A distributed implementation of shared uirtual memory with strong and weak coherence

Abstract

A virtual shared memory architecture (VSMA) is a distributed memory architecture that looks to the application software as if it were a shared memory system. The major problem with such a system is to maintain the coherence of the distributed data entities. Shared virtual memory means that the shared data entities are pages of local virtual memories with demand paging. Memory coherence may be strong or weak. Strong coherence is a scheme where all the shared data entities look from the outside as if they were stored in one coherent memory. This simplifies programming of a distributed memory system at the cost of a high message traffic in the system, needed to maintain the strong coherence. The efficiency of the system can be increased by adding a weak coherence scheme that allows for multiple writes by different threads of control into the same page. The price of the weak coherence scheme is the need for explicit program synchronizations, needed to reestablish at the end the strong coherence of the result. For the computer architect, the challenging question is how to implement a VSMA most efficiently and, specifically, by what architectural means to support the implementation. In the paper a new solution to this question is presented based upon an innovative distributed memory architecture in which communication is conducted by a dedicated communication processor in each node rather than by the node CPU. This will make the exchange of short, fixed-size messages, e.g., invalidation notices, very efficient. Therefore, it becomes more appropriate to minimize the overall administrative overhead, even at the cost of more message traffic. On that rationale, a novel, capability-based mechanism for both strong and weak coherence of shared virtual memory is presented. The weak coherence scheme is built on top of the strong coherence, utilizing its mechanisms. The proposed implementation is totally distributed and based on a strict need to know philosophy. Consequently, the elaborate pointer lists and their handling at runtime typical for other solutions is not needed.