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In computer science, a cache 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. Once data is stored in the cache, future use of it can be made by accessing the cached copy rather than refetching or recomputing the original data, so that the average access time is lower. Caches have proved extremely effective in many areas of computing, because access patterns in typical computer applications have locality of reference. There are several sorts of locality, but we mainly mean that the same data is often used several times, with accesses that are close together in time, or that data near to each other are accessed close together in time.
CPU cachesMain 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. Disk buffer(also known as disk cache or cache buffer) Hard disks have historically often been packaged with embedded computers used for control and interface protocols. Since the late 1980s, nearly all disks sold have these embedded computers and either an ATA, SCSI, or Fibre Channel interface. The embedded computer usually has some small amount of memory which it uses to store the bits going to and coming from the disk platter. The disk buffer is physically distinct from and is used differently than the page cache typically kept by the operating system in the computer's main memory. The disk buffer is controlled by the embedded computer in the disk drive, and the page cache is controlled by the computer to which that disk is attached. The disk buffer is usually quite small, 2 to 8 MB, and the page cache is generally all unused physical memory, which in a 2004 PC may be between 20 and 2000 MB. And while data in the page cache is reused multiple times, the data in the disk buffer is typically never reused. In this sense, the phrases disk cache and cache buffer are misnomers, and the embedded computer's memory is more appropriately called the disk buffer. The disk buffer has multiple uses:
Other cachesCPU caches are generally managed entirely by hardware. Other caches are managed by a variety of software. The cache of disk sectors in main memory is usually managed by the operating system kernel or file system. The BIND DNS daemon caches a mapping of domain names to IP addresses, as does a resolver library. 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. 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 ISPs use proxy caches to save bandwidth on frequently-accessed web pages. The search engine Google keeps a cached copy of each page it examines on the web. These copies are used by the Google indexing software, but they are also made available to Google users, in case the original page is unavailable. If you click on the "Cached" link in a Google search result, you will see the web page as it looked when Google indexed it. Ccache is a program that caches the output of the compilation to speed up the second-time compilation. See alsoExternal links
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:: About Us This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cache". |