Lightweight hardware distributed shared memory supported by generalized combining

On a large scale parallel computer system, shared memory provides a general and convenient programming environment. The paper describes a lightweight method for constructing an efficient shared memory system supported by hierarchical coherence management and generalized combining. The hierarchical management technique and generalized combining cooperate with each other. We eliminate the following heavyweight and high cost factors: a large amount of directory memory which is proportional to the number of processors, a separate memory component for the directory, tag/state information, and a protocol processor. In our method, the amount of memory required for the directory is proportional to the logarithm of the number of processors. This implies that a single word for each memory block is sufficient for covering a massively parallel system and that the access costs of the directory are small. Moreover, our combining technique, generalized combining, does not expect the accidental events which existing combining networks do, that is, events that messages meet each other at a switching node. A switching node can combine succeeding messages with a preceding one even after the preceding message leaves the node. This can increase the rate of successful combining. We have developed a prototype parallel computer OCHANOMIZ-5, that implements this lightweight distributed shared memory and generalized combining with simple hardware. The results of evaluating the prototype's performance using several programs show that our methodology provides the advantages of parallelization.