Amdahl's law

http://en.wikipedia.org/wiki/Amdahl%27s_law

It is used to find the maximum expected improvement to an overall system when only part of the system is improved.

It is often used in parallel computing to predict the theoretical maximum speedup using multiple processors.

文中的那个图实在是很棒，下面的说明必须得看。

The speedup of a program using multiple processors in parallel computing is limited by the sequential fraction of the program. For example, if 95% of the program can be parallelized, the theoretical maximum speedup using parallel computing would be 20× as shown in the diagram, no matter how many processors are used.