A Knowledge Based Analysis of Cache Coherence

This paper presents a case study of the application of the knowledge-based approach to concurrent systems specification, design and verification. A highly abstract solution to the cache coherence problem is first presented, in the form of a knowledge-based program, that formalises the intuitions underlying the MOESI [Sweazey & Smith, 1986] characterisation of cache coherency protocols. It is shown that any concrete implementation of this knowledge-based program, which relates a cache’s actions to its knowledge about the status of other caches, is a correct solution of the cache coherence problem. Three existing protocols in the MOESI class are shown to be such implementations. The knowledgebased characterisation furthermore raises the question of whether these protocols are optimal in their use of information available to the caches. This question is investigated using by the model checker MCK, which is able to verify specifications in the logic of knowledge and time.