Bus sniffing

Bus sniffing or Bus snooping is a technique used in distributed shared memory systems and multiprocessors aimed at achieving cache coherence. Every cache controller monitors bus awaiting for broadcasts which may cause him to invalidate his cache line.

Cache line most commonly is in states "dirty", "invalid" and "valid" or "shared". On read miss request for read is broadcasted on a bus. All cache controllers are monitoring the bus. The one having the copy in the state "dirty" changes it state to "valid" and send the copy to requesting node. On write miss invalidation of all copies is additionally performed. When writing block in state valid it's state is changed into dirty and broadcast causes all cache controllers invalidate their copies.

Since snooping does not scale well, larger ccNuma systems tend to use Directory-based coherence protocols.