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Computing (FOLDOC) dictionary (also found in English - English (Wordnet), )
parallel processing
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parallel 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 ("singleinstruction/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
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.
A huge number of 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 explicitparallelism 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 and monitors
to allow processes to share resources without conflict.
Load balancing attempts to keep all processors busy by
moving tasks from heavily loaded processors to less loaded