In previous post, we briefly discussed some variants of cache coherence protocol. We also pointed out that, in widely adopted NUMA architecture, snoop-based cache coherence scheme is typically combined with home directories associated with each distributed home memory. Although sounds intuitive, there are a couple of home directory implementation issues we need to consider, including
Status tracked by home directory
Non atomic operations
Status Tracked by Home Directory
Home directory can receive read request, write request or invalidate from local cache. For a read or write request, it needs to decide whether the data is in the home memory or in a remote cache; for a write or invalidate request, it needs to invalidate the remote cache blocks. This means, the home directory has to keep track of cache block state, as well as which cache has the cache block copy.