Practice question · Multiple choice
A program spends 95% of its time in one function. You optimise everything else until it is instant. How much faster is the program?
Hints
- Take the original runtime as 100 units and write down what remains after the change.
- The part you did not touch sets a floor no amount of work elsewhere can go below.
Show the answer
A. About 5% faster: the untouched 95% still has to run
Why
100 units become 95, so the gain is about 5%. Amdahl’s law: the untouched fraction bounds the total speed-up however perfectly the rest is optimised. This is the argument for profiling before optimising: effort spent outside the hot path is capped before it starts, and intuitions about where time goes are usually wrong.
Practise Integrated computational problem solving
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