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Encyclopedia > Cache
Look up cache in Wiktionary, the free dictionary.

In computer science, a cache (IPA:/kæʃ/, like "cash-ay" [1]) is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive to fetch (due to longer access time) or to compute, compared to the cost of reading the cache. In other words, a cache is a temporary storage area where frequently accessed data can be stored for rapid access. Once the data is stored in the cache, future use can be made by accessing the cached copy rather than re-fetching or recomputing the original data, so that the average access time is shorter. Cache, therefore, helps expedite data access that the CPU would otherwise need to fetch from main memory. Usually cache refers to the computer memory, cache. ... Wikipedia does not have an article with this exact name. ... Wiktionary (a portmanteau of wiki and dictionary) is a multilingual, Web-based project to create a free content dictionary, available in over 150 languages. ... Computer science, or computing science, is the study of the theoretical foundations of information and computation and their implementation and application in computer systems. ... Symbols of the International Phonetic Alphabet. ... In telecommunication, the term access time has the following meanings: In a telecommunication system, the elapsed time between the start of an access attempt and successful access. ...


Cache have proven to be extremely effective in many areas of computing because access patterns in typical computer applications have locality of reference. There are several kinds of locality, but this article primarily deals with data that are accessed close together in time (temporal locality). The data might or might not be located physically close to each other (spatial locality). This article does not cite any references or sources. ... It has been suggested that this article or section be merged with Memory locality. ... Memory Locality is a term in computer science used to denote the temporal or spatial proximity of memory access. ... Memory Locality is a term in computer science used to denote the temporal or spatial closeness of memory lookups in the upper memory hierarchy. ...

Contents

History

Use of the word “cache” in the computer context originated in 1967 during preparation of an article for publication in the IBM Systems Journal. The paper concerned an exciting memory improvement in Model 85, a latecomer in the IBM System/360 product line. The Journal editor, Lyle R. Johnson, pleaded for a more descriptive term than “high-speed buffer”; when none was forthcoming, he suggested “cache.” The paper was published in early 1968, the authors were honored by IBM, their work was widely welcomed and subsequently improved upon, and “cache” soon became standard usage in computer literature.[2]


Operation

Diagram of a CPU memory cache
Diagram of a CPU memory cache

A cache is a block of memory for temporary storage of data likely to be used again. The CPU and hard drive frequently use a cache, as do web browsers and web servers. Image File history File links This is a lossless scalable vector image. ... CPU can stand for: in computing: Central processing unit in journalism: Commonwealth Press Union in law enforcement: Crime prevention unit in software: Critical patch update, a type of software patch distributed by Oracle Corporation in Macleans College is often known as Ash Lim. ... Typical hard drives of the mid-1990s. ...


A cache is made up of a pool of entries. Each entry has a datum (a nugget of data) which is a copy of the datum in some backing store. Each entry also has a tag, which specifies the identity of the datum in the backing store of which the entry is a copy.


When the cache client (a CPU, web browser, operating system) wishes to access a datum presumably in the backing store, it first checks the cache. If an entry can be found with a tag matching that of the desired datum, the datum in the entry is used instead. This situation is known as a cache hit. So, for example, a web browser program might check its local cache on disk to see if it has a local copy of the contents of a web page at a particular URL. In this example, the URL is the tag, and the contents of the web page is the datum. The percentage of accesses that result in cache hits is known as the hit rate or hit ratio of the cache.


The alternative situation, when the cache is consulted and found not to contain a datum with the desired tag, is known as a cache miss. The datum fetched from the backing store during miss handling is usually inserted into the cache, ready for the next access.


If the cache has limited storage, it may have to eject some other entry in order to make room. The heuristic used to select the entry to eject is known as the replacement policy. One popular replacement policy, least recently used (LRU), replaces the least recently used entry (see cache algorithms). More efficient caches compute use frequency against the size of the stored contents, as well as the latencies and throughputs for both the cache and the backing store. While this works well for larger amounts of data, long latencies, and slow throughputs, such as experienced with a hard drive and the Internet, it's not efficient to use this for cached main memory (RAM).[citation needed] In computer science, besides the common use as rule of thumb (see heuristic), the term heuristic has two well-defined technical meanings. ... It has been suggested that this article or section be merged into cache algorithms. ... It has been suggested that page replacement algorithm be merged into this article or section. ... Primary storage is a category of computer storage, often called main memory. ...


