Cache MemoryMost PCs are held back

Cache MemoryMost PCs are held back not by the speed of their main processor, but by the time it takes to move data in and out of memory. One of the most important techniques for getting around this bottleneck is the memory cache.The idea is to use a small number of very fast memory chips as a buffer or cache between main memory and the processor. Whenever the processor needs to read data it looks in this cache area first. If it finds the data in the cache then this counts as a 'cache hit' and the processor need not go through the more laborious process of reading data from the main memory. Only if the data is not in the cache does it need to access main memory, but in the process it copies whatever it finds into the cache so that it is there ready for the next time it is needed. The whole process is controlled by a group of logic circuits called the cache controller.One of the cache controller's main jobs is to look after 'cache coherency' which means ensuring that any changes written to main memory are reflected within the cache and vice versa. There are several techniques for achieving this, the most obvious being for the processor to write directly to both the cache and main memory at the same time. This is known as a “write-through” cache and is the safest solution, but also the slowest.The main alternative is the “write-back” cache which allows the processor to write changes only to the cache and not to main memory. Cache entries that have changed are flagged as 'dirty', telling the cache controller to write their contents back to main memory before using the space to cache new data. A write-back cache speeds up the write process, but does require a more intelligent cache controller.Most cache controllers move a 'line' of data rather than just a single item each time they need to transfer data between main memory and the cache. This tends to improve the chance of a cache hit as most programs spend their time stepping through instructions stored sequentially in memory, rather than jumping about from one area to another. The amount of data transferred each time is known as the 'line size'.How a Disk Cache WorksDisk caching works in essentially the same way whether you have a cache on your disk controller or you are using a software-based solution. The CPU requests specific data from the cache. In some cases, the information will already be there and the request can be met without accessing the hard disk.If the requested information isn't in the cache, the data is read from the disk along with a large chunk of adjacent information. The cache then makes room for the new data by replacing old. Depending on the algorithm that is being applied, this may be the information that has been in the cache the longest, or the information that is the least recently used. The CPU's request can then be met, and the cache already has the adjacent data loaded in anticipation of that information being requested next.

Результаты (суахили) 1:

Cache Kumbukumbu

Wengi PC hufanyika nyuma si kwa kasi ya processor kuu, lakini kwa muda ambao unachukua kuhamisha data ndani na nje ya kumbukumbu. Moja ya mbinu zaidi muhimu kwa ajili ya kupata kuzunguka vikwazo huu ni kumbukumbu akiba.

Wazo ni kutumia idadi ndogo ya chips haraka sana kumbukumbu kama kinga au cache kati kumbukumbu kuu na processor. Kila processor inahitaji kusoma data inaonekana katika eneo hili cache kwanza. Kama anaona data katika cache basi makosa haya kama 'cache hit' na processor haja si kupitia mchakato zaidi yoyote ya utumishi wa kusoma data kutoka kumbukumbu kuu. Ikiwa tu data si katika akiba gani atahitaji kufikia kumbukumbu kuu, lakini katika mchakato ni nakala yoyote anaona katika cache hivyo kuwa ni pale tayari kwa ajili ya wakati ujao ni inahitajika. mchakato mzima ni kudhibitiwa na kundi la nyaya mantiki kuitwa cache mtawala.

Moja ya kazi cache mtawala kuu ni kuangalia baada ya 'cache coherency' ambayo ina maana ya kuhakikisha kwamba mabadiliko yoyote imeandikwa kwa kumbukumbu kuu ni yalijitokeza katika cache na kinyume chake. Kuna mbinu kadhaa kwa ajili kufanikisha hili, wengi dhahiri kuwa kwa processor kuandika moja kwa moja kwa zote mbili cache na kumbukumbu kuu kwa wakati mmoja. Hii inajulikana kama "kuandika-kwa njia ya" cache na ni suluhisho salama, lakini pia polepole.

Mbadala kuu ni "kuandika-nyuma" cache ambayo inaruhusu processor kuandika mabadiliko tu kwa cache na si kwa kumbukumbu kuu. Cache entries kwamba yamebadilika ni alama kuwa 'chafu', kuwaambia cache mtawala kwa kuandika maudhui yao nyuma kwa kumbukumbu kuu kabla ya kutumia nafasi ya cache takwimu mpya. Cache kuandika-nyuma kasi ya kuandika mchakato, lakini haina zinahitaji zaidi akili cache mtawala.

Controllers nyingi cache hoja 'line' ya data kuliko tu kitu kimoja kila wakati unahitaji kuhamisha data kati ya kumbukumbu kuu na akiba. Hii huelekea kuboresha nafasi ya cache hit kama programu nyingi kutumia muda wao wanazidi kupitia maelekezo kuhifadhiwa sequentially katika kumbukumbu, badala ya kuruka juu kutoka eneo moja hadi nyingine. Kiasi cha data kuhamishiwa kila wakati hujulikana kama 'mstari ukubwa.

Jinsi Disk Cache Works

Disk Caching kazi katika kimsingi njia hiyo kama una akiba kwenye disk mtawala wako au unatumia programu makao ufumbuzi. CPU maombi data maalum kutoka kwenye akiba. Wakati mwingine, habari itakuwa tayari kuwa huko na ombi inaweza kuwa alikutana bila kupata disk ngumu.

Kama taarifa ya ombi sio katika akiba, data kusoma kutoka disk pamoja na chunk kubwa ya habari karibu. cache basi hufanya chumba kwa data mpya kwa kuondoa zamani. Kulingana na algorithm kwamba unatumika, hii inaweza kuwa habari kwamba amekuwa katika cache mrefu, au taarifa ambayo angalau hivi karibuni kutumika. Ombi CPU ya basi kufikiwa, na cache tayari ina data ya karibu kubeba kwa kutarajia kwamba kuwa maelezo yaliyotakiwa ijayo.