I can tell you two common mistakes people make when profiling. The first is measuring CPU time in an application that is I/O bound. Devel::DProf shows CPU time by default, and that's pretty much useless for an application that uses a database or does other slow activities that are light on CPU.

The other problem I see is when people try to profile mod_perl apps but load their code before initializing the debugger. If you load code during startup in mod_perl, you have to start the debugger first, because the profiler uses the debug hooks. If you don't do that, you only get back info on the code loaded later, and it looks wrong and useless. (This is all described in the mod_perl docs.)

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