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SIMD vs MIMD
Panllel computers cm be classified into two groups by Fiynn [1966]: SiMD (Single
Instruction, Multiple Data) and MiMD (Multiple instruction, Multiple Data).
A SLMD machine consists of a number of identical processors doing the same things to
different data at any given point of time. Typical SIMD machines have large numbers of relatively
simple and affordable processors ~sulting in fine-gnined parallelism. which distributes the data
as widely as possible with each processor performing the simplest operations.
in MIMD machines, the most widely employed parallel machine architecture. each
processor enecutes a possibly different program on different data under the conuol of different
instruction asynchronously. The MIMD machines generally have fewer but mon powerful
processon than SIMD machines.
Shared Memory vs Message Passing
Another architecturai classification is whether the parallel computer is a shared memory
machine or a message passing machine according to how the processors cornrnunicate with each
other.
Shared memory computen have global memory that can be directly accessed by dl
processon. Shared memocy computen are not very scalrble. panicularly when the entire global
memory is equally accessible to dl of luge number of processors. They also impose an inherent
concem of s ynchronization. i .e. how different processors can read and write the data in the same
location of memory simultaneously without conflict. Currently, most shared memory computers
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