You can speed up an algorithm 1000 fold but if it only accounts for 1% of your total execution time you've only gained a 1% speedup.
In a pure program, yes. In an impure one, not necessarily. The change in algorithm could result in some different data structures or layout, and this could cause another part of the program to run much faster or slower (e.g. because of cache locality issues).
It's one way of measuring, and can really help narrow things down in larger programs.
i.e., pipeline takes 300s to run. Suspect the final output may be worth profiling and optimizing. Stick a 'return' at the top of the output function and see the pipeline now takes 295s to run. Conclusion: final output not the problem.
For example, one function thrash the cache, which ends up greatly slowing down a second function. If you remove the first function entirely (comment it out, etc), the change will reflect that, but a flame graph won't (since it only shows the actual runtime for the second function: it has no way to determine how much impact it is having on other functions indirectly).
Beware measuring, though. Sometimes, just thinking about the problem ahead of time can clearly expose where you are spending time. At least, it can give a hypothesis to test for where you are spending time. With measurements to confirm/refute. :)
"Longest total." I don't care if its a big slice of pie or a lot of little slices. But it better be one of those.
> Sometimes, just thinking about the problem ahead of time can clearly expose where you are spending time.
That's certainly true. But 99% of people overestimate their ability to do this. And are surprised by the results.
If so, good, but it wasn't clear to me that that's what you meant. If you mean something else then I don't know what you mean at all.
To that end, I was taking it to mean that longest "synchronous" path through your system. Not necessarily a single method, by any measure. But, systems have plenty of what I will call "checkpoints" where code can be restarted/rerun with no hard to recover penalty. That is what I took to mean by longest.
Why beware measuring? In my experience, people are generally terrible at guessing where the bulk of the time is being spent. Measure as soon as you care.