Distributed Shared Memory

Distributed shared memory in distributed systems

Definition Of distributed shared memory

A form of memory architecture where the physically separated memory can be addressed as one logically shared space is called distributed shared memory. The address space is shared same physical address on two processors refers to the same location in memory.

A distributed shared memory is a mechanism allowing end-users' processes to access shared data without using inter-process communications. In other words, the goal of a DSM system is to make inter-process communications transparent to end-users. Both hardware and software implementations have been proposed in the literature. From a programming point of view, two approaches have been studied:

* Shared virtual memory: This notion is very similar to the well-known concept of paged virtual memory implemented in mono-processor systems. The basic idea is to group all distributed memories together into a single wide address space. Drawbacks: such systems do not allow to take into account the semantics of shared data: the data granularity is arbitrarily fixed to some page size whatever the type and the actual size of the shared data might be. The programmer has no means to provide informations about these data.

* Object DSM: in that class of approaches, shared data are objects i.e. variables with access functions. In his applications, the user has only to define which data (objects) are shared. The whole management of the shared objects (creation, access, modification) is handled by the DSM system. In opposite of SVM systems which work at operating system layer, objects DSM systems actually propose a programming model alternative to the classical message-passing.

Traditionally, distributed computing is based on message passing. DSM provides the virtual address space that is shared among all nodes in a distributed system. With DSM, programs access data in shared address space just as they access data in the...