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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 [8] 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.
File-backed mapping maps an area of the process's virtual memory to files; that is, reading those areas of memory causes the file to be read. It is the default mapping type. Anonymous mapping maps an area of the process's virtual memory not backed by any file, made available via the MAP_ANONYMOUS/MAP_ANON flags.
Memory-mapped I/O is preferred in IA-32 and x86-64 based architectures because the instructions that perform port-based I/O are limited to one register: EAX, AX, and AL are the only registers that data can be moved into or out of, and either a byte-sized immediate value in the instruction or a value in register DX determines which port is the source or destination port of the transfer.
In computing, memory mapping may refer to: Memory-mapped file , also known as mmap() Memory-mapped I/O , an alternative to port I/O; a communication between CPU and peripheral device using the same instructions, and same bus, as between CPU and memory
A process can store data in memory-mapped files on memory-backed file systems, such as the tmpfs file system or file systems on a RAM drive, and map files into and out of the address space as needed. A set of processes may still depend upon the enhanced security features page-based isolation may bring to a multitasking environment.
Lightning Memory-Mapped Database (LMDB) is an embedded transactional database in the form of a key-value store. LMDB is written in C with API bindings for several programming languages . LMDB stores arbitrary key/data pairs as byte arrays, has a range-based search capability, supports multiple data items for a single key and has a special mode ...
The corresponding performance counter is called "Committed Bytes". Limit is the maximum possible value for Total; it is the sum of the current pagefile size plus the physical memory available for pageable contents (this excludes RAM that is assigned to non-pageable areas). The corresponding performance counter is called "Commit Limit".
When the operating system requested memory to load a program, or a program requested more memory to hold data from a file for instance, it would call the memory handling library. This examined the mappings to look for an area in main memory large enough to hold the request. If such a block was found, a new entry was entered into the table.