DeinoMPI

The Great and Terrible implementation of MPI-2

MPI_Cart_shift

Returns the shifted source and destination ranks, given a shift direction and amount

int MPI_Cart_shift(
  MPI_Comm comm,
  int direction,
  int displ,
  int *source,
  int *dest
);

Parameters

comm: [in] communicator with cartesian structure (handle)
direction: [in] coordinate dimension of shift (integer)
displ: [in] displacement (> 0: upwards shift, < 0: downwards shift) (integer)
source: [out] rank of source process (integer)
dest: [out] rank of destination process (integer)

Remarks

If the process topology is a cartesian structure, a MPI_SENDRECV operation is likely to be used along a coordinate direction to perform a shift of data. As input, MPI_SENDRECV takes the rank of a source process for the receive, and the rank of a destination process for the send. If the function MPI_CART_SHIFT is called for a cartesian process group, it provides the calling process with the above identifiers, which then can be passed to MPI_SENDRECV. The user specifies the coordinate direction and the size of the step (positive or negative). The function is local.

The direction argument indicates the dimension of the shift, i.e., the coordinate which value is modified by the shift. The coordinates are numbered from 0 to ndims-1, when ndims is the number of dimensions.

Depending on the periodicity of the cartesian group in the specified coordinate direction, MPI_CART_SHIFT provides the identifiers for a circular or an end-off shift. In the case of an end-off shift, the value MPI_PROC_NULL may be returned in rank_source or rank_dest, indicating that the source or the destination for the shift is out of range.

Advice to users.

In Fortran, the dimension indicated by DIRECTION = i has DIMS(i+1) nodes, where DIMS is the array that was used to create the grid. In C, the dimension indicated by direction = i is the dimension specified by dims[i].

Thread and Interrupt Safety

This routine is both thread- and interrupt-safe. This means that this routine may safely be used by multiple threads and from within a signal handler.

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.

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_TOPOLOGY: Invalid topology. Either there is no topology associated with this communicator, or it is not the correct type (e.g., MPI_CART when expecting MPI_GRAPH).

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_ARG: Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

Example Code

The following sample code illustrates MPI_Cart_shift.

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

int main( int argc, char *argv[] )
{
    int errs = 0;
    int size, rank;
    int source, dest;
    int dims[2], periods[2];
    MPI_Comm comm;

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

    dims[0] = size;
    periods[0] = 1;
    MPI_Cart_create( MPI_COMM_WORLD, 1, dims, periods, 0, &comm );
    MPI_Cart_shift( comm, 0, 1, &source, &dest );
    if (source != ((rank - 1 + size) % size)) {
        errs++;
        printf( "source for shift 1 is %d\n", source );fflush(stdout);
    }
    if (dest != ((rank + 1) % size)) {
        errs++;
        printf( "dest for shift 1 is %d\n", dest );fflush(stdout);
    }
    MPI_Cart_shift( comm, 0, 0, &source, &dest );
    if (source != rank) {
        errs++;
        printf( "Source for shift 0 is %d\n", source );fflush(stdout);
    }
    if (dest != rank) {
        errs++;
        printf( "Dest for shift 0 is %d\n", dest );fflush(stdout);
    }
    MPI_Cart_shift( comm, 0, -1, &source, &dest );
    if (source != ((rank + 1) % size)) {
        errs++;
        printf( "source for shift -1 is %d\n", source );fflush(stdout);
    }
    if (dest != ((rank - 1 + size) % size)) {
        errs++;
        printf( "dest for shift -1 is %d\n", dest );fflush(stdout);
    }

    /* Now, with non-periodic */
    MPI_Comm_free( &comm );
    periods[0] = 0;
    MPI_Cart_create( MPI_COMM_WORLD, 1, dims, periods, 0, &comm );
    MPI_Cart_shift( comm, 0, 1, &source, &dest );
    if ((rank > 0 && source != (rank - 1)) || (rank == 0 && source != MPI_PROC_NULL)) {
        errs++;
        printf( "source for non-periodic shift 1 is %d\n", source );fflush(stdout);
    }
    if ((rank < size-1 && dest != rank + 1) || ((rank == size-1) && dest != MPI_PROC_NULL)) {
        errs++;
        printf( "dest for non-periodic shift 1 is %d\n", dest );fflush(stdout);
    }
    MPI_Cart_shift( comm, 0, 0, &source, &dest );
    if (source != rank) {
        errs++;
        printf( "Source for non-periodic shift 0 is %d\n", source );fflush(stdout);
    }
    if (dest != rank) {
        errs++;
        printf( "Dest for non-periodic shift 0 is %d\n", dest );fflush(stdout);
    }
    MPI_Cart_shift( comm, 0, -1, &source, &dest );
    if ((rank < size - 1 && source != rank + 1) || (rank == size - 1 && source != MPI_PROC_NULL)) {
        errs++;
        printf( "source for non-periodic shift -1 is %d\n", source );fflush(stdout);
    }
    if ((rank > 0 && dest != rank - 1) || (rank == 0 && dest != MPI_PROC_NULL)) {
        errs++;
        printf( "dest for non-periodic shift -1 is %d\n", dest );fflush(stdout);
    }
    MPI_Comm_free( &comm );

    MPI_Finalize();
    return 0;
}

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