:ref:`MPI_Reduce_scatter`, :ref:`MPI_Ireduce_scatter`, :ref:`MPI_Reduce_scatter_init` - Combines values and scatters the results.
#include <mpi.h>
int MPI_Reduce_scatter(const void *sendbuf, void *recvbuf, const int recvcounts[],
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
int MPI_Ireduce_scatter(const void *sendbuf, void *recvbuf, const int recvcounts[],
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm, MPI_Request *request)
int MPI_Reduce_scatter_init(const void *sendbuf, void *recvbuf,
const int recvcounts[], MPI_Datatype datatype, MPI_Op op,
MPI_Comm comm, MPI_Info info, MPI_Request *request)
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_REDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, IERROR
MPI_IREDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
COMM, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, REQUEST, IERROR
MPI_REDUCE_SCATTER_INIT(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
COMM, INFO, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, INFO, REQUEST, IERROR
USE mpi_f08
MPI_Reduce_scatter(sendbuf, recvbuf, recvcounts, datatype, op, comm,
ierror)
TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
TYPE(*), DIMENSION(..) :: recvbuf
INTEGER, INTENT(IN) :: recvcounts(*)
TYPE(MPI_Datatype), INTENT(IN) :: datatype
TYPE(MPI_Op), INTENT(IN) :: op
TYPE(MPI_Comm), INTENT(IN) :: comm
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Ireduce_scatter(sendbuf, recvbuf, recvcounts, datatype, op, comm,
request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*)
TYPE(MPI_Datatype), INTENT(IN) :: datatype
TYPE(MPI_Op), INTENT(IN) :: op
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Reduce_scatter_init(sendbuf, recvbuf, recvcounts, datatype, op, comm,
info, request, ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*)
TYPE(MPI_Datatype), INTENT(IN) :: datatype
TYPE(MPI_Op), INTENT(IN) :: op
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Info), INTENT(IN) :: info
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
sendbuf
: Starting address of send buffer (choice).recvcounts
: Integer array specifying the number of elements in result distributed to each process. Array must be identical on all calling processes.datatype
: Datatype of elements of input buffer (handle).op
: Operation (handle).comm
: Communicator (handle).info
: Info (handle, persistent).
recvbuf
: Starting address of receive buffer (choice).request
: Request (handle, non-blocking only).ierror
: Fortran only: Error status (integer).
:ref:`MPI_Reduce_scatter` first does an element-wise reduction on vector of count = S(i)recvcounts[i] elements in the send buffer defined by sendbuf, count, and datatype. Next, the resulting vector of results is split into n disjoint segments, where n is the number of processes in the group. Segment i contains recvcounts[i] elements. The ith segment is sent to process i and stored in the receive buffer defined by recvbuf, recvcounts[i], and datatype.
When the communicator is an intracommunicator, you can perform a reduce-scatter operation in-place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of the sendbuf. In this case, the input data is taken from the top of the receive buffer. The area occupied by the input data may be either longer or shorter than the data filled by the output data.
When the communicator is an inter-communicator, the reduce-scatter operation occurs in two phases. First, the result of the reduction performed on the data provided by the processes in the first group is scattered among the processes in the second group. Then the reverse occurs: the reduction performed on the data provided by the processes in the second group is scattered among the processes in the first group. For each group, all processes provide the same recvcounts argument, and the sum of the recvcounts values should be the same for both groups.
The reduction functions ( MPI_Op ) do not return an error value. As a result, if the functions detect an error, all they can do is either call :ref:`MPI_Abort` or silently skip the problem. Thus, if you change the error handler from MPI_ERRORS_ARE_FATAL to something else, for example, MPI_ERRORS_RETURN , then no error may be indicated.
The reason for this is the performance problems in ensuring that all collective routines return the same error value.