A proposal for a DSM architecture suitable for a widely distributed environment and its evaluation

To realize functionally distributed computing in a wide area distributed environment, distributed shared memory (DSM) is an attractive option due to the simplicity and flexibility in software programming. DSM has mainly been studied in a local environment. In a widely distributed environment, the latency of communication impacts system performance, even if a high bandwidth network is used. DSM models in a widely distributed environment are discussed and evaluated in this paper. First, two existing DSM models are examined: shared virtual memory and replicated shared memory. Next, an innovative replicated shared memory model, which uses internal machine memory, is proposed. A prototype of this model using multi-thread programming was implemented on multi-CPU SPARCstations. These DSM models are compared with SCRAMNet, whose mechanism is based on replicated shared memory. Results from this evaluation show the superiority of the replicated shared memory compared to shared virtual memory when the length of the network is large. While replicated shared memory using external memory is influenced by the ratio of local and global accesses, replicated shared memory using internal machine memory is suitable for a wide variety of cases. The replicated shared memory model is considered to be suitable particularly for applications which impose real time operation in a widely distributed environment, since some latency hiding techniques such as context switching or data prefetching are not effective for real time demands.