Why is there a difference in execution time while running the same program multiple times?

Question

Probably a dumb question. I'm noticing a difference in execution time while running a simple Hello World program in C on a Linux machine( It's not language specific though).

Program:

#include<stdio.h>
#include<time.h>

int main()
{
    clock_t begin, end;
    double time_spent;

    begin = clock();

    printf("%s", "Hello World\n");
    end = clock();
    time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
    printf("%f\n", time_spent);
    return 0;
}

o/p:

$ ./hello 
Hello World
0.000061
$ ./hello 
Hello World
0.000057
$ ./hello 
Hello World
0.000099 

This is tested on a quad core machine with a load average of 0.4 and enough free memory. Though the difference is pretty small, what could be the reason behind it?

Answer 1

Unless you're running a real-time operating system, you're going to see at least a slight variation in run times. This is due to OS scheduling, any I/O that might be happening at around that time, etc.

A difference of 0.04 ms is not a big difference at all.

If your program runs in a loop for at least several seconds, the percentage of variation should be reduced.