Software DSM

Software DSM is a runtime system that provides the shared address space abstraction across a message-passing based cluster of computers. The basic idea suggested by Kai Li [21], is to use the virtual memory page protection mechanism to implement an invalidation-based coherence protocol similar to directory-based cache coherence, but at page granularity and completely in software. Since the unit of coherence is a virtual memory page, false sharing occurs when multiple unrelated shared objects lie on the same page. To alleviate the message traffic that would be generated in the presence of false sharing, several relaxed consistency models have been proposed  [16,4,19,6,18]. These consistency models define a memory model for programmers in which they agree to exclusively use explicit synchronization. Under this assumption, the coherence protocol can delay the invalidation messages until a synchronization operation is performed, thus reducing both the protocol messages as well as the extra communication that an early invalidation would have unnecessarily caused.