|
« Return to documentation listing
Table of Contents
MPI_Reduce_scatter_block, MPI_Ireduce_scatter_block - Combines
values and scatters the results in blocks.
#include <mpi.h>
int MPI_Reduce_scatter_block(const void *sendbuf, void *recvbuf, int recvcount,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
int MPI_Ireduce_scatter_block(const void *sendbuf, void *recvbuf, int recvcount,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm, MPI_Request *request)
INCLUDE ’mpif.h’
MPI_REDUCE_SCATTER_BLOCK(SENDBUF, RECVBUF, RECVCOUNT, DATATYPE, OP,
COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER RECVCOUNT, DATATYPE, OP, COMM, IERROR
MPI_IREDUCE_SCATTER_BLOCK(SENDBUF, RECVBUF, RECVCOUNT, DATATYPE, OP,
COMM, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER RECVCOUNT, DATATYPE, OP, COMM, REQUEST, IERROR
- sendbuf
- Starting address of send buffer (choice).
- recvcount
- lement count per block (non-negative integer).
- 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_block first does an element-wise reduction
on vector of count = n * revcount elements in the send buffer defined by
sendbuf, count, and
datatype, using the operation op, where n is the number of processes in
the group of comm. Next, the resulting vector of results is split into n
disjoint segments, where n is the number of processes in the group. Each
segments contains recvcount elements. The ith segment is sent to process
i and stored in the receive buffer defined by recvbuf, recvcount, 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.
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.
MPI_Reduce_scatter
Table of Contents
« Return to documentation listing
|
