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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.
– 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
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.
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 ]
The cache that accesses a memory block first is the root node. Each memory block has the root node information (HEAD) and Sharing counter field (SC). The SC field has the number of caches that share the block. Each cache entry has pointers to the next sharing caches known as L-CHD and R-CHD. A condition for this directory is that the binary ...
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 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]
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.