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Cache coherence is a key problem in building shared memory multiprocessors. It was traditionally implemented in hardware via coherence protocols. However, the advent of systems like Cedar allowed one to consider a compiler-assisted implementation of cache coherence for parallel programs, [81] with minimal and completely local hardware support ...
A cache coherence protocol is used to maintain cache coherency. The two main types are snooping and directory-based protocols. Cache coherence is of particular relevance in multiprocessing systems, where each CPU may have its own local cache of a shared memory resource. Coherent caches: The value in all the caches' copies is the same.
The traffic can be reduced by using a cache that acts as a "filter" versus the shared memory, that is the cache is an essential element for shared-memory in SMP systems. In multiprocessor systems with separate caches that share a common memory, a same datum can be stored in more than one cache.
A cache line in the O state is dirty and must be written back to memory before being discarded. The F state in the MESIF protocol is simply a way to choose one of the sharers of a clean cache line to respond to a read request for data using a direct cache-to-cache transfer instead of waiting for the data to come from the main memory.
Memory coherence is an issue that affects the design of computer systems in which two or more processors or cores share a common area of memory. [1] [2] [3] [4]In a uniprocessor system (where there exists only one core), there is only one processing element doing all the work and therefore only one processing element that can read or write from/to a given memory location.
In MSI, each block contained inside a cache can have one of three possible states: Modified: The block has been modified in the cache. The data in the cache is then inconsistent with the backing store (e.g. memory). A cache with a block in the "M" state has the responsibility to write the block to the backing store when it is evicted.
Alternatively, cache coherency protocols such as the MESIF protocol attempt to reduce the communication required to maintain cache coherency. Scalable Coherent Interface (SCI) is an IEEE standard defining a directory-based cache coherency protocol to avoid scalability limitations found in earlier multiprocessor systems. For example, SCI is used ...
The Scalable Coherent Interface or Scalable Coherent Interconnect (SCI), is a high-speed interconnect standard for shared memory multiprocessing and message passing.The goal was to scale well, provide system-wide memory coherence and a simple interface; i.e. a standard to replace existing buses in multiprocessor systems with one with no inherent scalability and performance limitations.