Snooping (cache coherence)
Beginner
A way to keep copies in agreement where every cache listens to every request, like everyone hearing an announcement.
Novice
A coherence scheme in which each miss is broadcast on a shared bus or network and every cache checks (snoops) its tags. Simple and fast for a few cores; the broadcasts don’t scale to many.
Expert
Relies on an ordered broadcast medium as the serialization point. Traffic and tag-lookup energy grow with core count, so large chips use directories or snoop filters that track which caches might hold a line.
Explained in Caches and coherence (Architectures).
See also: Cache coherence, MESI protocol.