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MPI_Reduce_scatter, MPI_Ireduce_scatter - 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)
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
#include <mpi.h>
void MPI::Comm::Reduce_scatter(const void* sendbuf, void* recvbuf,
int recvcounts[], const MPI::Datatype& datatype,
const MPI::Op& op) const
- 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).
- recvbuf
- Starting address of receive
buffer (choice).
- request
- Request (handle, non-blocking only).
- IERROR
- Fortran
only: Error status (integer).
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 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.
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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