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 nonce, the estimated figures are usually more useful than misleading.