[scott_s]

Amdahl's law is not about overhead - it is true that parallelizing a computation usually has some cost, and you have to make sure what you're parallelizing is a heavy-enough computation that you still win in the presence of that cost. But that's not what Amdahl's law is about.

It is about the limitation of overall performance improvement when improving the performance of a component. If, say, you improve the performance of a piece of your application that only accounts for 10% of the running time, then you're limited by a 10% total performance improvement. This matters in a parallel context because if you have heavily parallelized your application, and it has actually improved performance, you're likely to eventually be limited by the performance of the sequential parts of your application. See https://en.wikipedia.org/wiki/Amdahl%27s_law ; although the last line of the introduction reads like nonsense.