Parallel slowdown is a phenomenon in
parallel computing
Parallel computing is a type of computing, computation in which many calculations or Process (computing), processes are carried out simultaneously. Large problems can often be divided into smaller ones, which can then be solved at the same time. ...
where parallelization of a
parallel algorithm
In computer science, a parallel algorithm, as opposed to a traditional serial algorithm, is an algorithm which can do multiple operations in a given time. It has been a tradition of computer science to describe serial algorithms in abstract mach ...
beyond a certain point causes the program to run slower (take more time to run to completion).
Parallel slowdown is typically the result of a communications bottleneck. As more
processor nodes are added, each processing node spends progressively more time doing communication than useful processing. At some point, the communications overhead created by adding another processing node surpasses the increased processing power that node provides, and parallel slowdown occurs.
Parallel slowdown occurs when the algorithm requires significant communication, particularly of intermediate results. Some problems, known as
embarrassingly parallel problems, do not require such communication, and thus are not affected by slowdown.
References
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See also
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Mythical man month, an analogous situation for a team programmers where productivity is affected by human communication.
Analysis of parallel algorithms