Cache Coherence

The majority of coherency protocols that support multiprocessors use a sequential consistency standard. DSM systems use a weak or release consistency standard. The following methods are used for cache coherence management and consistency in read/write (R/W) and instantaneous operations: Written data locations are sequenced. Write operations occur instantaneously. Program order preservation is maintained with RW data. A coherent memory view is maintained, where consistent values are provided through shared memory. Several types of cache coherency may be utilized by different structures, as follows: Directory based coherence: References a filter in which memory data is accessible to all processors. When memory area data changes, the cache is updated or invalidated. Bus snooping: Monitors and manages all cache memory and notifies the processor when there is a write operation. Used in smaller systems with fewer processors. Snarfing: Self-monitors and updates its address and data versions. Requires large amounts of bandwidth and resources compared to directory based coherence and bus snooping.