GCC Profiling

Overview

To use profiling, the program must be compiled and linked with the -qg profiling option:

We will use an called profiling_test.c (full code can be found at https://github.com/gbmhunter/BlogAssets/tree/master/Programming/ProfilingGprof):

#include <stdio.h>

int fibonacci(int n) {
    if(n == 0)
        return 0;
    else if(n == 1)
        return 1;
    else
        return(fibonacci(n-1) + fibonacci(n-2));
} 

int loop100M() {
    int val = 0;
    for(int i = 0; i < 100000000; i++) {
    if(i % 10 == 0)
        val++;
    else if(i % 3)
        val--;
    }
    return val;
}

int main (void) {  
    printf("Fibonacci value = %u\n", fibonacci(40));          
    printf("Loop value = %u\n", loop100M());

    return 0;
}

We will then compile it with the command:

$ gcc -pg profiling_test.c -o profiling_test

This creates what is called an instrumented executable. It contains additional code which records the time spent in each function.

When run, the program will produce a file gmon.out in the same directory as it is run. You can pass your program to gprof to display the profiling results:

$ gprof ./profiling_test
Flat profile:

Each sample counts as 0.01 seconds.
    %   cumulative   self              self     total           
    time   seconds   seconds    calls  ms/call  ms/call  name    
    60.96      0.68     0.68        1   676.63   676.63  fibonacci
    31.84      1.03     0.35        1   353.47   353.47  loop100M
    8.19      1.12     0.09                             frame_dummy
...

You can see above that approximately 60% of the time was spent calculating the Fibonacci sequence, while 30% was spent looping 100 million times. If this was a real life scenario, you could now start to optimise your code!

If you find text hard to analyze, see the gprof2dot section below on how to create a visualization of the above results.

The above command will write the profiling results to the terminal. Instead, if you wish to write the results to a file, use the following command:

$ gprof profiling_test > profiling_results.txt

Clean Exiting

gmon.out is only written to if your C/C++ program exits cleanly, that is, it either calls exit() or returns from main().

Here is the relevant info from the gprof manual:

The profiled program must call "exit"(2) or return normally for the profiling information to be saved in the gmon.out file.

Your program doesn’t count as a clean exit if it is running in a Linux terminal and Ctrl-C is pressed! However, there is a way to fix this, by catching the Ctrl-C signal and writing to the file before exiting…

#include <dlfcn.h>
#include <stdio.h>
#include <unistd.h>

void SigIntHandler(int sig) {
    fprintf(stderr, "Exiting on SIGUSR1\n");
    void (*_mcleanup)(void);
    _mcleanup = (void (*)(void)) dlsym(RTLD_DEFAULT, "_mcleanup");
    if (_mcleanup == NULL)
        fprintf(stderr, "Unable to find gprof exit hook\n");
    else _mcleanup();
    _exit(0);
}

int main() {
    signal(SIGINT, SigIntHandler);

    ... code that does not return here
}

gprof2dot

gprof2dot is a tool that can create a visualization of the gprof output. TO install gprof2dot:

$ pip install gprof2dot

To install graphviz (which is needed if you are going to make “dot” graphs like below):

$ sudo apt install graphviz

To create a dot graph image:

$ gprof2dot ./profiling.txt | dot -Tpng -o profiling.png

This created the below image for the example code above: