Manpages - MPI_Allreduce.3

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

  #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)

  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

  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

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

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).

recvbuf

Starting address of receive buffer (choice).

request

Request (handle, non-blocking only).

IERROR

Fortran only: Error status (integer).

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

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 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.

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.

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.