Re: [OMPI users] execuation time is not stable with 2 processes

[Mark Potts]

Hi,
   I'd opt for the fact that tv0 is given value only on rank 0 and tv1 is
   only given value on rank 1.  Kind of hard to get a diff betwn the two
   on either rank with that setup.  You need to determine the tv0 and tv1
   on both ranks.
   In addition, there are a number of other errors in the code (such as
   MPI_Finalize() as an errant function outside of main), etc.
         regards,

Ralph Castain wrote:

Did you bind the processes? If not you may be seeing the impact of having 
processes bouncing between cpus, and/or processes not being local to their 
memory. Try adding -bind-to-core or -bind-to-socket to your cmd line and see if 
things smooth out. I'm assuming, of course, that you are running on a system 
that supports binding...

The time can also be impacted by other things running on your cpu - could be 
context switching.

Final point: since both processes are running on the same node, IB will have no 
involvement - the messages are going to flow over shared memory.


On Aug 4, 2010, at 6:51 AM, Tad Lake wrote:

Hi,

 I have a little program for execution time.
=================================================
#include "mpi.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
int main (int argc, char *argv[]) {
MPI_Status Stat;
struct timeval tv0, tv1;

long int totaltime = 0;
int i, j;
int buf[10240];
 int numtasks, rank;

MPI_Init (&argc, &argv);
MPI_Comm_size (MPI_COMM_WORLD, &numtasks);
MPI_Comm_rank (MPI_COMM_WORLD, &rank);


if (rank == 0) {

gettimeofday("&tv0, NULL); for(i=0;i<1000;i++){

  MPI_Send (buf, 10240, MPI_INT, 1, tag, MPI_COMM_WORLD);
  MPI_Recv (buf, 10240, MPI_INT, 1, tag,MPI_COMM_WORLD, &Stat);
  }
  gettimeofday (&tv1, NULL);
}else{
 gettimeofday(&tv0, NULL);
 for(i=0;i<1000;i++){
   MPI_Recv(buf, 10240,MPI_INT, 0, tag, MPI_COMM_WORLD, &Stat);
   MPI_Send(buf, 10240, MPI_INT, 1, tag, MPI_COMM_WORLD);
 }
 gettimeofday(&tv1, NULL);
}

totaltime = (tv1.tv_sec - tv0.tv_sec) *  1000000 + (tv1.tv_usec - tv0.tv_usec);
fprintf (stdout, "rank %d with total time is %d",rank, totaltime);
}

MPI_Finalize ();

return 0;

} =======================================================

I run it :
 mpirun -np 2 --host node2 ./a.out

But the result of time is not stable with difference of 100 times. For example, 
the max value of time can be 3000, meanwhile the min is 100.

Is there anything wrong ?
I am using 1.4.2 and openib.






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>> Mark J. Potts, PhD
>>
>> HPC Applications Inc.
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