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→Optimisation: advice on optimisation
==Optimisation==
==Optimisation==
In a long-running calculation, the choice of a good algorithm is essential. Changing to a better algorithm can often provide much better performance gains than is possible with compiler optimisation.
Programs often obey the 80-20 rule, where 20% of the code accounts for 80% of the CPU usage. Programmers usually do a poor job of guessing as to which 20% of the code is causing performance problems, so before doing any hand optimisation it is crucial to (use a tool to) measure the program performance and identify the hot spots.
Many compilers have one or more optional optimisation levels. As the level/amount of optimisation increases the compiler tends to spend more and more time doing the optimisation. The word 'optimisation' is perhaps misleading - perfect optimisation is a very hard problem ([[NP-complete]] or even [[undecidable]]), so a conceptually better term would be 'improvement'.
Many compilers have one or more optional optimisation levels. As the level/amount of optimisation increases the compiler tends to spend more and more time doing the optimisation. The word 'optimisation' is perhaps misleading - perfect optimisation is a very hard problem ([[NP-complete]] or even [[undecidable]]), so a conceptually better term would be 'improvement'.