306 lines
9.5 KiB
Groff
306 lines
9.5 KiB
Groff
.\" Man page generated from reStructuredText.
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.TH "MPI_ALLREDUCE" "3" "Feb 14, 2025" "" "Open MPI"
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.sp
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\fI\%MPI_Allreduce\fP, \fI\%MPI_Iallreduce\fP, \fI\%MPI_Allreduce_init\fP \- Combines values
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from all processes and distributes the result back to all processes.
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.SH SYNTAX
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.SS C Syntax
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.INDENT 0.0
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.INDENT 3.5
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.sp
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.nf
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.ft C
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#include <mpi.h>
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int MPI_Allreduce(const void *sendbuf, void *recvbuf, int count,
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MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
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int MPI_Iallreduce(const void *sendbuf, void *recvbuf, int count,
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MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
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MPI_Request *request)
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int MPI_Allreduce_init(const void *sendbuf, void *recvbuf, int count,
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MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
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MPI_Info info, MPI_Request *request)
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.ft P
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.fi
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.UNINDENT
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.UNINDENT
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.SS Fortran Syntax
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.INDENT 0.0
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.INDENT 3.5
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.sp
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.nf
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.ft C
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USE MPI
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! or the older form: INCLUDE \(aqmpif.h\(aq
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MPI_ALLREDUCE(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
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<type> SENDBUF(*), RECVBUF(*)
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INTEGER COUNT, DATATYPE, OP, COMM, IERROR
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MPI_IALLREDUCE(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, REQUEST, IERROR)
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<type> SENDBUF(*), RECVBUF(*)
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INTEGER COUNT, DATATYPE, OP, COMM, REQUEST, IERROR
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MPI_ALLREDUCE_INIT(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, INFO, REQUEST, IERROR)
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<type> SENDBUF(*), RECVBUF(*)
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INTEGER COUNT, DATATYPE, OP, COMM, INFO, REQUEST, IERROR
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.ft P
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.fi
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.UNINDENT
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.UNINDENT
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.SS Fortran 2008 Syntax
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.INDENT 0.0
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.INDENT 3.5
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.sp
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.nf
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.ft C
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USE mpi_f08
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MPI_Allreduce(sendbuf, recvbuf, count, datatype, op, comm, ierror)
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TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
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TYPE(*), DIMENSION(..) :: recvbuf
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INTEGER, INTENT(IN) :: count
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TYPE(MPI_Datatype), INTENT(IN) :: datatype
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TYPE(MPI_Op), INTENT(IN) :: op
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TYPE(MPI_Comm), INTENT(IN) :: comm
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INTEGER, OPTIONAL, INTENT(OUT) :: ierror
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MPI_Iallreduce(sendbuf, recvbuf, count, datatype, op, comm, request,
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ierror)
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TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
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TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
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INTEGER, INTENT(IN) :: count
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TYPE(MPI_Datatype), INTENT(IN) :: datatype
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TYPE(MPI_Op), INTENT(IN) :: op
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TYPE(MPI_Comm), INTENT(IN) :: comm
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TYPE(MPI_Request), INTENT(OUT) :: request
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INTEGER, OPTIONAL, INTENT(OUT) :: ierror
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MPI_Allreduce_init(sendbuf, recvbuf, count, datatype, op, comm, info, request,
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ierror)
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TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
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TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
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INTEGER, INTENT(IN) :: count
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TYPE(MPI_Datatype), INTENT(IN) :: datatype
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TYPE(MPI_Op), INTENT(IN) :: op
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TYPE(MPI_Comm), INTENT(IN) :: comm
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TYPE(MPI_Info), INTENT(IN) :: info
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TYPE(MPI_Request), INTENT(OUT) :: request
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INTEGER, OPTIONAL, INTENT(OUT) :: ierror
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.ft P
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.fi
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.UNINDENT
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.UNINDENT
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.SH INPUT PARAMETERS
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.INDENT 0.0
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.IP \(bu 2
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\fBsendbuf\fP: Starting address of send buffer (choice).
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.IP \(bu 2
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\fBcount\fP: Number of elements in send buffer (integer).
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.IP \(bu 2
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\fBdatatype\fP: Datatype of elements of send buffer (handle).
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.IP \(bu 2
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\fBop\fP: Operation (handle).
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.IP \(bu 2
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\fBcomm\fP: Communicator (handle).
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.IP \(bu 2
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\fBinfo\fP: Info (handle, persistent only).
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.UNINDENT
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.SH OUTPUT PARAMETERS
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.INDENT 0.0
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.IP \(bu 2
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\fBrecvbuf\fP: Starting address of receive buffer (choice).
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.IP \(bu 2
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\fBrequest\fP: Request (handle, non\-blocking only).
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.IP \(bu 2
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\fBierror\fP: Fortran only: Error status (integer).
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.UNINDENT
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.SH DESCRIPTION
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.sp
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Same as \fI\%MPI_Reduce\fP except that the result appears in the receive buffer
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of all the group members.
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.sp
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\fBExample 1:\fP A routine that computes the product of a vector and an
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array that are distributed across a group of processes and returns the
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answer at all nodes (compare with Example 2, with \fI\%MPI_Reduce\fP, below).
