Method for reducing directory writes and latency in a high performance, directory-based, coherency protocol

Abstract

A computer system has a plurality of processors wherein each processor preferably has its own cache memory. Each processor or group of processors may have a memory controller that interfaces to a main memory. Each main memory includes a “directory” that maintains the directory coherence state of each block of that memory. One or more of the processors are members of a “local” group of processors. Processors outside a local group are referred to as “remote” processors with respect to that local group. Whenever a remote processor performs a memory reference for a particular block of memory, the processor that maintains the directory for that block normally updates the directory to reflect that the remote processor now has exclusive ownership of the block. However, memory references between processors within a local group do not result in directory writes. Instead, the cache memory of the local processor that initiated the memory requests places or updates a copy of the requested data in its cache memory and also sets associated tag control bits to reflect the same or similar information as would have been written to the directory. If a subsequent request is received for that same block, the local processor that previously accessed the block examines its cache for the associated tag control bits. Using those bits, that processor will determine that it currently has the block exclusive and provides the requested data to the new processor that is requesting the data.