A Study on Memory-Based Communications and Synchronization in Distributed Systems

In terms of facilities for communications and synchronization in parallel programs, the descriptive power of the shared-memory model is equal to that of the message-passing-style (send-and-receive-type) model. From the viewpoint of performance, however,the situation is different. In modern architectures for distributedmemory multicomputers, memory is an important and fundamental building-block of the nodes. It is thus directly accessed by processors that use their memory management units for protection and virtualization. Therefore, a very convenient way of improving performance or increasing the variety of functions is to have the communication/synchronization subsystems handle information related to the memory locations at which target data are stored. In more concrete terms, if communication/synchronization subsystems do so, they can use the information to reduce the number of copies of data. Moreover, these subsystems have been made capable of directly accessing data in user tasks, it becomes possible to implement advanced synchronization functions (e.g., atomic operations, queue operations and so forth), as well as simple read/write operations, within the subsystems. In short, communication/synchronization subsystems based on the shared-memory model are superior in this way to message-passing-style subsystems which simply relay data from one task to another. This conclusion holds whether the subsystem is implemented in hardware or software