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MPI_Cart_shift - Returns the shifted source and destination ranks,
given a shift direction and amount.
#include <mpi.h>
int MPI_Cart_shift(MPI_Comm comm, int direction, int disp,
int *rank_source, int *rank_dest)
INCLUDE ’mpif.h’
MPI_CART_SHIFT(COMM, DIRECTION, DISP, RANK_SOURCE,
RANK_DEST, IERROR)
INTEGER COMM, DIRECTION, DISP, RANK_SOURCE
INTEGER RANK_DEST, IERROR
#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).
- rank_source
- Rank
of source process (integer).
- rank_dest
- Rank of destination process (integer).
- IERROR
- Fortran only: Error status (integer).
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)
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].
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.
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