Shared vs. Private in CMPs

• Shared caches are often very large in the CMPs
  • They are banked to avoid worst-case wire delay
  • The banks are usually distributed across the floor of the chip on an interconnect

• In shared caches, getting a block from a remote bank takes time proportional to the physical distance between the requester and the bank
  • Non-uniform cache architecture (NUCA)
• This is same for private caches, if the data resides in a remote cache
• Shared cache may have higher average hit latency than the private cache
  • Hopefully most hits in the latter will be local
• Shared caches are most likely to have less misses than private caches
  • Latter wastes space due to replication