In
computer science
Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...
, distributed memory refers to a
multiprocessor computer system in which each
processor has its own private
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
.
Computational tasks can only operate on local data, and if remote data are required, the computational task must communicate with one or more remote processors. In contrast, a
shared memory multiprocessor offers a single memory space used by all processors. Processors do not have to be aware where data resides, except that there may be performance penalties, and that race conditions are to be avoided.
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
Network topology is the arrangement of the elements (Data link, links, Node (networking), nodes, etc.) of a communication network. Network topology can be used to define or describe the arrangement of various types of telecommunication networks, ...
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
Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
), using bespoke network links (used in for example the
transputer
The transputer is a series of pioneering microprocessors from the 1980s, intended for parallel computing. To support this, each transputer had its own integrated memory and serial communication links to exchange data with other transputers. ...
), 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.
Distributed shared memory
Similarly, in
distributed shared memory each node of a cluster has access to a large shared memory in addition to each node's limited non-shared private memory.
Shared memory vs. distributed memory vs. distributed shared memory
* The advantage of (distributed) shared memory is that it offers a unified address space in which all data can be found.
* The advantage of distributed memory is that it excludes race conditions, and that it forces the programmer to think about data distribution.
* The advantage of distributed (shared) memory is that it is easier to design a machine that scales with the algorithm
Distributed shared memory hides the mechanism of communication, it does not hide the latency of communication.
See also
*
Memory virtualization
*
Distributed cache
References
{{DEFAULTSORT:Distributed Memory
Parallel computing
Distributed computing architecture