Parallel Computer Architecture

8,4 Design Challenges for Directory Protocols 589

a bus within a node has some particular disadvantages. First, if the bus has to

accommodate several processors, it becomes longer and is not likely to be contained

in a single board or other packaging unit. These effects slow the bus down, increas-

ing the latency to both local and remote data. Second, if the bus supports snooping

coherence within the node, a request that must be satisfied remotely typically has to

wait for local snoop results to be reported before it is sent out to the network, caus-

ing unnecessary delays. Third, with a snooping bus at the remote node too, many

references tha ...