6. Inaccuracy of gprof Output

6.1 Statistical Sampling Error		Statistical margins of error
6.2 Estimating children Times		Estimating children times

6.1 Statistical Sampling Error

The run-time figures that gprof 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.

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.

The sampling period 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.

The actual amount of error can be predicted. For n samples, the expected error is the square-root of n. For example, if the sampling period is 0.01 seconds and foo's run-time is 1 second, n is 100 samples (1 second/0.01 seconds), sqrt(n) is 10 samples, so the expected error in foo'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 bar's run-time is 100 seconds, n is 10000 samples, sqrt(n) is 100 samples, so the expected error in bar's run-time is 1 second, or one percent of the observed value. It is likely to vary this much on the average from one profiling run to the next. (Sometimes it will vary more.)

This does not mean that a small run-time figure is devoid of information. If the program's total 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.

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 `-s' option of gprof. Here is how:

1. Run your program once.

2. Issue the command `mv gmon.out gmon.sum'.

3. Run your program again, the same as before.

4. Merge the new data in `gmon.out' into `gmon.sum' with this command:

   gprof -s executable-file gmon.out gmon.sum

5. Repeat the last two steps as often as you wish.

6. Analyze the cumulative data using this command:

   gprof executable-file gmon.sum > output-file

6.2 Estimating children Times

Some of the figures in the call graph are estimates--for example, the children time values and all the the time figures in caller and subroutine lines.

There is no direct information about these measurements in the profile data itself. Instead, gprof estimates them by making an assumption about your program that might or might not be true.

The assumption made is that the average time spent in each call to any function foo is not correlated with who called foo. If foo used 5 seconds in all, and 2/5 of the calls to foo came from a, then foo contributes 2 seconds to a's children time, by assumption.

This assumption is usually true enough, but for some programs it is far from true. Suppose that foo returns very quickly when its argument is zero; suppose that a always passes zero as an argument, while other callers of foo pass other arguments. In this program, all the time spent in foo is in the calls from callers other than a. But gprof has no way of knowing this; it will blindly and incorrectly charge 2 seconds of time in foo to the children of a.

We hope some day to put more complete data into `gmon.out', so that this assumption is no longer needed, if we can figure out how. For the nonce, the estimated figures are usually more useful than misleading.