78 lines
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78 lines
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<title>Assumptions - GNU gprof</title>
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<a name="Assumptions"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Sampling-Error.html#Sampling-Error">Sampling Error</a>,
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Up: <a rel="up" accesskey="u" href="Inaccuracy.html#Inaccuracy">Inaccuracy</a>
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<h3 class="section">6.2 Estimating <code>children</code> Times</h3>
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<p>Some of the figures in the call graph are estimates—for example, the
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<code>children</code> time values and all the time figures in caller and
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subroutine lines.
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<p>There is no direct information about these measurements in the profile
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data itself. Instead, <code>gprof</code> estimates them by making an assumption
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about your program that might or might not be true.
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<p>The assumption made is that the average time spent in each call to any
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function <code>foo</code> is not correlated with who called <code>foo</code>. If
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<code>foo</code> used 5 seconds in all, and 2/5 of the calls to <code>foo</code> came
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from <code>a</code>, then <code>foo</code> contributes 2 seconds to <code>a</code>'s
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<code>children</code> time, by assumption.
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<p>This assumption is usually true enough, but for some programs it is far
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from true. Suppose that <code>foo</code> returns very quickly when its argument
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is zero; suppose that <code>a</code> always passes zero as an argument, while
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other callers of <code>foo</code> pass other arguments. In this program, all the
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time spent in <code>foo</code> is in the calls from callers other than <code>a</code>.
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But <code>gprof</code> has no way of knowing this; it will blindly and
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incorrectly charge 2 seconds of time in <code>foo</code> to the children of
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<code>a</code>.
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<!-- FIXME - has this been fixed? -->
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<p>We hope some day to put more complete data into <samp><span class="file">gmon.out</span></samp>, so that
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this assumption is no longer needed, if we can figure out how. For the
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novice, the estimated figures are usually more useful than misleading.
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</body></html>
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