timing floating point calculation with clock_gettime in C, time keeps changing? - c

Question

I'm trying to calculate the time of a piece of code (floating point multiplications) using clock_gettime(). I randomly generate 10 arrays of 100000 floats and then multiply them all together, ten at a time. What I want to do is measure the time it takes to perform ten floating point multiplies, then add that time value to the total time, and then print it all out at the end.
I thought it was all working, but I discovered that if I remove my printf() statements from the while loop, the time decreases from 28125692 to 17490394 nanoseconds! This shouldn't be happening. I placed the clock_gettime() calls at the beginning and end of the floating point multiplications, so theoretically, the printf() statement should not effect the elapsed time!
Any ideas? Thank you.
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <time.h>
int main(int argc, char *argv[])
{
int i;
int j;
float alpha[100000];
float bravo[100000];
float charlie[100000];
float delta[100000];
float echo[100000];
float foxtrot[100000];
float golf[100000];
float hotel[100000];
float india[100000];
float juliet[100000];
float max;
long long num_calcs;
struct timespec start_time;
struct timespec end_time;
int diff_seconds;
long diff_nanoseconds;
long long total_calcs;
long long total_seconds;
long long total_nanoseconds;
num_calcs = 100000;
max = 1000.0;
printf("\n%lld floating point calculations requested.", num_calcs);
printf("\nGenerating random floating point values...");
//initialize random seed
srand((unsigned int)time(NULL));
//generate random floating point values
for (i = 0; i < 100000; i++)
{
alpha[i] = ((float) rand()) / ((float) (RAND_MAX)) * max;
bravo[i] = ((float) rand()) / ((float) (RAND_MAX)) * max;
charlie[i] = ((float) rand()) / ((float) (RAND_MAX)) * max;
delta[i] = ((float) rand()) / ((float) (RAND_MAX)) * max;
echo[i] = ((float) rand()) / ((float) (RAND_MAX)) * max;
foxtrot[i] = ((float) rand()) / ((float) (RAND_MAX)) * max;
golf[i] = ((float) rand()) / ((float) (RAND_MAX)) * max;
hotel[i] = ((float) rand()) / ((float) (RAND_MAX)) * max;
india[i] = ((float) rand()) / ((float) (RAND_MAX)) * max;
juliet[i] = ((float) rand()) / ((float) (RAND_MAX)) * max;
}
printf("done!");
printf("\nRunning %lld floating point multiplications...", num_calcs);
//run calculations
i = 0;
total_calcs = 0;
total_seconds = 0;
total_nanoseconds = 0;
while (total_calcs < num_calcs)
{
printf("\n\nRunning 10 calculations...");
//do 10 multiplications
//start the timer
if (clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start_time) < 0)
{
printf("\nclock_gettime for start_time failed, exiting...");
return -1;
}
alpha[i] * bravo[i] * charlie[i] * delta[i] * echo[i] *
foxtrot[i] * golf[i] * hotel[i] * india[i] * juliet[i];
//stop the timer
if (clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end_time) < 0)
{
printf("\nclock_gettime for end_time failed, exiting...");
return -1;
}
printf("done!");
total_calcs = total_calcs + 10; //increase total calculations
i++; //increment array index
//show timing statistics
printf("\nSTART TIME tv_sec: %d", (int) start_time.tv_sec);
printf("\nSTART TIME tv_nsec: %ld", start_time.tv_nsec);
printf("\nEND TIME tv_sec: %d", (int) end_time.tv_sec);
printf("\nEND TIME tv_nsec: %ld", end_time.tv_nsec);
//calculate time it took to run 10 floating point caculcations
if ((end_time.tv_nsec - start_time.tv_nsec) < 0)
{
diff_seconds = end_time.tv_sec - start_time.tv_sec - 1;
diff_nanoseconds = 1000000000 + end_time.tv_nsec - start_time.tv_nsec;
}
else
{
diff_seconds = end_time.tv_sec - start_time.tv_sec;
diff_nanoseconds = end_time.tv_nsec - start_time.tv_nsec;
}
//add elapsed time for the last 10 calculations to total elapsed time
total_seconds = total_seconds + diff_seconds;
total_nanoseconds = total_nanoseconds + diff_nanoseconds;
printf("\nPerformed 10 floating point multiplications in %d seconds and %ld nanoseconds.", diff_seconds, diff_nanoseconds);
printf("\nPerformed %lld floating point multiplications in %lld seconds and %lld nanoseconds.", total_calcs, total_seconds, total_nanoseconds);
}
printf("done!");
printf("\nPerformed %lld floating point multiplications in %lld seconds and %lld nanoseconds.\n", total_calcs, total_seconds, total_nanoseconds);
return 0;
}

Answer 1

The statement
alpha[i] * bravo[i] * charlie[i] * delta[i] * echo[i] *
foxtrot[i] * golf[i] * hotel[i] * india[i] * juliet[i];
is a no-op. An optimizing compiler will omit this from the executable.
To make it work, try adding the result to another variable and print the sum at the end of the program.