The performance impact of block sizes and fetch strategies

The interactions between a cache´s block size, fetch size, and fetch policy from the perspective of maximizing system-level performance are explored. It has been previously noted that, given a simple fetch strategy, the performance optimal block size is almost always four or eight words. If there is even a small cycle time penalty associated with either longer blocks or fetches, then the performance optimal size is noticeably reduced. In split cache organizations, where the fetch and block sizes of instruction and data caches are all independent design variables, instruction cache block size and fetch size should be the same. For the workload and write-back write policy used in this trace-driven simulation study, the instruction cache block size should be about a factor of 2 greater than the data cache fetch size, which in turn should be equal to or double the data cache block size. The simplest fetch strategy of fetching only on a miss and stalling the CPU until the fetch is complete works well. Complicated fetch strategies do not produce the performance improvements indicated by the accompanying reductions in miss ratios because of limited memory resources and a strong temporal clustering of cache misses. For the environments simulated, the most effective fetch strategy improved performance by between 1.7% and 4.5% over the simplest strategy described above