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NAME
MPI_Reduce_scatter - Combines values and scatters the results.
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Reduce_scatter(void *sendbuf, void *recvbuf, int *recvcounts,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
Fortran Syntax
INCLUDE 'mpif.h'
MPI_REDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, IERROR
C++ Syntax
#include <mpi.h>
void MPI::Comm::Reduce_scatter(const void* sendbuf, void* recvbuf,
int recvcounts[], const MPI::Datatype& datatype,
const MPI::Op& op) const
INPUT PARAMETERS
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).
OUTPUT PARAMETERS
recvbuf Starting address of receive buffer (choice).
IERROR Fortran only: Error status (integer).
DESCRIPTION
MPI_Reduce_scatter first does an element-wise reduction on vector of
count = S(i)revcounts[i] elements in the send buffer defined by send-
buf, 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 COMMUNICATOR IS AN INTER-COMMUNICATOR
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 argu-
ment, and the sum of the recvcounts values should be the same for both
groups.
NOTES ON COLLECTIVE OPERATIONS
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 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.
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++ func-
tions 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.
1.3.4 Nov 11, 2009 MPI_Reduce_scatter(3)
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