When a datum is written to the cache, it must at some point be written to the backing store as well. The timing of this write is controlled by what is known as the write policy.


In a write-through cache, every write to the cache causes a synchronous write to the backing store.


Alternatively, in a write-back (or write-behind) cache, writes are not immediately mirrored to the store. Instead, the cache tracks which of its locations have been written over (these locations are marked dirty). The data in these locations is written back to the backing store when those data are evicted from the cache. For this reason, a miss in a write-back cache will often require two memory accesses to service: one to retrieve the needed datum, and one to write replaced data from the cache to the store.


Data write-back may be triggered by other policies as well. The client may make many changes to a datum in the cache, and then explicitly notify the cache to write back the datum.


No-write allocation is a cache policy where only processor reads are cached, thus avoiding the need for write-back or write-through when the old value of the datum was absent from the cache prior to the write.


The data in the backing store may be changed by entities other than the cache, in which case the copy in the cache may become out-of-date or stale. Alternatively, when the client updates the data in the cache, copies of that data in other caches will become stale. Communication protocols between the cache managers which keep the data consistent are known as coherency protocols. Cache coherence refers to the integrity of data stored in local caches of a shared resource. ...


Applications

CPU caches

Main article: CPU cache

Small memories on or close to the CPU chip can be made faster than the much larger main memory. Most CPUs since the 1980s have used one or more caches, and modern general-purpose CPUs inside personal computers may have as many as half a dozen, each specialized to a different part of the problem of executing programs. Diagram of a CPU memory cache A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. ... A microprocessor is a programmable digital electronic component that incorporates the functions of a central processing unit (CPU) on a single semiconducting integrated circuit (IC). ...


Disk cache

CPU caches are generally managed entirely by hardware; other caches are managed by a variety of software. The page cache which is a cache of disk pages in main memory, is usually managed by the operating system kernel or file system. In computing, page cache, sometimes ambiguously called disk cache, is a transparent cache of disk-backed pages kept in primary storage (RAM) for quicker access. ... Primary storage is a category of computer storage, often called main memory. ... A kernel connects the application software to the hardware of a computer. ... It has been suggested that Crash counting be merged into this article or section. ...


While the hardware disk buffer is also misleadingly referred to as "disk cache", its main function is write buffering and read prebuffering. Repeated cache hits are low due to the disproportionally large amount of main memory. Look up cache in Wiktionary, the free dictionary. ... Primary storage is a category of computer storage, often called main memory. ...


In turn, fast local hard disk can be used to cache information held on even slower data storage devices, such as tape drives or optical jukeboxes. Such a scheme is the main concept of hierarchical storage management. DDS tape drive. ... An optical jukebox or autochanger is a computer device used for massive data storage. ... Hierarchical Storage Management (HSM) is a data storage technique that automatically moves data between high-cost and low-cost storage media. ...


Other caches

The BIND DNS daemon caches a mapping of domain names to IP addresses, as does a resolver library. It has been suggested that this article be split into multiple articles. ... This article or section does not cite any references or sources. ...


Write-through operation is common when operating over unreliable networks (like an ethernet LAN), because of the enormous complexity of the coherency protocol required between multiple write-back caches when communication is unreliable. For instance, web page caches and client-side network file system caches (like those in NFS or SMB) are typically read-only or write-through specifically to keep the network protocol simple and reliable. Cache coherence refers to the integrity of data stored in local caches of a shared resource. ... Client/Server is a network application architecture which separates the client (usually the graphical user interface) from the server. ... For the protocol of this name, see Network File System (protocol). ... Network File System (NFS) is a network file system protocol originally developed by Sun Microsystems in 1984, allowing a user on a client computer to access files over a network as easily as if the network devices were attached to its local disks. ... Server Message Block (SMB) is an application-level network protocol mainly applied to shared access to files, printers, serial ports, and miscellaneous communications between nodes on a network. ...


