[issue3865] explain that profilers should be used for profiling, not benchmarking

[Fredrik Lundh]

Fredrik Lundh <effbot at users.sourceforge.net> added the comment:

(the reason this is extra bad for C modules is that the profilers
introduce overhead for Python code, but not for C-level functions.  For
example, using the standard profiler to benchmark parser performance for
xml.etree.ElementTree vs. xml.etree.cElementTree will make ET appear to
be about 10 times slower than it actually is.)

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue3865>
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