Editing Bus snooping (section)

== Snoop filter ==
When a bus transaction occurs to a specific cache block, all snoopers must snoop the bus transaction. Then the snoopers look up their corresponding [[Cache (computing)|cache tag]] to check whether it has the same cache block. In most cases, the caches do not have the cache block since a well optimized parallel program doesn’t share much data among threads. Thus the cache tag lookup by the snooper is usually an unnecessary work for the cache who does not have the cache block. But the tag lookup disturbs the cache access by a processor and incurs additional power consumption.

One way to reduce the unnecessary snooping is to use a snoop filter. A snoop filter determines whether a snooper needs to check its cache tag or not. A snoop filter is a directory-based structure and monitors all coherent traffic in order to keep track of the coherency states of cache blocks. It means that the snoop filter knows the caches that have a copy of a cache block. Thus it can prevent the caches that do not have the copy of a cache block from making the unnecessary snooping. There are three types of filters depending on the location of the snoop filters. One is a source filter that is located at a cache side and performs filtering before coherence traffic reaches the shared bus. Another is a destination filter that is located at receiver caches and prevents unnecessary cache-tag look-ups at the receiver core, but this type of filtering fails to prevent the initial coherence message from the source. Lastly, in-network filters prune coherence traffic dynamically inside the shared bus.<ref>{{Cite book|last1=Agarwal|first1=N.|last2=Peh|first2=L.|last3=Jha|first3=N. K.|title=Proceedings of the 42nd Annual IEEE/ACM International Symposium on Microarchitecture |chapter=In-network coherence filtering |date=December 2009|chapter-url=https://ieeexplore.ieee.org/document/5375372|pages=232–243|doi=10.1145/1669112.1669143|hdl=1721.1/58870|isbn=9781605587981 |s2cid=6626465 |hdl-access=free}}</ref> The snoop filter is also categorized as inclusive and exclusive. The inclusive snoop filter keeps track of the presence of cache blocks in caches. However, the exclusive snoop filter monitors the absence of cache blocks in caches. In other words, a hit in the inclusive snoop filter means that the corresponding cache block is held by caches. On the other hand, a hit in the exclusive snoop filter means that no cache has the requested cache block.<ref>{{Cite book|title=Design of a Snoop Filter for Snoop-Based Cache Coherency Protocols|last=Ulfsnes|first=Rasmus|date=June 2013|location=Norwegian University of Science and Technology}}</ref>