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Subject: Re: [OMPI users] random MPI_UNIVERSE_SIZE and inter-commicator created by MPI_Comm_spawn
From: Mathieu Gontier (mg.mailing-list_at_[hidden])
Date: 2010-02-19 01:34:09


Thank you very much for you well argumented answer. Indeed, I missed something because I used MPI_Comm_remote_size in my spawned application.


George Bosilca wrote:

Your MPI_COMM_UNIVERSE is a inter-communicator, and therefore the MPI_Comm_size and MPI_Comm_rank return the size, respectively the rank, in the local group. There is a special accessor for getting the remote group size (MPI_Comm_remote_size).

Now regarding the previous question (about the attribute MPI_UNIVERSE_SIZE), your answer is in the MPI standard 2.2 page 346 line 35. The MPI_UNIVERSE_SIZE is not set in Open MPI.


On Feb 18, 2010, at 10:18 , Mathieu Gontier wrote:

Another question on the same example.

When I ask the size on the inter-communitator (MPI_COMM_UNIVERSE in the example) between the spaner/parent and the spawned/children processes, the same number of processes than in MPI_COMM_WORLD is returned. I do not really understand because I expected my MPI_COMM_UNIVERSE be a higher communicator including here the group of MPI_COMM_SELF and the group of MPI_COMM_WORLD of my spawned application (./worker).
I think I missed something. Does someone can help me?

Thank you.

Mathieu Gontier wrote:

I am trying to use MPI_Comm_spawn (MPI-2 standard only) and I have an problem when I use MPI_UNIVERSE_SIZE. Here my code:

int main( int argc, char *argv[] )
 int wsize=0, wrank=-1 ;
 int usize=0, urank=-1 ;
 int ier ;
 int usize_attr=0, flag=0 ;
 ier = MPI_Init( &argc, &argv ) ;
 ier = MPI_Comm_size( MPI_COMM_WORLD, &wsize ) ;
 ier = MPI_Comm_rank( MPI_COMM_WORLD, &wrank ) ;
 ier = MPI_Comm_get_attr( MPI_COMM_WORLD, MPI_UNIVERSE_SIZE, (void*) &usize_attr, &flag ) ;
 if ( !flag ) { printf( "[manager][error] This MPI does not support MPI_UNIVERSE_SIZE.\n" ) ; return 1 ; }
 if( wrank==0 ) {
     ier = MPI_Comm_spawn( "./worker", argv, ( wsize/2 > 2 ? wsize/2 : 2 ), MPI_INFO_NULL, 0, MPI_COMM_SELF, &MPI_COMM_UNIVERSE, MPI_ERRCODES_IGNORE);
     ier = MPI_Comm_size( MPI_COMM_UNIVERSE, &usize ) ;
     ier = MPI_Comm_rank( MPI_COMM_UNIVERSE, &urank ) ;
 printf( "[manager] I am the process %d on %d in world (universe attr size: %d) and  %d on %d in universe\n", wrank, wsize, usize_attr, urank, usize ) ;
 return 0;

When I call MPI_Comm_get_attr() with MPI_UNIVERSE_SIZE, my variable u_size_attr is random. Any idea?

Thank you.

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