How cache Coherence problem effects parallel processing and what are solution to this problem, Explain in detail with the help of diagram and example. ?
Answers
Answer:
In a multiprocessor system, data inconsistency may occur among adjacent levels or within the same level of the memory hierarchy. For example, the cache and the main memory may have inconsistent copies of the same object.
As multiple processors operate in parallel, and independently multiple caches may possess different copies of the same memory block, this creates cache coherence problem. Cache coherence schemes help to avoid this problem by maintaining a uniform state for each cached block of data.
Let X be an element of shared data which has been referenced by two processors, P1 and P2. In the beginning, three copies of X are consistent. If the processor P1 writes a new data X1 into the cache, by using write-through policy, the same copy will be written immediately into the shared memory. In this case, inconsistency occurs between cache memory and the main memory. When a write-back policy is used, the main memory will be updated when the modified data in the cache is replaced or invalidated.
In general, there are three sources of inconsistency problem −
Sharing of writable data
Process migration
I/O activity
Explanation:
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Explanation:
computing, a cache (/kæʃ/ (About this soundlisten) kash,[1] or /ˈkeɪʃ/ kaysh in Australian English[2]) is a hardware or software component that stores data so that future requests for that data can be served faster; the data stored in a cache might be the result of an earlier computation or a copy of data stored elsewhere. A cache hit occurs when the requested data can be found in a cache, while a cache miss occurs when it cannot. Cache hits are served by reading data from the cache, which is faster than recomputing a result or reading from a slower data store; thus, the more requests that can be served from the cache, the faster the system performs.
To be cost-effective and to enable efficient use of data, caches must be relatively small. Nevertheless, caches have proven themselves in many areas of computing, because typical computer applications access data with a high degree of locality of reference. Such access patterns exhibit temporal locality, where data is requested that has been recently requested already, and spatial locality, where data is requested that is stored physically close to data that has already been requested.