligible for removal from process working sets.