Getting getrusage() to measure system time in C

Question

I would like to measure the system time it takes to execute some code. To do this I know I would sandwich said code between two calls to getrusage(), but I get some unexpected results...

#include <sys/time.h>
#include <sys/resource.h>
#include <unistd.h>
#include <stdio.h>

int main() {
  struct rusage usage;
  struct timeval start, end;
  int i, j, k = 0;

  getrusage(RUSAGE_SELF, &usage);
  start = usage.ru_stime;
  for (i = 0; i < 10000; i++) {
    /* Double loop for more interesting results. */
    for (j = 0; j < 10000; j++) {
      k += 20; 
    }
  }
  getrusage(RUSAGE_SELF, &usage);
  end = usage.ru_stime;

  printf("Started at: %ld.%lds\n", start.tv_sec, start.tv_usec);
  printf("Ended at: %ld.%lds\n", end.tv_sec, end.tv_usec);
  return 0;
}

I would hope that this produces two different numbers, but alas! After seeing my computer think for a second or two, this is the result:

Started at: 0.1999s
Ended at: 0.1999s

Am I not using getrusage() right? Why shouldn't these two numbers be different? If I am fundamentally wrong, is there another way to use getrusage() to measure the system time of some source code? Thank you for reading.

Whitehall

Please observe that tv_usec are microseconds, so your format should be %lu.%06u. – Jin

Answer 1

You're misunderstanding the difference between "user" and "system" time. Your example code is executing primarily in user-mode (ie, running your application code) while you are measuring, but "system" time is a measure of time spent executing in kernel-mode (ie, processing system calls).

ru_stime is the correct field to measure system time. Your test application just happens not to accrue any such time between the two points you check.

Heckle

Thank you! You explained this better than my professor could. – Whitehall

Answer 2

You should use usage.ru_utime, which is user CPU time used, instead.

Emiliaemiliaromagna

This indeed does give me more preferable results. However I am still obligated to find a working way to measure the system time of some code. What is the use of usage.ru_stime if it does not work? – Whitehall

If the program is designed to run under kernel mode, then use ru_stime. Otherwise, use ru_utime. – Emiliaemiliaromagna

Answer 3

Use gprof. This will give give you the time taken by each function. Install gprof and use these flags for compilation: -pg -fprofile-arcs -ftest-coverage.

Cincinnati