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Concept
Gprof
Summary
Gprof is a performance analysis tool for Unix applications. It used a hybrid of instrumentation and sampling and was created as an extended version of the older "prof" tool. Unlike prof, gprof is capable of limited call graph collecting and printing.
GPROF was originally written by a group led by Susan L. Graham at the University of California, Berkeley for Berkeley Unix (4.2BSD). Another implementation was written as part of the GNU project for GNU Binutils in 1988 by Jay Fenlason.
Instrumentation code is automatically inserted into the program code during compilation (for example, by using the '-pg' option of the gcc compiler), to gather caller-function data. A call to the monitor function 'mcount' is inserted before each function call.
Sampling data is saved in 'gmon.out' or in progname''.gmon' file just before the program exits, and can be analyzed with the 'gprof' command-line tool. Several gmon files can be combined with 'gprof -s' to accumulate data from several runs of a program.
GPROF output consists of two parts: the flat profile and the call graph. The flat profile gives the total execution time spent in each function and its percentage of the total running time. Function call counts are also reported. Output is sorted by percentage, with hot spots at the top of the list.
The second part of the output is the textual call graph, which shows for each function who called it (parent) and who it called (child subroutines). There is an external tool called gprof2dot capable of converting the call graph from gprof into graphical form.
At run-time, timing values are obtained by statistical sampling. Sampling is done by probing the target program's program counter at regular intervals using operating system interrupts (programmed via profil(2) or setitimer(2) syscalls). The resulting data is not exact, rather a statistical approximation. The amount of error is usually more than one sampling period. If a value is n times the sampling period, the expected error in the value is the square root of n sampling periods.
Official source
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