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Subject: Re: [OMPI users] Help!
From: guosong (guosong1079_at_[hidden])
Date: 2009-09-03 21:17:53


Thanks. I thought those threads will not interfere with each other since they have no common variables except std IO.
 

Date: Thu, 3 Sep 2009 09:01:28 -0700
From: Eugene.Loh_at_[hidden]
To: users_at_[hidden]
Subject: Re: [OMPI users] Help!

guosong wrote:

I am new to open MPI
I am not, but I'm not real familiar with thread-safe MPI programming. Still, I saw no other replies to your question, so I'll make an attempt to answer.

MPI does not guarantee thread safety. E.g., see http://www.mpi-forum.org/docs/mpi-20-html/node162.htm#Node162 and the ensuing pages.

To do what you want to do, you need to make sure you have thread safety. There is a standard MPI interface for doing so. I'm not good at C++, so here is a C example:

#include <stdio.h>
#include <mpi.h>

int main(int argc, char **argv) {
  int provided;

  /* start MPI, asking for support for multiple threads */
  MPI_Init_thread(&argc,&argv,MPI_THREAD_MULTIPLE,&provided);

  /* report what level of support is actually provided */
  if ( MPI_THREAD_SINGLE == provided ) printf(" MPI_THREAD_SINGLE\n");
  if ( MPI_THREAD_FUNNELED == provided ) printf(" MPI_THREAD_FUNNELED\n");
  if ( MPI_THREAD_SERIALIZED == provided ) printf(" MPI_THREAD_SERIALIZED\n");
  if ( MPI_THREAD_MULTIPLE == provided ) printf(" MPI_THREAD_MULTIPLE\n");

  /* exit */
  MPI_Finalize();

  return 0;
}

Run this program. If the program does not print out "MPI_THREAD_MULTIPLE", you're not getting the level of thread support you require and your program is not guaranteed to run properly. You have to make sure you're using a thread-safe MPI before you can expect your program to work.

The next question is whether Open MPI is thread safe, and I'll leave you with this:
http://www.open-mpi.org/faq/?category=supported-systems#thread-support

Good luck. Again, I'm not experienced with thread-safe MPI programming (though I understand the general concepts).

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