Distributed memory

In a distributed memory system there is typically a processor, a memory, and some form of interconnection that allows programs on each processor to interact with each other. The interconnect can be organised with point to point links or separate hardware can provide a switching network. The network topology is a key factor in determining how the multiprocessor machine scales. The links between nodes can be implemented using some standard network protocol (for example Ethernet), using bespoke network links (used in for example the transputer), or using dual-ported memories.

Programming distributed memory machines

The key issue in programming distributed memory systems is how to distribute the data over the memories. Depending on the problem solved, the data can be distributed statically, or it can be moved through the nodes. Data can be moved on demand, or data can be pushed to the new nodes in advance.

As an example, if a problem can be described as a pipeline where data x is processed subsequently through functions f, g, h, etc. (the result is h(g(f(x)))), then this can be expressed as a distributed memory problem where the data is transmitted first to the node that performs f that passes the result onto the second node that computes g, and finally to the third node that computes h. This is also known as systolic computation.

Data can be kept statically in nodes if most computations happen locally, and only changes on edges have to be reported to other nodes. An example of this is simulation where data is modeled using a grid, and each node simulates a small part of the larger grid. On every iteration, nodes inform all neighboring nodes of the new edge data.