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In computer science, cache coherence is the consistency of shared resource data that ends up stored in multiple local caches. When clients in a system maintain caches of a common memory resource, problems may arise with inconsistent data, which is particularly the case with CPUs in a multiprocessing system. In the illustration on the right, if the client on the top has a copy of a memory block from a previous read and the client on the bottom changes that memory block, the client on the top could be left with an invalid cache of memory without any notification of the change. Cache coherence is intended to manage such conflicts and maintain consistency between the cache and memory. ==Overview== In a shared memory multiprocessor system with a separate cache memory for each processor, it is possible to have many copies of any one instruction operand: one copy in the main memory and one in each cache memory. When one copy of an operand is changed, the other copies of the operand must be changed also. Cache coherence is the discipline that ensures that changes in the values of shared operands are propagated throughout the system in a timely fashion. There are three distinct levels of cache coherence:〔 〕 # every write operation appears to occur instantaneously # all processors see exactly the same sequence of changes of values for each separate operand # different processors may see an operation and assume different sequences of values; this is considered to be a noncoherent behavior. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cache coherence」の詳細全文を読む スポンサード リンク
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