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A cache coherence protocol is used to maintain cache coherency. The two main types are snooping and directory-based protocols. Cache coherence is of particular relevance in multiprocessing systems, where each CPU may have its own local cache of a shared memory resource. Coherent caches: The value in all the caches' copies is the same.
Bus snooping or bus sniffing is a scheme by which a coherency controller (snooper) in a cache (a snoopy cache) monitors or snoops the bus transactions, and its goal is to maintain a cache coherency in distributed shared memory systems. This scheme was introduced by Ravishankar and Goodman in 1983, under the name "write-once" cache coherency. [1]
– The cache is set M (D) if the "shared line" is off, otherwise is set O (SD). All the other copies are set S (V) Cache in E (R) or M (D) state (exclusiveness) – The write can take place locally without any other action. The state is set (or remains) M (D) Write Miss – Write Allocate – Read with Intent to Modified operation
The Scalable Coherent Interface or Scalable Coherent Interconnect (SCI), is a high-speed interconnect standard for shared memory multiprocessing and message passing.The goal was to scale well, provide system-wide memory coherence and a simple interface; i.e. a standard to replace existing buses in multiprocessor systems with one with no inherent scalability and performance limitations.
In MSI, each block contained inside a cache can have one of three possible states: Modified: The block has been modified in the cache. The data in the cache is then inconsistent with the backing store (e.g. memory). A cache with a block in the "M" state has the responsibility to write the block to the backing store when it is evicted.
The MESIF protocol is a cache coherency and memory coherence protocol developed by Intel for cache coherent non-uniform memory architectures. [1] The protocol consists of five states, Modified (M), Exclusive (E), Shared (S), Invalid (I) and Forward (F).
The MESI protocol is an invalidate-based cache coherence protocol, and is one of the most common protocols that support write-back caches.It is also known as the Illinois protocol due to its development at the University of Illinois at Urbana-Champaign. [1]
This cache does not have permission to modify the copy. Unlike the MESI protocol, a shared cache line may be dirty with respect to memory; if it is, one cache has a copy in the Owned state, and that cache is responsible for eventually updating main memory. If no cache holds the line in the Owned state, the memory copy is up to date.