Gustafson's Law Calculator (Scaled Speedup)

What this tool does

Amdahl's law is gloomy: with 5 % of the work serial, no number of cores can make a job more than 20 times faster. John Gustafson pointed out in 1988 that people rarely run the same small job on a bigger machine; they run a bigger job in the same time, and then the serial part shrinks in relative terms. With 64 cores and 5 % serial time, the scaled speedup is about 61 instead of Amdahl's 15. That is weak scaling, the model behind supercomputer benchmarks; Amdahl describes strong scaling, which is what you get when you only want the same answer faster. Here the serial share is measured on the parallel machine, as Gustafson defined it.

Formula

S = N − s (N − 1) · Amdahl: S = 1 ÷ (s + (1 − s) ÷ N)

Variables

Worked example

Limitations

• The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
• The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.