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A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. [1] A cache is a smaller, faster memory, located closer to a processor core, which stores copies of the data from frequently used main memory locations.
The page cache in main memory is managed by the operating system kernel. While the disk buffer, which is an integrated part of the hard disk drive or solid state drive, is sometimes misleadingly referred to as disk cache, its main functions are write sequencing and read prefetching.
Hit Time cache + (Miss Rate cache × Miss Penalty time taken to go to main memory after missing cache). [further explanation needed] The hit time for caches is less than the hit time for the main memory, so the AAT for data retrieval is significantly lower when accessing data through the cache rather than main memory. [14]
The resulting load on memory use is known as pressure (respectively register pressure, cache pressure, and (main) memory pressure). Terms for data being missing from a higher level and needing to be fetched from a lower level are, respectively: register spilling (due to register pressure : register to cache), cache miss (cache to main memory ...
The gap between processor speed and main memory speed has grown exponentially. Until 2001–05, CPU speed, as measured by clock frequency, grew annually by 55%, whereas memory speed only grew by 7%. [1] This problem is known as the memory wall. The motivation for a cache and its hierarchy is to bridge this speed gap and overcome the memory wall.
The term memory is often synonymous with the terms RAM, main memory, or primary storage. Archaic synonyms for main memory include core (for magnetic core memory) and store. [3] Main memory operates at a high speed compared to mass storage which is slower but less expensive per bit and higher
Pages in the page cache modified after being brought in are called dirty pages. [5] Since non-dirty pages in the page cache have identical copies in secondary storage (e.g. hard disk drive or solid-state drive), discarding and reusing their space is much quicker than paging out application memory, and is often preferred over flushing the dirty pages into secondary storage and reusing their space.
Cache placement policies are policies that determine where a particular memory block can be placed when it goes into a CPU cache.A block of memory cannot necessarily be placed at an arbitrary location in the cache; it may be restricted to a particular cache line or a set of cache lines [1] by the cache's placement policy.