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Cache hierarchy, or multi-level cache, is a memory architecture that uses a hierarchy of memory stores based on varying access speeds to cache data. Highly requested data is cached in high-speed access memory stores, allowing swifter access by central processing unit (CPU) cores.
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.
Consider the case when L2 is exclusive of L1. Suppose there is a processor read request for block X. If the block is found in L1 cache, then the data is read from L1 cache and returned to the processor. If the block is not found in the L1 cache, but present in the L2 cache, then the cache block is moved from the L2 cache to the L1 cache.
Memory hierarchy of an AMD Bulldozer server. The number of levels in the memory hierarchy and the performance at each level has increased over time. The type of memory or storage components also change historically. [6] For example, the memory hierarchy of an Intel Haswell Mobile [7] processor circa 2013 is:
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.
Diagram of a CPU memory cache operation. In computing, a cache (/ k æ ʃ / ⓘ KASH) [1] is a hardware or software component that stores data so that future requests for that data can be served faster; the data stored in a cache might be the result of an earlier computation or a copy of data stored elsewhere.
AMAT's three parameters hit time (or hit latency), miss rate, and miss penalty provide a quick analysis of memory systems. Hit latency (H) is the time to hit in the cache. Miss rate (MR) is the frequency of cache misses, while average miss penalty (AMP) is the cost of a cache miss in terms of time. Concretely it can be defined as follows.
Processor performance increase due to cache hierarchy depends on the number of accesses to the cache that satisfy block requests from the cache (cache hits) versus those that do not. Unsuccessful attempts to read or write data from the cache (cache misses) result in lower level or main memory access, which increases latency.