linux - Erratic average execution times for C function -

i'm trying optimize chunk of code given me friend baseline average execution times of extremely erratic , i'm lost why/how fix it.

code:

#include <sys/time.h> #include <time.h> #include <stdio.h> #include "wall.h" /* code */  int main() {     int average;     struct timeval tv;     int i;      for(i = 0; < 1000; i++) /* running code 1,000 times */     {         gettimeofday(&tv, null); /* starting time */           start(); /* launching code */           int ret = tv.tv_usec; /* finishing time */          ret /= 1000; /* converting milliseconds */          average += ret; /* adding average */      }     printf("average execution time: %d milliseconds\n", average/1000);       return 0; } 

output of 5 different runs:

• 804 milliseconds
• 702 milliseconds
• 394 milliseconds
• 642 milliseconds
• 705 milliseconds

i've tried multiple different ways of getting average execution time, each 1 either doesn't give me precise enough answer or gives me erratic one. i'm lost now, appreciated!

i know these types of benchmarks system dependent, i've listed system specs below:

• ubuntu 12.10 x64
• 7.8 gib ram
• intel core i7-3770 cpu @ 3.40ghz x 8
• geforce gt 620/pcie/sse2

edit

thank input decided go gprof instead of constructing own. thank you, once again!

there several problems here, including 0 details on code you're benchmarking doing, , you're using "gettimeofday()" incorrectly (and, perhaps, inappropriately).

suggestions:

1) don't use "gettimeofday()":

http://blog.habets.pp.se/2010/09/gettimeofday-should-never-be-used-to-measure-time

2) supplement "time elapsed" gprof:

http://www.cs.duke.edu/~ola/courses/programming/gprof.html