Previous: Sampling Error, Up: Inaccuracy

6.2 Estimating children Times

Some of the figures in the call graph are estimates—for example, the children time values and all 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 novice, the estimated figures are usually more useful than misleading.