A cache of recently visited web pages can be managed by your web browser. Some browsers are configured to use an external proxy web cache, a server program through which all web requests are routed so that it can cache frequently accessed pages for everyone in an organization. Many internet service providers use proxy caches to save bandwidth on frequently-accessed web pages. An example of a Web browser (Mozilla Firefox) A web browser is a software application that enables a user to display and interact with text, images, videos, music and other information typically located on a Web page at a website on the World Wide Web or a local area network. ... In computer networks, a proxy server is a server (a computer system or an application program) which services the requests of its clients by forwarding requests to other servers. ... Web caching is the caching of web documents (e. ... “ISP” redirects here. ... -1...


Search engines also frequently make web pages they have indexed available from their cache. For example, Google provides a "Cached" link next to each search result. This is useful when web pages are temporarily inaccessible from a web server. This article is about search engines. ... A screenshot of a web page. ... This article is about the corporation. ... The inside/front of a Dell PowerEdge web server The term Web server can mean one of two things: A computer program that is responsible for accepting HTTP requests from clients, which are known as Web browsers, and serving them HTTP responses along with optional data contents, which usually are...


Another type of caching is storing computed results that will likely be needed again, or memoization. An example of this type of caching is ccache, a program that caches the output of the compilation to speed up the second-time compilation. Memoization is a technique used to speed up computer programs by storing the results of functions for later reuse, rather than recomputing them. ... ccache is a computer program which caches the output of C/C++ compilation so that the next time, the same compilation can be omitted. ...


The difference between buffer and cache

The terms are not mutually exclusive and the functions are frequently combined; however, there is a difference in intent. A buffer is a temporary memory location, that is traditionally used because CPU instructions just cannot directly address data stored in peripheral devices. Thus, addressable memory is used as intermediate stage. Additionally such buffer may be feasible when a large block of data is assembled or disassembled (as required by a storage device), or when data must be delivered in a different order than that in which it is produced. Also a whole buffer of data is usually transferred sequentially (for example to hard disk), so buffering itself sometimes increases transfer performance. These benefits are present even if the buffered data are written to the buffer once and read from the buffer once. In computing, a buffer is a region of memory used to temporarily hold output or input data, comparable to buffers in telecommunication. ... In computer science, an instruction typically refers to a single operation of a processor within a computer architecture. ...


A cache also increases transfer performance. A part of the increase similarly comes from the possiblity that multiple small transfers will combine into one large block. But the main performance gain occurs because there is a good chance that the same datum will be read from cache multiple times, or that written data will soon be read. Cache's sole purpose is to reduce accesses to the underlying slower storage. Cache is also usually an abstraction layer that is designed to be invisible from perspective of neighboring layers. An abstraction layer is a way of hiding the implementation details of a particular set of functionality. ...


References

  1. ^ or, formerly, /kɑːʃ/Oxford English Dictionary cache (restricted); also Dictionary.com (unrestricted). Although the pronunciation /ˈkæʃeɪ/ is sometimes heard in English, it properly only represents the French adjective 'caché(e)', meaning 'hidden'. /keɪʃ/ is a mispronunciation.
  2. ^ G. C. Stierhoff and A. G. Davis. A History of the IBM Systems Journal. IEEE Annals of the History of Computing, Vol. 20, No. 1 (Jan. 1998), pages 29-35. [1]

// In grammar, an adjective is a word whose main syntactic role is to modify a noun or pronoun (called the adjectives subject), giving more information about what the noun or pronoun refers to. ...

See also


  Results from FactBites:
 
Cache - Wikipedia, the free encyclopedia (1705 words)
In computer science, a cache (pronounced "cash") is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive (usually in terms of access time) to fetch or compute relative to reading the cache.
Caches have proven extremely effective in many areas of computing because access patterns in typical computer applications have locality of reference.
The cache of disk sectors in main memory is usually managed by the operating system kernel or file system.
CPU cache - Wikipedia, the free encyclopedia (6998 words)
A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory.
A victim cache is a cache used to hold blocks evicted from a CPU cache due to a conflict or capacity miss.
This cache is generally implemented on a separate chip from the CPU, and, as of 2004, may range in size from 2 to 256 megabytes.
  More results at FactBites »

 
 

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