[Omp] slow performance

[andrew wang]

>From: Dieter an Mey <anmey at rz.rwth-aachen.de>
>To: Herbert Fruechtl <Herbert.Fruchtl at uk.fujitsu.com>
>CC: andrew wang <mcwang88 at hotmail.com>, omp at openmp.org
>Subject: Re: [Omp] slow performance
>Date: Thu, 16 Dec 2004 17:10:28 +0100
>
>Good point!
>Sorry that I didn't see it.
>
>Dieter
>
>Herbert Fruechtl wrote:
>
>>You are not doing anything in parallel, but only replicating your work.
>>You need either an "omp parallel for" pragma, or "parallel" AND "for"
>>pragmas.


#pragma omp parallel private (jj,kk,x, sum)
	  {

	      id = omp_get_thread_num();

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

...

are equivalent to "omp parallel for"


>>Also be careful with accumulating your result. The statement
>>
>>	pi += sum;
>>
>>needds an "atomic" pragma. Or better still, specify pi as "accumulate"
>>in your omp pragma.


Yes, I should do that. But for this testing program, I simply igonore it at 
this  moment.


>>
>>Hope this helps,
>>
>>   Herbert
>>
>>
>>
>>>-----Original Message-----
>>>From: Omp-bounces at openmp.org [mailto:Omp-bounces at openmp.org] On Behalf Of 
>>>andrew wang
>>>Sent: 15 December 2004 14:53
>>>To: omp at openmp.org
>>>Subject: [Omp] slow performance
>>>
>>>
>>>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
>>>
>>>
>>>
>>>_______________________________________________
>>>Omp mailing list
>>>Omp at openmp.org http://openmp.org/mailman/listinfo/omp_openmp.org
>>>
>>
>>
>>_______________________________________________
>>Omp mailing list
>>Omp at openmp.org
>>http://openmp.org/mailman/listinfo/omp_openmp.org
>>
>
>
>--
>--------------------------------------------------------------------
>Dieter an Mey
>High Performance Computing               Hochleistungsrechnen
>RWTH Aachen University                   Rechen- und Kommunikations-
>Center for Computing and Communication   zentrum der RWTH Aachen
>phone: ++49-(0)241-80-24377              Seffenter Weg 23
>fax:   ++49-(0)241-80-22134              52074 Aachen, Germany
>email: anmey at rz.rwth-aachen.de
>--------------------------------------------------------------------
>