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Improving Cache Average Access Time

  • Reduce Miss Rate
  • Miss Rates can be:
    • Compulsory Misses: first access will always miss
    • Capacity Misses: when the cache is too small and when it is full
    • Conflict Misses: when the cache has mapping scheme constraints (eviction in direct or set associative)
  • Higher miss rate when the block size is too small (not enough data per access) or too large (time spent getting data I wouldn't use, and it takes up space that will prevent other data)
  • Larger cache size reduces capacity misses
  • High associativity reduces conflict misses. At reasonable cache sizes, associativity above K=2K=2 or 4โˆ’way4-way does not improve much.

![Smith, Allan J.ย ย Disk Cache โ€“ Miss Ratio Analysis and Design Considerations. 1985](../assets/F22233.png

Hit Timeโ€‹

  • higher when the cache is larger (large and fast memories are expensive)
  • higher when the cache has high associativity (i.e., large KK, more time spent searching one set with KK lines).

Miss Penaltyโ€‹

  • Higher when the cache has large blocks (long transfer times, but latency is always paid)