Cache

In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster(SONY VGP-BPS13Q Battery). The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere. If requested data is contained in the cache (cache hit) (SONY VGP-BPS13B/Q Battery), this request can be served by simply reading the cache, which is comparatively faster. Otherwise (cache miss), the data has to be recomputed or fetched from its original storage location, which is comparatively slower. Hence, the more requests can be served from the cache the faster the overall system performance is(SONY VGN NR11S/S battery).

As opposed to a buffer, which is managed explicitly by a client[citation needed], a cache stores data transparently: This means that a client who is requesting data from a system is not aware that the cache exists, which is the origin of the name cache (from French "cacher", to conceal) (SONY VGN NR11M/S battery).

To be cost efficient and to enable an efficient use of data, caches are relatively small. Nevertheless, caches have proven themselves in many areas of computing because access patterns in typical computer applications have locality of reference(SONY VGN NR11Z/S battery). References exhibit temporal locality if data is requested again that has been recently requested already. References exhibit spatial locality if data is requested that is physically stored close to data that has been requested already(SONY VGN NR11Z/T battery).

Operation

Hardware implements cache as a block of memory for temporary storage of data likely to be used again. CPUs and hard drives frequently use a cache, as do web browsers and web servers(SONY VGP-BPS13A/Q Battery).

A cache is made up of a pool of entries. Each entry has a datum (a nugget of data) - a copy of the same 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(SONY Vaio PCG-F305 battery).

When the cache client (a CPU, web browser, operating system) needs to access a datum presumed to exist in the backing store, it first checks the cache. If an entry can be found with a tag matching that of the desired datum(SONY VGP-BPS21/S Battery), 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(SONY VGP-BPS21 Battery). 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(SONY VGP-BPS21B Battery).

The alternative situation, when the cache is consulted and found not to contain a datum with the desired tag, has become known as a cache miss. The previously uncached datum fetched from the backing store during miss handling is usually copied into the cache, ready for the next access(SONY VGP-BPS21A/B Battery).

During a cache miss, the CPU usually ejects some other entry in order to make room for the previously uncached datum. The heuristic used to select the entry to eject is known as the replacement policy(SONY VGP-BPS13B/S Battery). 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(SONY VGP-BPS13S Battery). 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 is not efficient for use with a CPU cache(SONY VGP-BPS13A/S Battery).

When a system writes a datum to the cache, it must at some point write that datum to the backing store as well. The timing of this write is controlled by what is known as the write policy(Sony VAIO VGN-FZ15G Battery).

In a write-through cache, every write to the cache causes a synchronous write to the backing store(Sony VAIO VGN-FZ15T Battery).

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 and marks these locations as dirty(SONY VGP-BPS13/B Battery). The data in these locations are written back to the backing store when those data are evicted from the cache, an effect referred to as a lazy write(Dell N3010 Battery). For this reason, a read miss in a write-back cache (which requires a block to be replaced by another) 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(SONY VGP-BPS13B/B Battery).

Other policies may also trigger data write-back. The client may make many changes to a datum in the cache, and then explicitly notify the cache to write back the datum(SONY VGP-BPS13 Battery).

No-write allocation (a.k.a. write-no-allocate) is a cache policy which caches only processor reads, i.e. on a write-miss:

Datum is written directly to memory,

Datum at the missed-write location is not added to cache(SONY VGP-BPS13 Battery).

This avoids the need for write-back or write-through when the old value of the datum was absent from the cache prior to the write(SONY VAIO VGN-FZ31B Battery).

Entities other than the cache may change the data in the backing store, 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 those data in other caches will become stale(SONY VAIO VGN-FZ31M Battery). Communication protocols between the cache managers which keep the data consistent are known as coherency protocols(SONY VAIO VGN-FZ31J Battery).

CPU cache

Main article: CPU cache

Small memories on or close to the CPU can operate faster than the much larger main memory. Most CPUs since the 1980s have used one or more caches, and modern high-end embedded(SONY VAIO VGN-FZ31E Battery), desktop and server microprocessors may have as many as half a dozen, each specialized for a specific function. Examples of caches with a specific function are the D-cache and I-cache (data cache and instruction cache) (Sony Vaio VGN-FZ31S battery).

