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| <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> |
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