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Remote access cache
- Essentially a large tertiary cache
- Captures remote cache blocks evicted from local cache hierarchy
- Also visible to the coherence protocol: so inclusion must be maintained with processor caches
- Must be highly associative and larger than the outermost level of cache
- Usually part of DRAM is reserved for RAC
- For multiprocessor nodes, requests from different processors to the same cache block can be merged together; also there is a prefetching effect
- Used in Stanford DASH
- Disadvantage: latency and space
COMA
- Cache-only memory architecture
- Solves the space problem of RAC
- Home node only maintains the directory entries, but may not have the cache block in memory
- A node requesting a cache block brings it to its local memory and local cache as usual
- Entire memory is treated as a large tertiary cache
- Known as the attraction memory (AM)
- Home as well as any node having a cache block maintain a directory entry for the cache block
- A request first looks up AM directory state and, if unowned, gets forwarded to home which, in turn, forwards it to one of the sharers
- Cache-only memory architecture
- To start with home has the cache blocks
- It retains a cache block until it is replaced by some other migration
- There is always a master copy of each cache block
- What happens on a replacement of the master copy?
- Swap with source of migrating cache block
- Latency problem remains at the requester
- Inclusion problems between AM and processor cache hierarchy
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