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Man Pages


Manual Reference Pages  -  MPI_ALLREDUCE (3)

NAME

MPI_Allreduce, MPI_Iallreduce - Combines values from all processes and distributes the result back to all processes.

CONTENTS

Syntax
C Syntax
Fortran Syntax
C++ Syntax
Input Parameters
Output Parameters
Description
Use Of In-place Option
When Communicator Is An Inter-communicator
Notes On Collective Operations
Errors

SYNTAX

C Syntax

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

int MPI_Iallreduce(const void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype, MPI_Op op, MPI_Comm comm, MPI_Request *request)

Fortran Syntax

INCLUDE ’mpif.h’
MPI_ALLREDUCE(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
        <type>  SENDBUF(*), RECVBUF(*)
        INTEGER COUNT, DATATYPE, OP, COMM, IERROR

MPI_IALLREDUCE(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, REQUEST, IERROR)         <type>  SENDBUF(*), RECVBUF(*)         INTEGER COUNT, DATATYPE, OP, COMM, REQUEST, IERROR

C++ Syntax

#include <mpi.h>
void MPI::Comm::Allreduce(const void* sendbuf, void* recvbuf,
        int count, const MPI::Datatype& datatype, const
        MPI::Op& op) const=0

INPUT PARAMETERS

sendbuf Starting address of send buffer (choice).
count Number of elements in send buffer (integer).
datatype Datatype of elements of send buffer (handle).
op Operation (handle).
comm Communicator (handle).

OUTPUT PARAMETERS

recvbuf Starting address of receive buffer (choice).
request Request (handle, non-blocking only).
IERROR Fortran only: Error status (integer).

DESCRIPTION

Same as MPI_Reduce except that the result appears in the receive buffer of all the group members.

Example 1: A routine that computes the product of a vector and an array that are distributed across a group of processes and returns the answer at all nodes (compare with Example 2, with MPI_Reduce, below).

SUBROUTINE PAR_BLAS2(m, n, a, b, c, comm)
REAL a(m), b(m,n)    ! local slice of array
REAL c(n)            ! result
REAL sum(n)
INTEGER n, comm, i, j, ierr

! local sum DO j= 1, n sum(j) = 0.0 DO i = 1, m sum(j) = sum(j) + a(i)*b(i,j) END DO END DO

! global sum CALL MPI_ALLREDUCE(sum, c, n, MPI_REAL, MPI_SUM, comm, ierr)

! return result at all nodes RETURN

Example 2: A routine that computes the product of a vector and an array that are distributed across a group of processes and returns the answer at node zero.

SUBROUTINE PAR_BLAS2(m, n, a, b, c, comm)
REAL a(m), b(m,n)    ! local slice of array
REAL c(n)            ! result
REAL sum(n)
INTEGER n, comm, i, j, ierr

! local sum DO j= 1, n sum(j) = 0.0 DO i = 1, m sum(j) = sum(j) + a(i)*b(i,j) END DO END DO

! global sum CALL MPI_REDUCE(sum, c, n, MPI_REAL, MPI_SUM, 0, comm, ierr)

! return result at node zero (and garbage at the other nodes) RETURN

USE OF IN-PLACE OPTION

When the communicator is an intracommunicator, you can perform an all-reduce operation in-place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of sendbuf at all processes.

Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM.

Because the in-place option converts the receive buffer into a send-and-receive buffer, a Fortran binding that includes INTENT must mark these as INOUT, not OUT.

WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

When the communicator is an inter-communicator, the reduce operation occurs in two phases. The data is reduced from all the members of the first group and received by all the members of the second group. Then the data is reduced from all the members of the second group and received by all the members of the first. The operation exhibits a symmetric, full-duplex behavior.

When the communicator is an intra-communicator, these groups are the same, and the operation occurs in a single phase.

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.

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.

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1.10.2 MPI_ALLREDUCE (3) Jan 21, 2016

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