# 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 $K=2$ or $4-way$ does not improve much.

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

## Hit Time​

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

## Miss Penalty​

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