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