#include <mpi.h>
int MPI_Reduce_scatter(void *sendbuf, void *recvbuf, int *recvcounts,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
INCLUDE 'mpif.h'
MPI_REDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, 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
MPI_Reduce_scatter first does an element-wise reduction on vector of count
= S(i)revcounts[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 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.
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.