命中率、缺失率、缺失损失-南航计算机组成原理课件7

命中率、缺失率、缺失损失 Hit: 要访问的信息在Cache中 Hit Rate(命中率)：在Cache中的概率 Hit Time (命中时间) ：在Cache中的访问时间，包括： Time to determine hit/miss + Cache access time (即： 判断时间 + Cache访问) Miss: 要找的信息不在Cache中 Miss Rate (缺失率) = 1 - (Hit Rate) Miss Penalty (缺失损失)：访问一个主存块所花时间 Hit Time << Miss Penalty （Why?） * A HIT is when the data the processor wants to access is found in the upper level (Blk X). The fraction of the memory access that are HIT is defined as HIT rate. HIT Time is the time to access the Upper Level where the data is found (X). It consists of: (a) Time to access this level. (b) AND the time to determine if this is a Hit or Miss. If the data the processor wants cannot be found in the Upper level. Then we have a miss and we need to retrieve the data (Blk Y) from the lower level. By definition (definition of Hit: Fraction), the miss rate is just 1 minus the hit rate. This miss penalty also consists of two parts: (a) The time it takes to replace a block (Blk Y to BlkX) in the upper level. (b) And then the time it takes to deliver this new block to the processor. It is very important that your Hit Time to be much much smaller than your miss penalty. Otherwise, there will be no reason to build a memory hierarchy. +2 = 14 min. (X:54)