Taxonomy-based comparison of several distributed shared memory systems

Two possible modes of Input/Output (I/O) are 'sequential' and 'random-access', and there is an extremely strong conceptual link between I/O and communication. Sequential communication, typified in the I/O setting by magnetic tape, is typified in the communication setting by a stream, e.g., a UNIX pipe. Random-access communication, typified in the I/O setting by a drum or disk device, is typified in the communication setting by shared memory. In this paper, we study and survey the extension of the random-access model to distributed computer systems. A Distributed Shared Memory (DSM) is a memory area shared by processes running on computers connected by a network. DSM provides direct system support of the shared memory programming model. When assisted by hardware, it can also provide a low-overhead interprocess communication (IPC) mechanism to software. Shared pages are migrated on demand between the hosts. Since computer network latency is typically much larger than that of a shared bus, caching in DSM is necessary for performance. We use caching and issues such as address space structure and page replacement schemes to define a taxonomy. Based on the taxonomy we examine three DSM efforts in detail, namely: IVY, Clouds and MemNet.