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As a general rule, 90% of the execution time of your program will be spent in 10% of its code. Profilers are tools that help you identify the 10% of hot spots that constrain the speed of your program. This is a good thing for making it faster.
But in the Unix tradition, profilers have a far more important function. They enable you not to optimize the other 90%! This is good, and not just because it saves you work. The really valuable effect is that not optimizing that 90% holds down global complexity and reduces bugs.
You may recall that we quoted Donald Knuth observing “Premature optimization is the root of all evil” in Chapter═1, and that Rob Pike and Ken Thompson had a few pungent observations on the topic as well. These were the voices of experience. Do good design. Think about what's right first. Tune for efficiency later.
Profilers help you do this. If you get in the good habit of using them, you can get rid of the bad habit of premature optimization. Profilers don't just change the way you work; they change how you think.
Profilers for compiled languages rely on instrumenting object code, so they are even more platform-dependent than compilers. On the other hand, a compiled-language profiler doesn't care about the source language of the programs it instruments. Under Unix, the single profiler gprof(1) handles C, C++, and all other compiled languages.
Perl, Python, and Emacs Lisp have their own profilers included in their basic distributions; these are portable across all platforms on which the host languages themselves run. Java has built-in profiling. Tcl has no profiling support as yet.