April 2016
Intermediate to advanced
170 pages
3h 48m
English
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Amdahl's law
The last important concept of this chapter is a behavior known as Amdahl's law. In simple terms, Amdahl's law states that we can parallelize/distribute our computations as much as we want, gaining in performance as we add compute resources. However, our code cannot be faster than the speed of its combined sequential (that is, non parallelizable) parts on a single processor.
Put more formally, Amdahl's law has the following formulation. Given an algorithm that is partially parallel, let's call P its parallel fraction and S its serial (that is, non parallel) fraction (clearly, S+P=100%). Furthermore, let's call T(n) the runtime (in seconds) of the algorithm when using n processors. Then, the following relation holds:
The preceding relation, ...