113 lines
5.1 KiB
HTML
113 lines
5.1 KiB
HTML
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>Sampling Error - GNU gprof</title>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html">
|
||
|
|
<meta name="description" content="GNU gprof">
|
||
|
|
<meta name="generator" content="makeinfo 4.13">
|
||
|
|
<link title="Top" rel="start" href="index.html#Top">
|
||
|
|
<link rel="up" href="Inaccuracy.html#Inaccuracy" title="Inaccuracy">
|
||
|
|
<link rel="next" href="Assumptions.html#Assumptions" title="Assumptions">
|
||
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
||
|
|
<!--
|
||
|
|
This file documents the gprof profiler of the GNU system.
|
||
|
|
|
||
|
|
Copyright (C) 1988-2019 Free Software Foundation, Inc.
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3
|
||
|
|
or any later version published by the Free Software Foundation;
|
||
|
|
with no Invariant Sections, with no Front-Cover Texts, and with no
|
||
|
|
Back-Cover Texts. A copy of the license is included in the
|
||
|
|
section entitled ``GNU Free Documentation License''.
|
||
|
|
|
||
|
|
-->
|
||
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
||
|
|
<style type="text/css"><!--
|
||
|
|
pre.display { font-family:inherit }
|
||
|
|
pre.format { font-family:inherit }
|
||
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallexample { font-size:smaller }
|
||
|
|
pre.smalllisp { font-size:smaller }
|
||
|
|
span.sc { font-variant:small-caps }
|
||
|
|
span.roman { font-family:serif; font-weight:normal; }
|
||
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
||
|
|
--></style>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<div class="node">
|
||
|
|
<a name="Sampling-Error"></a>
|
||
|
|
<p>
|
||
|
|
Next: <a rel="next" accesskey="n" href="Assumptions.html#Assumptions">Assumptions</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="Inaccuracy.html#Inaccuracy">Inaccuracy</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h3 class="section">6.1 Statistical Sampling Error</h3>
|
||
|
|
|
||
|
|
<p>The run-time figures that <code>gprof</code> gives you are based on a sampling
|
||
|
|
process, so they are subject to statistical inaccuracy. If a function runs
|
||
|
|
only a small amount of time, so that on the average the sampling process
|
||
|
|
ought to catch that function in the act only once, there is a pretty good
|
||
|
|
chance it will actually find that function zero times, or twice.
|
||
|
|
|
||
|
|
<p>By contrast, the number-of-calls and basic-block figures are derived
|
||
|
|
by counting, not sampling. They are completely accurate and will not
|
||
|
|
vary from run to run if your program is deterministic and single
|
||
|
|
threaded. In multi-threaded applications, or single threaded
|
||
|
|
applications that link with multi-threaded libraries, the counts are
|
||
|
|
only deterministic if the counting function is thread-safe. (Note:
|
||
|
|
beware that the mcount counting function in glibc is <em>not</em>
|
||
|
|
thread-safe). See <a href="Implementation.html#Implementation">Implementation of Profiling</a>.
|
||
|
|
|
||
|
|
<p>The <dfn>sampling period</dfn> that is printed at the beginning of the flat
|
||
|
|
profile says how often samples are taken. The rule of thumb is that a
|
||
|
|
run-time figure is accurate if it is considerably bigger than the sampling
|
||
|
|
period.
|
||
|
|
|
||
|
|
<p>The actual amount of error can be predicted.
|
||
|
|
For <var>n</var> samples, the <em>expected</em> error
|
||
|
|
is the square-root of <var>n</var>. For example,
|
||
|
|
if the sampling period is 0.01 seconds and <code>foo</code>'s run-time is 1 second,
|
||
|
|
<var>n</var> is 100 samples (1 second/0.01 seconds), sqrt(<var>n</var>) is 10 samples, so
|
||
|
|
the expected error in <code>foo</code>'s run-time is 0.1 seconds (10*0.01 seconds),
|
||
|
|
or ten percent of the observed value.
|
||
|
|
Again, if the sampling period is 0.01 seconds and <code>bar</code>'s run-time is
|
||
|
|
100 seconds, <var>n</var> is 10000 samples, sqrt(<var>n</var>) is 100 samples, so
|
||
|
|
the expected error in <code>bar</code>'s run-time is 1 second,
|
||
|
|
or one percent of the observed value.
|
||
|
|
It is likely to
|
||
|
|
vary this much <em>on the average</em> from one profiling run to the next.
|
||
|
|
(<em>Sometimes</em> it will vary more.)
|
||
|
|
|
||
|
|
<p>This does not mean that a small run-time figure is devoid of information.
|
||
|
|
If the program's <em>total</em> run-time is large, a small run-time for one
|
||
|
|
function does tell you that that function used an insignificant fraction of
|
||
|
|
the whole program's time. Usually this means it is not worth optimizing.
|
||
|
|
|
||
|
|
<p>One way to get more accuracy is to give your program more (but similar)
|
||
|
|
input data so it will take longer. Another way is to combine the data from
|
||
|
|
several runs, using the ‘<samp><span class="samp">-s</span></samp>’ option of <code>gprof</code>. Here is how:
|
||
|
|
|
||
|
|
<ol type=1 start=1>
|
||
|
|
<li>Run your program once.
|
||
|
|
|
||
|
|
<li>Issue the command ‘<samp><span class="samp">mv gmon.out gmon.sum</span></samp>’.
|
||
|
|
|
||
|
|
<li>Run your program again, the same as before.
|
||
|
|
|
||
|
|
<li>Merge the new data in <samp><span class="file">gmon.out</span></samp> into <samp><span class="file">gmon.sum</span></samp> with this command:
|
||
|
|
|
||
|
|
<pre class="example"> gprof -s <var>executable-file</var> gmon.out gmon.sum
|
||
|
|
</pre>
|
||
|
|
<li>Repeat the last two steps as often as you wish.
|
||
|
|
|
||
|
|
<li>Analyze the cumulative data using this command:
|
||
|
|
|
||
|
|
<pre class="example"> gprof <var>executable-file</var> gmon.sum > <var>output-file</var>
|
||
|
|
</pre>
|
||
|
|
</ol>
|
||
|
|
|
||
|
|
</body></html>
|
||
|
|
|