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Concept
Memory-mapped file
Summary
A memory-mapped file is a segment of virtual memory that has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on disk, but can also be a device, shared memory object, or other resource that the operating system can reference through a . Once present, this correlation between the file and the memory space permits applications to treat the mapped portion as if it were primary memory.
An early (1969) implementation of this was the PMAP system call on the DEC-20's TOPS-20 operating system, a feature used by Software House's System-1022 database system.
SunOS 4 introduced Unix's mmap, which permitted programs "to map files into memory."
Two decades after the release of TOPS-20's PMAP, Windows NT was given Growable Memory-Mapped Files (GMMF).
Since " function requires a size to be passed to it" and altering
a file's size is not readily accommodated, a GMMF API was developed. Use of GMMF requires
declaring the maximum to which the file size can grow, but no unused space is wasted.
The benefit of memory mapping a file is increasing I/O performance, especially when used on large files. For small files, memory-mapped files can result in a waste of slack space as memory maps are always aligned to the page size, which is mostly 4 KiB. Therefore, a 5 KiB file will allocate 8 KiB and thus 3 KiB are wasted. Accessing memory mapped files is faster than using direct read and write operations for two reasons. Firstly, a system call is orders of magnitude slower than a simple change to a program's local memory. Secondly, in most operating systems the memory region mapped actually is the kernel's page cache (file cache), meaning that no copies need to be created in user space.
Certain application-level memory-mapped file operations also perform better than their physical file counterparts. Applications can access and update data in the file directly and in-place, as opposed to seeking from the start of the file or rewriting the entire edited contents to a temporary location.
Official source
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