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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.
In computing, the MSI protocol - a basic cache-coherence protocol - operates in multiprocessor systems. As with other cache coherency protocols, the letters of the protocol name identify the possible states in which a cache line can be.
The traffic can be reduced by using a cache that acts as a "filter" versus the shared memory, that is the cache is an essential element for shared-memory in SMP systems. In multiprocessor systems with separate caches that share a common memory, a same datum can be stored in more than one cache.
Alternatively, cache coherency protocols such as the MESIF protocol attempt to reduce the communication required to maintain cache coherency. Scalable Coherent Interface (SCI) is an IEEE standard defining a directory-based cache coherency protocol to avoid scalability limitations found in earlier multiprocessor systems. For example, SCI is used ...
Directory-based coherence is a mechanism to handle cache coherence problem in distributed shared memory (DSM) a.k.a. non-uniform memory access (NUMA). Another popular way is to use a special type of computer bus between all the nodes as a "shared bus" (a.k.a. system bus). [1]
Memory coherence is an issue that affects the design of computer systems in which two or more processors or cores share a common area of memory. [1] [2] [3] [4]In a uniprocessor system (where there exists only one core), there is only one processing element doing all the work and therefore only one processing element that can read or write from/to a given memory location.
The Dragon Protocol [1] is an update based cache coherence protocol used in multi-processor systems. Write propagation is performed by directly updating all the cached values across multiple processors. Update based protocols such as the Dragon protocol perform efficiently when a write to a cache block is followed by several reads made by other ...
A cache line in the O state is dirty and must be written back to memory before being discarded. The F state in the MESIF protocol is simply a way to choose one of the sharers of a clean cache line to respond to a read request for data using a direct cache-to-cache transfer instead of waiting for the data to come from the main memory.