[Omp] slow performance

[andrew wang]

Hello,

I am pretty new to OpenMP, before do some real thing, I write a simple code 
to test my understanding. But to my
big suprise, I see that the result is quite different from what I can 
imagine. The more threads I have, the more
slow the calculation is.

Can you help me to take a look at the source code and the result I got? Is 
it too much fork/join, but little time on calculation in each thread?

Thanks
Andrew





source:

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

static long num_steps = 1000000000L;
double step;

int omp_threads = 1;

void main (int argc,char **argv) {

   double  x, sum, pi=0.0;
   int id;
   time_t t1,t2,t11,t22;
   int ii, jj, kk, i;
   int total_time=0;

   (void) time(&t1);


   omp_threads=atoi(argv[1]);


   printf("omp_get_num_procs=%d\n",omp_get_num_procs());

   step = 1.0/(double) num_steps;
   omp_set_num_threads(omp_threads);


   for (i=0;i<16200;i++) {

	   for (ii=0; ii<50; ii++)
	   {

	   	(void) time(&t11);
		#pragma omp parallel private (jj,kk,x, sum)
		{

	   		id = omp_get_thread_num();

	   		for (jj=id;jj<3; jj=jj+omp_threads )
	   		 {

			   	for (kk=0; kk< 50; kk ++){


			   		x = (kk+0.5)*step;
			   		sum += 4.0/(1.0+x*x);   // more complicated calculation here.
			   	}

			   	pi += sum;
			 }
		}

		(void) time(&t22);
                total_time += (long)(t22-t11);

	   }
   }


   id = omp_get_thread_num();


   printf("Parallel region time=%d seconds\n", (int) total_time);

   (void) time(&t2);
   printf("Total time = %d seconds\n", (int) (t2-t1));

}

result:


atlas1> a.out 1
omp_get_num_procs=4
Parallel region time=2 seconds
Total time = 3 seconds
atlas1> a.out 2
omp_get_num_procs=4
Parallel region time=41 seconds
Total time = 41 seconds
atlas1> a.out 3
omp_get_num_procs=4
Parallel region time=71 seconds
Total time = 72 seconds