Working set is the physical memory assigned to a process by the Microsoft Windows NT and Windows 2000 operating systems. The working set consists of pages, which are sections of code and application data that have recently been used by the process.
If the process requires code or data that is not in its working set, it generates a page fault, causing the Virtual Memory Manager to swap new pages into the working set. If page faults become excessive, application performance can be slowed. Adding more RAM is one solution to this problem, while increasing the maximum working set size can also be helpful.
If memory becomes scarce, the Virtual Memory Manager moves the memory pages referenced less recently from the working sets in order to recover memory for the system. Older pages are also removed as new pages are added. Efficient applications use and store data in sequence, with the result that fewer memory pages are needed by the application, allowing a small working set.