(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 and monitors
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,symmetric multi-processing.
Usenet newsgroup: news:comp.parallel.
Institutions,
research groups.