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
- 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
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.
USE OF IN-PLACE OPTION¶
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 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 argument, 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++ 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.