MPI_ALLREDUCE(3)				     Open MPI					  MPI_ALLREDUCE(3)

MPI_Allreduce,	MPI_Iallreduce	<#mpi-iallreduce>, MPI_Allreduce_init <#mpi-allreduce-init> - 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)

	  int MPI_Allreduce_init(const void *sendbuf, void *recvbuf, int count,
				 MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
				 MPI_Info info, 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

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

	  MPI_Allreduce_init(sendbuf, recvbuf, count, datatype, op, comm, info, 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_Info), INTENT(IN) :: info
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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).

       • info: Info (handle, persistent only).

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 <#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 <#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 <#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 return result of the function and  Fortran
       routines in the last argument.

       Before  the error value is returned, the current MPI error handler associated with the communication object
       (e.g., communicator, window, file) is called.  If no communication object is associated with the MPI  call,
       then  the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When
       MPI_COMM_SELF is not initialized (i.e.,	before	MPI_Init  <#mpi-init>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#mpi-finalize>,  or  when	using the Sessions Model exclusively) the error raises the
       initial error handler. The initial error handler can  be	 changed  by  calling  MPI_Comm_set_errhandler	<#
       mpi-comm-set-errhandler>	 on  MPI_COMM_SELF  when  using the World model, or the mpi_initial_errhandler CLI
       argument	 to  mpiexec  or  info	key   to   MPI_Comm_spawn   <#mpi-comm-spawn>/MPI_Comm_spawn_multiple	<#
       mpi-comm-spawn-multiple>.   If  no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
       error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error  handler  is  called  for  all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT  An  error handler that can be invoked on a communicator, window, file, or session. When
	 called on a communicator, it acts as if MPI_Abort <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the  group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					  MPI_ALLREDUCE(3)
