Provided by: openmpi-doc_4.1.6-7ubuntu2_all 
NAME
MPI_Allreduce, MPI_Iallreduce - Combines values from all processes and distributes the
result back to all processes.
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
USE MPI
! or the older form: 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
Fortran 2008 Syntax
USE mpi_f08
MPI_Allreduce(sendbuf, recvbuf, count, datatype, op, comm, ierror)
TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
TYPE(*), DIMENSION(..) :: recvbuf
INTEGER, INTENT(IN) :: count
TYPE(MPI_Datatype), INTENT(IN) :: datatype
TYPE(MPI_Op), INTENT(IN) :: op
TYPE(MPI_Comm), INTENT(IN) :: comm
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
MPI_Iallreduce(sendbuf, recvbuf, count, datatype, op, comm, request,
ierror)
TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
INTEGER, INTENT(IN) :: count
TYPE(MPI_Datatype), INTENT(IN) :: datatype
TYPE(MPI_Op), INTENT(IN) :: op
TYPE(MPI_Comm), INTENT(IN) :: comm
TYPE(MPI_Request), INTENT(OUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: 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.