NAME¶
MPI_Allreduce - Combines values from all processes and distributes the
result back to all processes.
SYNTAX¶
C Syntax¶
#include <mpi.h>
int MPI_Allreduce(void *sendbuf, void *recvbuf, int count,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
Fortran Syntax¶
INCLUDE 'mpif.h'
MPI_ALLREDUCE( SENDBUF, RECVBUF, COUNT, DATATYPE, OP,
COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER COUNT, DATATYPE, OP, COMM, 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
- 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).
- 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.