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otp: Update profiling doc with eprof
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@psyeugenic
psyeugenic committed Oct 24, 2011
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<p>Erlang/OTP contains several tools to help finding bottlenecks.</p>

<p><c>fprof</c> and <c>eprof</c> provide the most detailed information
about where the time is spent, but they significantly slow downs the
programs they profile.</p>
<p><c>fprof</c> provide the most detailed information
about where the time is spent, but it significantly slows down the
program it profiles.</p>

<p><c>eprof</c> provides time information of each function used
in the program. No callgraph is produced but <c>eprof</c> has
considerable less impact on the program profiled.</p>

<p>If the program is too big to be profiled by <c>fprof</c> or <c>eprof</c>,
<c>cover</c> and <c>cprof</c> could be used to locate parts of the
code that should be more thoroughly profiled using <c>fprof</c> or
<c>eprof</c>.</p>

<p><c>cover</c> provides execution counts per line per process,
with less overhead than <c>fprof/eprof</c>. Execution counts can
with less overhead than <c>fprof</c>. Execution counts can
with some caution be used to locate potential performance bottlenecks.
The most lightweight tool is <c>cprof</c>, but it only provides execution
counts on a function basis (for all processes, not per process).</p>
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<section>
<title>fprof</title>
<p><c>fprof</c> measures the execution time for each function,
<p>
<c>fprof</c> measures the execution time for each function,
both own time i.e how much time a function has used for its
own execution, and accumulated time i.e. including called
functions. The values are displayed per process. You also get
to know how many times each function has been
called. <c>fprof</c> is based on trace to file in order to
minimize runtime performance impact. Using fprof is just a
matter of calling a few library functions, see fprof manual
page under the application tools.</p>
<p><c>fprof</c> was introduced in version R8 of Erlang/OTP. Its
predecessor <c>eprof</c> that is based on the Erlang trace BIFs,
is still available, see eprof manual page under the
application tools. Eprof shows how much time has been used by
each process, and in which function calls this time has been
spent. Time is shown as percentage of total time, not as
absolute time.</p>
matter of calling a few library functions, see
<seealso marker="tools:fprof">fprof</seealso>
manual page under the application tools.<c>fprof</c> was introduced in
version R8 of Erlang/OTP.
</p>
</section>

<section>
<title>eprof</title>
<p>
<c>eprof</c> is based on the Erlang trace_info BIFs. Eprof shows how much time has been used by
each process, and in which function calls this time has been
spent. Time is shown as percentage of total time and absolute time.
See <seealso marker="tools:eprof">eprof</seealso> for
additional information.
</p>
</section>

<section>
<title>cover</title>
<p><c>cover</c>'s primary use is coverage analysis to verify
<p>
<c>cover</c>'s primary use is coverage analysis to verify
test cases, making sure all relevant code is covered.
<c>cover</c> counts how many times each executable line of
code is executed when a program is run. This is done on a per
module basis. Of course this information can be used to
determine what code is run very frequently and could therefore
be subject for optimization. Using cover is just a matter of
calling a few library functions, see cover manual
page under the application tools.</p>
calling a few library functions, see
<seealso marker="tools:cover">cover</seealso>
manual page under the application tools.</p>
</section>

<section>
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<c>cover</c> regarding features. It counts how many times each
function is called when the program is run, on a per module
basis. <c>cprof</c> has a low performance degradation effect (versus
<c>fprof</c> and <c>eprof</c>) and does not need to recompile
any modules to profile (versus <c>cover</c>).</p>
<c>fprof</c>) and does not need to recompile
any modules to profile (versus <c>cover</c>).
See <seealso marker="tools:cprof">cprof</seealso> manual page for additional
information.
</p>
</section>

<section>
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<cell align="left" valign="middle"><c>eprof </c></cell>
<cell align="left" valign="middle">per process/function to screen/file </cell>
<cell align="left" valign="middle">medium </cell>
<cell align="left" valign="middle">significant slowdown </cell>
<cell align="left" valign="middle">small slowdown </cell>
<cell align="left" valign="middle">yes </cell>
<cell align="left" valign="middle">only total </cell>
<cell align="left" valign="middle">no </cell>
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