In computing, page cache, sometimes ambiguously called disk cache, is a transparent cache of disk-backed pages kept in main memory (RAM) by the operating system for quicker access. In Computer science, a cache (kæʃ like "cash") is a collection of data duplicating original Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination Page cache is typically implemented in kernels with the paging memory management, and is completely transparent to applications. In Computer science, the kernel is the central component of most computer Operating systems (OS In Computer Operating systems that have their Main memory divided into pages, paging (sometimes called swapping) is a transfer All memory that is not directly allocated to applications, is usually utilized for page cache. Hard disk read speeds are low and random accesses require expensive disk seeks compared to main memory—this is why RAM upgrades usually yield significant improvements in computers' speed and responsiveness. A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device In Computer science, random access (sometimes called direct access) is the ability to access an arbitrary element of a sequence in equal time Seek time is one of the three delays associated with reading or writing Data on a Computer 's Disk drive, and somewhat similar for CD or Separate disk caching is provided on the hardware side, by dedicated RAM or NVRAM chips located either in disk controller (inside a hard disk drive; properly called disk buffer) or in a disk array controller. Non-volatile Random access memory ( NVRAM) is the general name used to describe any type of random access memory which does not lose its information The disk controller (or "hard disk controller" is the circuit which allows the CPU to communicate with a Hard disk, Floppy disk or In Computer storage, disk buffer (often ambiguously called disk cache or cache buffer) is the embedded memory in a Hard drive acting as A disk array controller is a device which manages the physical Disk drives and presents them to the computer as logical units. Such memory should not be confused with page cache.
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Memory conservation
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For more details on this topic, see Demand paging. In Computer Operating systems, demand paging is an application of Virtual memory.
Since non-dirty pages in the page cache have identical copies in secondary storage (hard disk), discarding and re-using their space is much quicker than paging out application memory, and is often preferred. In a context of Computer Virtual memory, a page, memory page, or virtual page is a fixed-length block of Main memory, that is contiguous Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital Executable binaries, such as applications and libraries, are also typically accessed through page cache and mapped to individual process spaces using virtual memory (this is done through the mmap syscall on Unix-like operating systems). In computing a process is an instance of a Computer program that is being sequentially executed by a computer system that has the ability to run several computer Virtual memory is a Computer system technique which gives an application program the impression that it has contiguous working memory while in fact it may be physically In Computing, mmap is a POSIX -compliant Unix System call that maps files or devices into memory This not only means that the binary files are shared between separate processes, but also that unused parts of binaries will be pushed out of main memory eventually, leading to memory conservation.
Since cache pages can be easily dropped and re-used, some operating systems, notably Windows NT, even report the page cache usage as "free" memory, while the memory is actually allocated to disk pages. Windows NT is a family of Operating systems produced by Microsoft, the first version of which was released in July 1993 This has led to some confusion about the utilization of page cache in Windows.
Page cache and disk writes
The page cache also aids in writing to a disk. Pages that have been modified in memory for writing to disk, are marked "dirty" and have to be flushed to disk before they can be freed. When a file write occurs, the page backing the particular block is looked up. If it is already found in cache, the write is done to that page in memory. Otherwise, when the write perfectly falls on page size boundaries, the page is not even read from disk, but allocated and immediately marked dirty. In a context of Computer Virtual memory, a page, memory page, or virtual page is a fixed-length block of Main memory, that is contiguous Otherwise, the page(s) are fetched from disk and requested modifications are done.
However, not all cached pages can be written to — often, program code is mapped as read-only or copy-on-write; in the latter case, modifications to code will only be visible to the process itself and will not be written to disk. Copy-on-write (sometimes referred to as "COW" is an optimization strategy used in Computer programming.
History
The first commercially available page cache (disk cache) for microcomputers was MicroCache from Microcosm Ltd. Microcosm Ltd is a UK company established in 1979. Its early claims to fame included Silicon Disk System in 1981 and Microcache (the world's first This appeared in 1982, initially for the CP/M operating system and later for MS-DOS. CP/M (Control Program for Microcomputers is an Operating system originally created for Intel 8080 / 85 based Microcomputers by Gary Kildall MS-DOS (short for M icro' s' oft D isk O perating S ystem is an Operating system commercialized by Microsoft.
Microsoft added a disk cache to MS-DOS (version 4. Microsoft Corporation is an American multinational Computer technology Corporation, which rose to dominate the Home computer 01) in 1988. They called it SmartDrive. SmartDrive is a Disk caching program that shipped with MS-DOS versions 4
See also
References
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