Message-Driven Relaxed Consistency in a Software Distributed Shared Memory

Message-passing and distributed shared memory have their respective advantages and disadvantages in distributed parallel programming. We approach the problem of integrating both mechanisms into a single system by proposing a new message-driven coherency mechanism. Messages carrying explicit causality annotations are exchanged to trigger memory coherency actions. By adding annotations to standard message-based protocols, it is easy to construct efficient implementations of common synchronization and communication mechanisms. Because these are user-level messages, the set of available primitives is extended easily with language- or application-specific mechanisms. CarlOS, an experimental prototype for evaluating this approach, is derived from the lazy release consistent memory of TreadMarks. We describe the message-driven coherency memory model used in CarlOS, and we examine the performance of several applications.