Windows NT

In NT-based versions of Windows (such as Windows XP and Windows Vista), the file used for paging is named pagefile.sys. The default location of the page file is in the root directory of the partition where Windows is installed. Windows can be configured to use free space on any available drives for pagefiles. It is required, however, for the boot partition (i.e. the drive containing the Windows directory) to have a pagefile on it if the system is configured to write either kernel or full memory dumps after a crash. Windows uses the paging file as temporary storage for the memory dump. When the system is rebooted, Windows copies the memory dump from the pagefile to a separate file and frees the space that was used in the pagefile.^[6]

Fragmentation

In Windows's default configuration the pagefile is allowed to expand beyond its initial allocation when necessary. If this happens gradually, it can become heavily fragmented which can potentially cause performance problems. The common advice given to avoid this is to set a single "locked" pagefile size so that Windows will not expand it. However, the pagefile only expands when it has been filled, which, in its default configuration, is 150% the total amount of physical memory.^[7] Thus the total demand for pagefile-backed virtual memory must exceed 250% of the computer's physical memory before the pagefile will expand.

The fragmentation of the pagefile that occurs when it expands is temporary. As soon as the expanded regions are no longer in use (at the next reboot, if not sooner) the additional disk space allocations are freed and the pagefile is back to its original state.

Locking a page file's size can be problematic in the case that a Windows application requests more memory than the total size of physical memory and the pagefile. In this case, requests to allocate memory fail, which may cause applications and system processes to fail. Supporters of this view will note that the pagefile is rarely read or written in sequential order, so the performance advantage of having a completely sequential pagefile is minimal. However, it is generally agreed that a large pagefile will allow use of memory-heavy applications, and there is no penalty except that more disk space is used.

Defragmenting the page file is also occasionally recommended to improve performance when a Windows system is chronically using much more memory than its total physical memory. This view ignores the fact that, aside from the temporary results of expansion, the pagefile does not become fragmented over time. In general, performance concerns related to pagefile access are much more effectively dealt with by adding more physical memory.

Implementations

Windows 3.x and Windows 9x

Virtual memory has been a feature of Microsoft Windows since Windows 3.0 in 1990. Microsoft introduced virtual memory in response to the failures of Windows 1.0 and Windows 2.0, attempting to slash resource requirements for the operating system.

Confusion abounds about Microsoft's decision to refer to the swap file as "virtual memory". Novices unfamiliar with the concept accept this definition without question, and speak of adjusting Windows' virtual memory size. In fact every process has a fixed, unchangeable virtual memory size, usually 2 GB. The user has only an option to change disk capacity dedicated to paging.

Windows 3.x creates a hidden file named 386SPART.PAR or WIN386.SWP for use as a swap file. It is generally found in the root directory, but it may appear elsewhere (typically in the WINDOWS directory). Its size depends on how much swap space the system has (a setting selected by the user under Control Panel → Enhanced under "Virtual Memory".) If the user moves or deletes this file, a blue screen will appear the next time Windows is started, with the error message "The permanent swap file is corrupt". The user will be prompted to choose whether or not to delete the file (whether or not it exists).

Windows 95, Windows 98 and Windows Me use a similar file, and the settings for it are located under Control Panel → System → Performance tab → Virtual Memory. Windows automatically sets the size of the page file to start at 1.5× the size of physical memory, and expand up to 3× physical memory if necessary. If a user runs memory-intensive applications on a system with low physical memory, it is preferable to manually set these sizes to a value higher than default.