Editing Cache coherence (section)

{{Short description|Equivalence of all cached copies of a memory location}}
[[File:Cache coherence.svg|thumb|upright=1.7|If two clients have a cached copy of a particular memory block and one client changes the block, the other client's copy must be invalidated or updated. If it is not, the system is in an incoherent state: it contains two different records of the same memory block which both claim to be up-to-date.]]
[[File:Non Coherent.gif|thumb|Incoherent caches: The caches have different values of a single address location.]]
In [[computer architecture]], '''cache coherence''' is the uniformity of shared resource data that is stored in multiple [[Cache (computing)|local caches]]. In a cache coherent system, if multiple clients have a cached copy of the same region of a shared memory resource, all copies are the same. Without cache coherence, a change made to the region by one client may not be seen by others, and errors can result when the data used by different clients is mismatched.<ref name="advances_in_computers">{{Cite book |last=Marowka |first=Ami |url=https://linkinghub.elsevier.com/retrieve/pii/S0065245810790021 |title=Advances in Computers |chapter=Chapter 2 - Pitfalls and Issues of Manycore Programming |date=2010-01-01 |publisher=Elsevier |volume=79 |pages=71–117 |doi=10.1016/s0065-2458(10)79002-1|isbn=978-0-12-381027-4 }}</ref>

A [[Cache coherency protocols (examples)|cache coherence protocol]] is used to maintain cache coherency. The two main types are [[Bus snooping|snooping]] and [[Directory-based cache coherence|directory-based]] protocols.

Cache coherence is of particular relevance in [[multiprocessing]] systems, where each [[Central processing unit|CPU]] may have its own local cache of a shared memory resource.

[[File:Coherent.gif|thumb|Coherent caches: The value in all the caches' copies is the same.]]