MPI_ALLREDUCE, MPI_Allreduce

Purpose

Applies a reduction operation to the vector sendbuf over the set of tasks
specified by comm and places the result in recvbuf on all of the tasks in
comm.

C synopsis

#include <mpi.h>
int MPI_Allreduce(void* sendbuf,void* recvbuf,int count,
                  MPI_Datatype datatype,MPI_Op op,MPI_Comm comm);

C++ synopsis

#include mpi.h
void MPI::Comm::Allreduce(const void* sendbuf, void* recvbuf, int count,
                          const MPI::Datatype& datatype, const MPI::Op& op)
                          const;

FORTRAN synopsis

include 'mpif.h' or use mpi
MPI_ALLREDUCE(CHOICE SENDBUF,CHOICE RECVBUF,INTEGER COUNT,
              INTEGER DATATYPE,INTEGER OP,INTEGER COMM,INTEGER IERROR)

Description

This subroutine applies a reduction operation to the vector sendbuf over
the set of tasks specified by comm and places the result in recvbuf on all
of the tasks.

This subroutine is similar to MPI_REDUCE except the result is returned to
the receive buffer of all the group members.

The "in place" option for intracommunicators is specified by passing the
value MPI_IN_PLACE to the argument sendbuf at the root. In this case, the
input data is taken at each task from the receive buffer, where it will be
replaced by the output data.

If comm is an intercommunicator, the result of the reduction of the data
provided by tasks in group A is stored at each task in group B, and vice
versa. Both groups should provide the same count value.

MPI_IN_PLACE is not supported for intercommunicators.

When you use this subroutine in a threads application, make sure all
collective operations on a particular communicator occur in the same order
at each task. See IBM Parallel Environment for AIX: MPI Programming Guide
for more information on programming with MPI in a threads environment.

Parameters

sendbuf
     is the starting address of the send buffer (choice) (IN)
recvbuf
     is the starting address of the receive buffer (choice) (OUT)
count
     is the number of elements in the send buffer (integer) (IN)
datatype
     is the datatype of elements in the send buffer (handle) (IN)
op
     is the reduction operation (handle) (IN)
comm
     is the communicator (handle) (IN)
IERROR
     is the FORTRAN return code. It is always the last argument.

Notes

See IBM Parallel Environment for AIX: MPI Programming Guide for information
about reduction functions.

The MPI standard urges MPI implementations to use the same evaluation order
for reductions every time, even if this negatively affects performance. PE
MPI adjusts its reduce algorithms for the optimal performance on a given
task distribution. The MPI standard suggests, but does not mandate, this
sacrifice of performance. PE MPI chooses to put performance ahead of the
MPI standard's recommendation. This means that two runs with the same task
count may produce results that differ in the least significant bits, due to
rounding effects when evaluation order changes. Two runs that use the same
task count and the same distribution across nodes will always give
identical results.

In the 64-bit library, this function uses a shared memory optimization
among the tasks on a node. This optimization is discussed in the chapter
"Using shared memory" of IBM Parallel Environment for AIX: MPI Programming
Guide, and is enabled by default. This optimization is not available to
32-bit programs.

Errors

Fatal errors:

Invalid count
     count < 0

Invalid datatype

Type not committed

Invalid op

Invalid communicator

Unequal message lengths

Invalid use of MPI_IN_PLACE

MPI not initialized

MPI already finalized

Develop mode error if:

Inconsistent op

Inconsistent datatype

Inconsistent message length

Related information

     MPE_IALLREDUCE
     MPI_OP_CREATE
     MPI_REDUCE
     MPI_REDUCE_SCATTER