(Or "multiprocessing") The simultaneous use of more than one computer to solve a problem. There are many different kinds of parallel computer (or "parallel processor"). They are distinguished by the kind of interconnection between processors (known as "processing elements" or PEs) and between processors and memory. Flynn's taxonomy
also classifies parallel (and serial) computers according to whether all processors execute the same instructions at the same time ("single instruction/multiple data
" - SIMD) or each processor executes different instructions ("multiple instruction/multiple data
" - MIMD).
The processors may either communicate in order to be able to cooperate in solving a problem or they may run completely independently, possibly under the control of another processor which distributes work to the others and collects results from them (a "processor farm
"). The difficulty of cooperative problem solving is aptly demonstrated by the following dubious reasoning:
If it takes one man one minute to dig a post-hole then sixty men can dig it in one second. Amdahl's Law
states this more formally.
Processors communicate via some kind of network or bus or a combination of both. Memory may be either shared memory
(all processors have equal access to all memory) or private (each processor has its own memory - "distributed memory
") or a combination of both.
Many different software systems have been designed for programming parallel computers, both at the operating system
and programming language level. These systems must provide mechanisms for partitioning the overall problem into separate tasks and allocating tasks to processors. Such mechanisms may provide either implicit parallelism
- the system (the compiler
or some other program) partitions the problem and allocates tasks to processors automatically or explicit parallelism
where the programmer must annotate his program to show how it is to be partitioned. It is also usual to provide synchronisation primitives such as semaphores
to allow processes to share resources without conflict. Load balancing
attempts to keep all processors busy by allocating new tasks, or by moving existing tasks between processors, according to some algorithm
Communication between tasks may be either via shared memory
or message passing
. Either may be implemented in terms of the other and in fact, at the lowest level, shared memory uses message passing since the address and data signals which flow between processor and memory may be considered as messages.
The terms "parallel processing" and "multiprocessing" imply multiple processors working on one task whereas "concurrent processing
" and "multitasking
" imply a single processor sharing its time between several tasks.
See also cellular automaton
Institutions (http://ccsf.caltech.edu/other_sites.html), research groups (http://cs.cmu.edu/~scandal/research-groups.html).