Directory-based coherence protocols

Directory-based coherence protocols is set of coherence protocols, that is protocols ensuring cache coherence or memory coherence between multiple nodes of multiprocessor or distributed shared memory system such as ccNUMA.

In such protocols directory keep traces of different pages. Every request (e.g. read miss) is send to directory, which forwards it to node having most recent copy (so called 3-hop-miss or cache-to-cache transfer), which then responds with data. In home-based protocols each page has its own home node and directory is located on that node, containing only traces of pages from this node.