Directory-based coherence protocols

Directory-based coherence protocols are a set of cache coherence protocols, that is, protocols which ensure cache coherence or memory coherence between multiple nodes of multiprocessor or distributed shared memory systems, such as ccNUMA.

In such protocols, directories track where each "page," or block of memory, has been cached. Every request for data (i.e., every "read miss") is sent to the directory, which in turn forwards information to the nodes that have cached that data. In some protocols, these nodes then respond with data (these are called 3-hop-miss or cache-to-cache transfer protocols). In home-based protocols, each page has its own home node, and its corresponding directory is located on that node, containing only traces of pages from this node.