NAME¶
MPI_Cart_shift - Returns the shifted source and destination ranks, given
a shift direction and amount.
SYNTAX¶
C Syntax¶
#include <mpi.h>
int MPI_Cart_shift(MPI_Comm comm, int direction, int disp,
int *rank_source, int *rank_dest)
Fortran Syntax¶
INCLUDE 'mpif.h'
MPI_CART_SHIFT( COMM, DIRECTION, DISP, RANK_SOURCE,
RANK_DEST, IERROR)
INTEGER COMM, DIRECTION, DISP, RANK_SOURCE
INTEGER RANK_DEST, IERROR
C++ Syntax¶
#include <mpi.h>
void Cartcomm::Shift(int direction, int disp, int& rank_source,
int& rank_dest) const
- comm
- Communicator with Cartesian structure (handle).
- direction
- Coordinate dimension of shift (integer).
- disp
- Displacement ( > 0: upward shift, < 0: downward
shift) (integer).
OUTPUT PARAMETERS¶
- rank_source
- Rank of source process (integer).
- rank_dest
- Rank of destination process (integer).
- IERROR
- Fortran only: Error status (integer).
DESCRIPTION¶
If the process topology is a Cartesian structure, an 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 whose value is modified by the shift. The coordinates are numbered
from 0 to ndims-1, where ndims is the number of dimensions.
Note: The direction argument is in the range [0, n-1] for an
n-dimensional Cartesian mesh.
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.
Example: The communicator, comm, has a two-dimensional, periodic,
Cartesian topology associated with it. A two-dimensional array of REALs is
stored one element per process, in variable A. One wishes to skew this array,
by shifting column i (vertically, i.e., along the column) by i steps.
....
C find process rank
CALL MPI_COMM_RANK(comm, rank, ierr))
C find Cartesian coordinates
CALL MPI_CART_COORDS(comm, rank, maxdims, coords,
ierr)
C compute shift source and destination
CALL MPI_CART_SHIFT(comm, 0, coords(2), source,
dest, ierr)
C skew array
CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, dest, 0,
source, 0, comm, status,
ierr)
NOTE¶
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].
ERRORS¶
Almost all MPI routines return an error value; C routines as the value of the
function and Fortran routines in the last argument. C++ functions do not
return errors. If the default error handler is set to
MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will
be used to throw an MPI:Exception object.
Before the error value is returned, the current MPI error handler is called. By
default, this error handler aborts the MPI job, except for I/O function
errors. The error handler may be changed with MPI_Comm_set_errhandler; the
predefined error handler MPI_ERRORS_RETURN may be used to cause error values
to be returned. Note that MPI does not guarantee that an MPI program can
continue past an error.