The disadvantage is that snooping cannot be extended. If there is enough bandwidth, snooping based protocols tend to be faster, because all transactions are requests/responses seen by all processors. Each mechanism has its own advantages and disadvantages. The two most common mechanisms for ensuring consistency are snooping and directory-based. Related post: An Introduction to Hard Drive Cache: Definition and Importance Cache Coherence Mechanisms However, they are insufficient because they do not meet the Transaction Serialization conditions. The above conditions meet the Write Propagation criteria required for cache coherence. If processor P1 reads the old value of X, even after P2 is written, it can be said that the memory is incoherent. Propagating writes operations to shared memory locations ensures that all caches have a coherent view of memory. This condition defines the concept of a coherent view of memory. Therefore, X must always return the value written by P2. After processor P1 reads location X and another processor P2 writes X, there are no other writes to processor X between the two accesses, and the read and write must be sufficiently separated.When the processor P reads the location X after the write of X by the same processor P, when the write of X by another processor does not occur between the write and the read instruction by P, X must always return the value written by P.To achieve cache coherence, the following conditions must be met: In a multi-processor system, consider that more than one processor has cached a copy of memory location X. A kind of data that appears in different caches at the same time is called cache coherence, which is called global memory in some systems. DefinitionĬoherence defines the behavior of reads and writes to a single address location. However, in practice, it is usually executed at the granularity of cache blocks. In theory, cache coherence can be enforced at load/store granularity.
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