[edit]Disk cache

Main article: Page cache

While CPU caches are generally managed entirely by hardware, a variety of software manages other caches. The page cache in main memory, which is an example of disk cache, is managed by the operating system kernel(SONY VAIO VGN-FZ31z Battery).

While the hard drive's hardware disk buffer is sometimes misleadingly referred to as "disk cache", its main functions are write sequencing and read prefetching. Repeated cache hits are relatively rare, due to the small size of the buffer in comparison to the drive's capacity(SONY VAIO VGN-FZ38M Battery). However, high-end disk controllers often have their own on-board cache of hard disk data blocks.

Finally, fast local hard disk can also cache information held on even slower data storage devices, such as remote servers (web cache) or local tape drives or optical jukeboxes. Such a scheme is the main concept of hierarchical storage management(SONY VAIO VGN-FZ21m Battery).

[edit]Web cache

Main article: Web cache

Web browsers and web proxy servers employ web caches to store previous responses from web servers, such as web pages. Web caches reduce the amount of information that needs to be transmitted across the network(SONY VAIO VGN-FZ18m Battery), as information previously stored in the cache can often be re-used. This reduces bandwidth and processing requirements of the web server, and helps to improve responsiveness for users of the web(SONY VAIO VGN-FZ11m Battery).

Web browsers employ a built-in web cache, but some internet service providers or organizations also use a caching proxy server, which is a web cache that is shared among all users of that network(SONY VAIO VGN-FZ11z Battery).

Another form of cache is P2P caching, where the files most sought for by peer-to-peer applications are stored in an ISP cache to accelerate P2P transfers(SONY VAIO VGN-FZ11l Battery).

There are three kinds of web caches: 1)Client Caching, 2)Proxy Caches, 3)Gateway caches.

[edit]Other caches

The BIND DNS daemon caches a mapping of domain names to IP addresses, as does a resolver library(SONY VAIO VGN-FZ150E Battery).

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(SONY VAIO VGN-FZ190 Battery). 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(SONY VAIO VGN-FZ460E Battery).

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 can prove useful when web pages from a web server are temporarily or permanently inaccessible(sony vgp-bpl9 battery).

Another type of caching is storing computed results that will likely be needed again, or memoization. ccache, a program that caches the output of the compilation to speed up the second-time compilation, exemplifies this type(SONY vgp-bps9 battery).

Database caching can substantially improve the throughput of database applications, for example in the processing of indexes, data dictionaries, and frequently used subsets of data(SONY VAIO VGN-FZ11S Battery).

Distributed caching uses caches spread across different networked hosts.

The difference between buffer and cache

The terms "buffer" and "cache" are not mutually exclusive and the functions are frequently combined; however, there is a difference in intent(SONY VAIO VGN-FZ Battery).

A buffer is a temporary memory location, that is traditionally used because CPU instructions cannot directly address data stored in peripheral devices. Thus, addressable memory is used as intermediate stage(SONY VGP-BPS8 battery). Additionally such a buffer may be feasible when a large block of data is assembled or disassembled (as required by a storage device), or when data may be delivered in a different order than that in which it is produced(Sony VAIO PCG-5K1L battery). Also a whole buffer of data is usually transferred sequentially (for example to hard disk), so buffering itself sometimes increases transfer performance or reduce the variation or jitter of the transfer's latency as opposed to caching where the intent is to reduce the latency(Sony VAIO PCG-6W2L battery). These benefits are present even if the buffered data are written to the buffer once and read from the buffer once.

A cache also increases transfer performance. A part of the increase similarly comes from the possibility that multiple small transfers will combine into one large block(Sony VAIO PCG-7112L battery). 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. A cache's sole purpose is to reduce accesses to the underlying slower storage(Sony VAIO PCG-8Z1L battery). Cache is also usually an abstraction layer that is designed to be invisible from the perspective of neighbouring layers.