> Measure the performance of the target code in a specific situation A difficult part of optimization is actually trying to make the code work well in multiple

> Measure the performance of the target code in a specific situation A difficult part of optimization is actually trying to make the code work well in multiple specific situations. This often happens in library code where different users call your code with very different sizes of inputs. Sometimes a dumb algorithm works better. Sometimes a fancier algorithm with better big-O but bigger constant factors works better. In practice people try to measure them according to the input size and dynamically choose the algorithm based on the size. This has the pitfall of the heuristic not keeping up with hardware. It also becomes intractable if the performance characteristics depend on multiple factors, then it's trying to encode the minimum in a multi-dimensional space. This work involved in optimization is just exhausting.