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MESIF protocol
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<p>The MESIF protocol is a <a href="/facts/Cache_coherency/MhUD576q">cache coherency</a> and <a href="/facts/Memory_coherence/eJD9GntF">memory coherence</a> protocol developed by <a href="/facts/Intel/SMF0gJJX">Intel</a> for <a href="/facts/Non-Uniform_Memory_Access/MpmN5mN0">cache coherent non-uniform memory architectures</a>. The protocol consists of five states, Modified (M), Exclusive (E), Shared (S), Invalid (I) and Forward (F). </p><p>The M, E, S and I states are the same as in the <a href="/facts/MESI_protocol/BUCBWvtS">MESI protocol</a>. The F state is a specialized form of the S state, and indicates that a cache should act as a designated responder for any requests for the given line. The protocol ensures that, if any cache holds a line in the S state, at most one (other) cache holds it in the F state. </p><p>In a system of caches employing the MESI protocol, a cache line request that is received by multiple caches holding a line in the S state will be serviced inefficiently. It may either be satisfied from (slow) main memory, or <i>all</i> the sharing caches could respond, bombarding the requestor with redundant responses. In a system of caches employing the MESIF protocol, a cache line request will be responded to only by the cache holding the line in the F state. This allows the requestor to receive a copy at cache-to-cache speeds, while allowing the use of as few multicast packets as the network topology will allow. </p> <table><tbody><tr><th></th><th> M </th><th> E </th><th> S </th><th> I </th><th> F </th></tr><tr><th> M </th><td>N</td><td>N</td><td>N</td><td>Y</td><td>N</td></tr><tr><th> E </th><td>N</td><td>N</td><td>N</td><td>Y</td><td>N</td></tr><tr><th> S </th><td>N</td><td>N</td><td>Y</td><td>Y</td><td>Y</td></tr><tr><th> I </th><td>Y</td><td>Y</td><td>Y</td><td>Y</td><td>Y</td></tr><tr><th> F </th><td>N</td><td>N</td><td>Y</td><td>Y</td><td>N</td></tr></tbody></table> <p>Because a cache may unilaterally discard (invalidate) a line in the S or F states, it is possible that no cache has a copy in the F state, even though copies in the S state exist. In this case, a request for the line is satisfied (less efficiently, but still correctly) from main memory. To minimize the chance of the F line being discarded due to lack of interest, the most recent requestor of a line is assigned the F state; when a cache in the F state responds, it gives up the F state to the new cache. </p><p>Thus, the main difference from the MESI protocol is that a request for a copy of the cache line for read always enters the cache in the F state. The only way to enter the S state is to satisfy a read request from another cache, transferring the F state to it. </p><p>For any given pair of caches, the permitted states of a given cache line are listed in the table on the right. The order in which the states are listed has no significance other than to make the acronym MESIF pronounceable. </p><p>There are other techniques for satisfying read requests from shared caches while suppressing redundant replies, but having only a single designated cache respond makes it easier to invalidate all copies when necessary to transition to the Exclusive state. </p>