DeinoMPI

The Great and Terrible implementation of MPI-2

MPI_Intercomm_merge

Creates an intracommuncator from an intercommunicator

int MPI_Intercomm_merge(
  MPI_Comm intercomm,
  int high,
  MPI_Comm *newintracomm
);

Parameters

comm: [in] Intercommunicator (handle)
high: [in] Used to order the groups within comm (logical) when creating the new communicator. This is a boolean value; the group that sets high true has its processes ordered after the group that sets this value to false. If all processes in the intercommunicator provide the same value, the choice of which group is ordered first is arbitrary.
comm_out: [out] Created intracommunicator (handle)

Remarks

This function creates an intra-communicator from the union of the two groups that are associated with intercomm. All processes should provide the same high value within each of the two groups. If processes in one group provided the value high = false and processes in the other group provided the value high = true then the union orders the "low" group before the "high" group. If all processes provided the same high argument then the order of the union is arbitrary. This call is blocking and collective within the union of the two groups.

While all processes may provide the same value for the high parameter, this requires the MPI implementation to determine which group of processes should be ranked first. The MPICH implementation uses various techniques to determine which group should go first, but there is a possibility that the implementation will be unable to break the tie. Robust applications should avoid using the same value for high in both groups.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as mallocor other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierris an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGERin Fortran.

Algorithm

s Allocate contexts Local and remote group leaders swap high values Determine the high value. Merge the two groups and make the intra-communicator e

Errors

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS: No error; MPI routine completed successfully.

MPI_ERR_COMM: Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).

MPI_ERR_INTERN: This error is returned when some part of the MPICH implementation is unable to acquire memory.

Example Code

The following sample code illustrates MPI_Intercomm_merge.

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

/* Test intercomm merge, including the choice of the high value */

void GetIntercomm( MPI_Comm *comm, int *isLeftGroup, int min_size )
{
    int size, rank, remsize, merr;
    int done=0;
    MPI_Comm mcomm;
    int rleader;

    /* Split comm world in half */
    MPI_Comm_rank( MPI_COMM_WORLD, &rank );
    MPI_Comm_size( MPI_COMM_WORLD, &size );
    if (size > 1) {
        merr = MPI_Comm_split( MPI_COMM_WORLD, (rank < size/2), rank, &mcomm );
        if (rank == 0) {
            rleader = size/2;
        }
        else if (rank == size/2) {
            rleader = 0;
        }
        else {
            /* Remote leader is signficant only for the processes designated local leaders */
            rleader = -1;
        }
        *isLeftGroup = rank < size/2;
        merr = MPI_Intercomm_create( mcomm, 0, MPI_COMM_WORLD, rleader, 12345, comm );
        merr = MPI_Comm_free( &mcomm );
    }
    else
        *comm = MPI_COMM_NULL;
}

int main( int argc, char *argv[] )
{
    int errs = 0;
    int rank, size, rsize;
    int nsize, nrank;
    int minsize = 2;
    int isLeft;
    MPI_Comm comm, comm1, comm2, comm3, comm4;

    MPI_Init( &argc, &argv );

    GetIntercomm( &comm, &isLeft, minsize ))
    /* Determine the sender and receiver */
    MPI_Comm_rank( comm, &rank );
    MPI_Comm_remote_size( comm, &rsize );
    MPI_Comm_size( comm, &size );

    /* Try building intercomms */
    MPI_Intercomm_merge( comm, isLeft, &comm1 );
    /* Check the size and ranks */
    MPI_Comm_size( comm1, &nsize );
    MPI_Comm_rank( comm1, &nrank );
    if (nsize != size + rsize) {
        errs++;
        printf( "(1) Comm size is %d but should be %d\n", nsize, size + rsize );fflush(stdout);
        if (isLeft) {
            /* The left processes should be high */
            if (nrank != rsize + rank) {
                errs++;
                printf( "(1) rank for high process is %d should be %d\n", nrank, rsize + rank );fflush(stdout);
            }
        }
        else {
            /* The right processes should be low */
            if (nrank != rank) {
                errs++;
                printf( "(1) rank for low process is %d should be %d\n", nrank, rank );fflush(stdout);
            }
        }
    }

    MPI_Intercomm_merge( comm, !isLeft, &comm2 );
    /* Check the size and ranks */
    MPI_Comm_size( comm1, &nsize );
    MPI_Comm_rank( comm1, &nrank );
    if (nsize != size + rsize) {
        errs++;
        printf( "(2) Comm size is %d but should be %d\n", nsize, size + rsize );fflush(stdout);
        if (!isLeft) {
            /* The right processes should be high */
            if (nrank != rsize + rank) {
                errs++;
                printf( "(2) rank for high process is %d should be %d\n", nrank, rsize + rank );fflush(stdout);
            }
        }
        else {
            /* The left processes should be low */
            if (nrank != rank) {
                errs++;
                printf( "(2) rank for low process is %d should be %d\n", nrank, rank );fflush(stdout);
            }
        }
    }

    MPI_Intercomm_merge( comm, 0, &comm3 );
    MPI_Intercomm_merge( comm, 1, &comm4 );

    MPI_Comm_free( &comm1 );
    MPI_Comm_free( &comm2 );
    MPI_Comm_free( &comm3 );
    MPI_Comm_free( &comm4 );

    MPI_Finalize();
    return errs;
}

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