cache

<memory management> /kash/ A small fast memory holding recently accessed data, designed to speed up subsequent access to the same data. Most often applied to processor-memory access but also used for a local copy of data accessible over a network etc. When data is read from, or written to, main memory a copy is also saved in the cache, along with the associated main memory address. The cache monitors addresses of subsequent reads to see if the required data is already in the cache. If it is (a cache hit) then it is returned immediately and the main memory read is aborted (or not started). If the data is not cached (a cache miss) then it is fetched from main memory and also saved in the cache. The cache is built from faster memory chips than main memory so a cache hit takes much less time to complete than a normal memory access. The cache may be located on the same integrated circuit as the CPU, in order to further reduce the access time. In this case it is often known as primary cache since there may be a larger, slower secondary cache outside the CPU chip. The most important characteristic of a cache is its hit rate - the fraction of all memory accesses which are satisfied from the cache. This in turn depends on the cache design but mostly on its size relative to the main memory. The size is limited by the cost of fast memory chips. The hit rate also depends on the access pattern of the particular program being run (the sequence of addresses being read and written). Caches rely on two properties of the access patterns of most programs: temporal locality - if something is accessed once, it is likely to be accessed again soon, and spatial locality - if one memory location is accessed then nearby memory locations are also likely to be accessed. In order to exploit spatial locality, caches often operate on several words at a time, a "cache line" or "cache block". Main memory reads and writes are whole cache lines. When the processor wants to write to main memory, the data is first written to the cache on the assumption that the processor will probably read it again soon. Various different policies are used. In a write-through cache, data is written to main memory at the same time as it is cached. In a write-back cache it is only written to main memory when it is forced out of the cache. If all accesses were writes then, with a write-through policy, every write to the cache would necessitate a main memory write, thus slowing the system down to main memory speed. However, statistically, most accesses are reads and most of these will be satisfied from the cache. Write-through is simpler than write-back because an entry that is to be replaced can just be overwritten in the cache as it will already have been copied to main memory whereas write-back requires the cache to initiate a main memory write of the flushed entry followed (for a processor read) by a main memory read. However, write-back is more efficient because an entry may be written many times in the cache without a main memory access. When the cache is full and it is desired to cache another line of data then a cache entry is selected to be written back to main memory or "flushed". The new line is then put in its place. Which entry is chosen to be flushed is determined by a "replacement algorithm". Some processors have separate instruction and data caches. Both can be active at the same time, allowing an instruction fetch to overlap with a data read or write. This separation also avoids the possibility of bad cache conflict between say the instructions in a loop and some data in an array which is accessed by that loop. See also direct mapped cache, fully associative cache, sector mapping, set associative cache. (1997-06-25)

Cache

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For information related to caching Wikipedia, see Wikipedia:Cache.

Cache may refer to:

Caching or hoarding (animal behavior), a food storing behavior of animals
Treasure trove, a valuable cache which has been lost, or left unclaimed by the owner, or a place where items are stored
Geocaching, an outdoor treasure-hunting game

Computing

Cache (computing), a collection of data duplicating original values stored elsewhere on a computer
- CPU cache, a small area of fast memory used by the central processing unit
- Disk buffer, the small amount of buffer memory present on a hard drive
- Page cache, the cache of disk pages kept by the operating systems, stored in unused main memory
- Web cache, a mechanism for the temporary storage of web documents to increase performance
- DNS cache, a server in the domain name system which stores queried results for a period of time
- P2P caching, a technique used to reduce bandwidth costs for content on peer-to-peer networks
- Database caching, a mechanism used to cache database content in multi-tier applications
InterSystems Caché, an object / relational database

Geography

Cache (Aosta), a frazione in the Province of Aosta in the Aosta Valley region of Italy
Cache, Illinois, an unincorporated community in Alexander County, Illinois, United States
Cache, Oklahoma, a city in Comanche County
Cache Creek (disambiguation), several places
Cache County, Utah, United States
Cache, Utah, Cache County, Utah, United States
Cache Peak (Idaho), a mountain in Castle Rocks State Park

Caché may refer to:

Caché (film), a 2005 film directed by Michael Haneke
Caché (software), a database management system from InterSystems
Caché (album), a 1993 album by Kirk Whalum
Caché, Inc., South Florida women's apparel company rescued by Andrew Saul
Benedikt de Caché (1740–1809), Austrian diplomat