A methodological construction of an efficient sequential consistency protocol

Abstract

A concurrent object is an object that can be concurrently accessed by several processes. Sequential consistency is a consistency criterion for such objects. Informally, it states that a multiprocess program executes correctly if its results could have been produced by executing that program on a single processor system. (Sequential consistency is weaker than atomic consistency -the usual consistency criterion- as it does not refer to real-time.) The paper proposes a simple protocol that ensures sequential consistency when the shared memory abstraction is supported by the local memories of nodes that can communicate only by exchanging messages through reliable channels. Differently from other sequential consistency protocols, the proposed protocol does not rely on a strong synchronization mechanism such as an atomic broadcast primitive or a central node managing a copy of every shared object. From a methodological point of view, the protocol is built incrementally starting from the very definition of sequential consistency. It lies the noteworthy property of providing fast writes operations (i.e., a process has never to wait when it writes a new value in a shared object). According to the current local state, some read operations can also be fast. An experimental evaluation of the protocol is also presented. The proposed protocol could be used to manage Web page caching.