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Since the cache exists to bridge the speed gap, its performance measurement and metrics are important in designing and choosing various parameters like cache size, associativity, replacement policy, etc. Cache performance depends on cache hits and cache misses, which are the factors that create constraints to system performance.
A cache hit occurs when the requested data can be found in a cache, while a cache miss occurs when it cannot. Cache hits are served by reading data from the cache, which is faster than recomputing a result or reading from a slower data store; thus, the more requests that can be served from the cache, the faster the system performs.
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 block occupies a cache line in set 0, determined by the replacement policy for the cache. Address 0x0004 (tag - 0b000_0000, index – 0b0_0001, offset – 0b00) corresponds to block 1 of the memory and maps to the set 1 of the cache. The block occupies a cache line in set 1, determined by the replacement policy for the cache.
A cache has two primary figures of merit: latency and hit ratio. A number of secondary factors also affect cache performance. [1] The hit ratio of a cache describes how often a searched-for item is found. More efficient replacement policies track more usage information to improve the hit rate for a given cache size.
However, in the case of the write back policy, the changed cache block will be updated in the lower-level hierarchy only when the cache block is evicted. A "dirty bit" is attached to each cache block and set whenever the cache block is modified. [27] During eviction, blocks with a set dirty bit will be written to the lower-level hierarchy.
It is different than the external memory model because cache-oblivious algorithms do not know the block size or the cache size. In particular, the cache-oblivious model is an abstract machine (i.e., a theoretical model of computation). It is similar to the RAM machine model which replaces the Turing machine's infinite tape with an infinite array.
Some architectures such as PowerPC attempt to avoid the need for cacheline locking or scratchpads through the use of cache control instructions.Marking an area of memory with "Data Cache Block: Zero" (allocating a line but setting its contents to zero instead of loading from main memory) and discarding it after use ('Data Cache Block: Invalidate', signaling that main memory didn't receive any ...