Representation of Coherency Classes for Parallel Systems

Some parallel applications do not require a precise imitation of the behaviour of the physically shared memory programming model. Consequently, certain parallel machine architectures have elected to emphasise different required coherency properties because of possible efficiency gains. This has led to various definitions of models of store coherency. These definitions have not been amenable t o detailed analysis and, consequently, inconsistencies have resulted. In this paper a unified framework is proposed in which different models of store coherency are developed systematically by progressively relaxing the constraints that they have t o satisfy. A demonstration is given of how formal reasoning can be cam’ed out to compare different models. Some real-life systems are considered and a definition of a version of weak coherency is found to be incomplete.