seconds_example.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>

/* crude wall clock time keeping, returns a double of seconds */
double
seconds()
{
   struct timeval tp;
   struct timezone tzp;
   gettimeofday (&tp, &tzp);
   return ((double) tp.tv_sec + (double) tp.tv_usec * 1.e-6);
}

/* allocate 3 512MB arrays, 64M each */
#define SIZE 67108864

main()
{
   double *a,*b,*c;
   double start,stop,diff;
   int i,mem;

   a=(double *) malloc(SIZE*sizeof(double));
   b=(double *) malloc(SIZE*sizeof(double));
   c=(double *) malloc(SIZE*sizeof(double));

   for (i=0;i<SIZE;i++)
   {
      b[i]=i*1.25;
      c[i]=i*1.9;
   }
   start=seconds();
   for (i=0;i<SIZE;i++)
   {
      a[i]=b[i]*c[i];
   }
   stop=seconds();
   diff=stop-start;
   mem=(SIZE*sizeof(double)*3);
   printf ("%d double flops took %f seconds\n",SIZE,diff);
   printf ("%f Mflops/sec\n",SIZE/(diff*1000000));
   printf ("total size of 3 arrays = %d MB\n",mem/(1024*1024));
   printf ("memory bandwidth = %5.2f GB/sec\n",(mem/(1048*1024*1024))/diff);
}  

$ ./a.out
67108864 double flops took 0.123589 seconds
543.000129 Mflops/sec
total size of 3 arrays = 1536 MB
memory bandwidth = 8.09 GB/sec