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.INDENT 0.0
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.INDENT 3.5
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.sp
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.nf
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.ft C
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SUBROUTINE PAR_BLAS2(m, n, a, b, c, comm)
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REAL a(m), b(m,n) ! local slice of array
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REAL c(n) ! result
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REAL sum(n)
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INTEGER n, comm, i, j, ierr
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! local sum
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DO j= 1, n
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sum(j) = 0.0
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DO i = 1, m
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sum(j) = sum(j) + a(i)*b(i,j)
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END DO
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END DO
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! global sum
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CALL MPI_ALLREDUCE(sum, c, n, MPI_REAL, MPI_SUM, comm, ierr)
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! return result at all nodes
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RETURN
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.ft P
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.fi
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.UNINDENT
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.UNINDENT
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.sp
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\fBExample 2:\fP A routine that computes the product of a vector and an
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array that are distributed across a group of processes and returns the
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answer at node zero.
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.INDENT 0.0
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.INDENT 3.5
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.sp
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.nf
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.ft C
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SUBROUTINE PAR_BLAS2(m, n, a, b, c, comm)
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REAL a(m), b(m,n) ! local slice of array
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REAL c(n) ! result
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REAL sum(n)
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INTEGER n, comm, i, j, ierr
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! local sum
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DO j= 1, n
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sum(j) = 0.0
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DO i = 1, m
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sum(j) = sum(j) + a(i)*b(i,j)
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END DO
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END DO
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! global sum
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CALL MPI_REDUCE(sum, c, n, MPI_REAL, MPI_SUM, 0, comm, ierr)
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! return result at node zero (and garbage at the other nodes)
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RETURN
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.ft P
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.fi
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.UNINDENT
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.UNINDENT
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.SH USE OF IN-PLACE OPTION
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.sp
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When the communicator is an intracommunicator, you can perform an
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all\-reduce operation in\-place (the output buffer is used as the input
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buffer). Use the variable MPI_IN_PLACE as the value of \fIsendbuf\fP at all
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processes.
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.sp
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Note that MPI_IN_PLACE is a special kind of value; it has the same
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restrictions on its use as MPI_BOTTOM.
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.sp
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Because the in\-place option converts the receive buffer into a
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send\-and\-receive buffer, a Fortran binding that includes INTENT must
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mark these as INOUT, not OUT.
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.SH WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
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.sp
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When the communicator is an inter\-communicator, the reduce operation
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occurs in two phases. The data is reduced from all the members of the
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first group and received by all the members of the second group. Then
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the data is reduced from all the members of the second group and
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received by all the members of the first. The operation exhibits a
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symmetric, full\-duplex behavior.
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.sp
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When the communicator is an intra\-communicator, these groups are the
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same, and the operation occurs in a single phase.
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.SH NOTES ON COLLECTIVE OPERATIONS
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.sp
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The reduction functions ( MPI_Op ) do not return an error value. As a
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result, if the functions detect an error, all they can do is either call
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\fI\%MPI_Abort\fP or silently skip the problem. Thus, if you change the error
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handler from MPI_ERRORS_ARE_FATAL to something else, for example,
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MPI_ERRORS_RETURN , then no error may be indicated.
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.SH ERRORS
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.sp
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Almost all MPI routines return an error value; C routines as the return result
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of the function and Fortran routines in the last argument.
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.sp
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Before the error value is returned, the current MPI error handler associated
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with the communication object (e.g., communicator, window, file) is called.
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If no communication object is associated with the MPI call, then the call is
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considered attached to MPI_COMM_SELF and will call the associated MPI error
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handler. When MPI_COMM_SELF is not initialized (i.e., before
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\fI\%MPI_Init\fP/\fI\%MPI_Init_thread\fP, after \fI\%MPI_Finalize\fP, or when using the Sessions
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Model exclusively) the error raises the initial error handler. The initial
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error handler can be changed by calling \fI\%MPI_Comm_set_errhandler\fP on
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MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI
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argument to mpiexec or info key to \fI\%MPI_Comm_spawn\fP/\fI\%MPI_Comm_spawn_multiple\fP\&.
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If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
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error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error
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handler is called for all other MPI functions.
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.sp
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Open MPI includes three predefined error handlers that can be used:
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.INDENT 0.0
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.IP \(bu 2
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\fBMPI_ERRORS_ARE_FATAL\fP
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Causes the program to abort all connected MPI processes.
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.IP \(bu 2
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\fBMPI_ERRORS_ABORT\fP
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An error handler that can be invoked on a communicator,
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window, file, or session. When called on a communicator, it
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acts as if \fI\%MPI_Abort\fP was called on that communicator. If
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called on a window or file, acts as if \fI\%MPI_Abort\fP was called
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on a communicator containing the group of processes in the
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corresponding window or file. If called on a session,
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aborts only the local process.
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.IP \(bu 2
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\fBMPI_ERRORS_RETURN\fP
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Returns an error code to the application.
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.UNINDENT
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.sp
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MPI applications can also implement their own error handlers by calling:
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.INDENT 0.0
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.IP \(bu 2
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\fI\%MPI_Comm_create_errhandler\fP then \fI\%MPI_Comm_set_errhandler\fP
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.IP \(bu 2
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\fI\%MPI_File_create_errhandler\fP then \fI\%MPI_File_set_errhandler\fP
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.IP \(bu 2
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\fI\%MPI_Session_create_errhandler\fP then \fI\%MPI_Session_set_errhandler\fP or at \fI\%MPI_Session_init\fP
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.IP \(bu 2
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\fI\%MPI_Win_create_errhandler\fP then \fI\%MPI_Win_set_errhandler\fP
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.UNINDENT
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.sp
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Note that MPI does not guarantee that an MPI program can continue past
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an error.
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.sp
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See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
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.sp
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See the Error Handling section of the MPI\-3.1 standard for
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more information.
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.SH COPYRIGHT
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2003-2025, The Open MPI Community
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.\" Generated by docutils manpage writer.
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