The following conditions are necessary to achieve cache coherence: In a multiprocessor system, consider that more than one processor has cached a copy of the memory location X. One type of data occurring simultaneously in different cache memory is called cache coherence, or in some systems, global memory. Definition Ĭoherence defines the behavior of reads and writes to a single address location. However, in practice it is generally performed at the granularity of cache blocks. Theoretically, coherence can be performed at the load/store granularity. Transaction Serialization Reads/Writes to a single memory location must be seen by all processors in the same order. The following are the requirements for cache coherence: Write Propagation Changes to the data in any cache must be propagated to other copies (of that cache line) in the peer caches. Cache coherence is the discipline which ensures that the changes in the values of shared operands (data) are propagated throughout the system in a timely fashion. When one of the copies of data is changed, the other copies must reflect that change. In a shared memory multiprocessor system with a separate cache memory for each processor, it is possible to have many copies of shared data: one copy in the main memory and one in the local cache of each processor that requested it. Coherent caches: The value in all the caches' copies is the same.
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