Cache coherence

In computer architecture, cache coherence is the uniformity 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 incoherent data, which is particularly the case with CPUs in a multiprocessing system. In the illustration on the right, consider both the clients have a cached copy of a particular memory block from a previous read. Suppose the client on the bottom updates/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 by maintaining a coherent view of the data values in multiple caches. 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. When one of the copies of data is changed, the other copies must reflect that change. 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. 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. Transaction Serialization Reads/Writes to a single memory location must be seen by all processors in the same order. Theoretically, coherence can be performed at the load/store granularity. However, in practice it is generally performed at the granularity of cache blocks. Coherence defines the behavior of reads and writes to a single address location. One type of data occurring simultaneously in different cache memory is called cache coherence, or in some systems, global memory. In a multiprocessor system, consider that more than one processor has cached a copy of the memory location X.