MPI_Allreduce

Combines values from all processes and distribute the result back to all processes

Synopsis

#include "mpi.h"
int MPI_Allreduce ( sendbuf, recvbuf, count, datatype, op, comm )
void             *sendbuf;
void             *recvbuf;
int               count;
MPI_Datatype      datatype;
MPI_Op            op;
MPI_Comm          comm;

Input Parameters

	sendbuf	starting address of send buffer (choice)
	count	number of elements in send buffer (integer)
	datatype	data type of elements of send buffer (handle)
	op	operation (handle)
	comm	communicator (handle)

Output Parameter

recvbuf: starting address of receive buffer (choice)

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) havean additional argument ierr at the end of the argument list. ierris an integer and has the same meaning as the return value of the routinein C. In Fortran, MPI routines are subroutines, and are invoked with thecall statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGERin Fortran.

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_Abortor silently skip the problem. Thus, if you change the error handler fromMPI_ERRORS_ARE_FATAL to something else, for example, MPI_ERRORS_RETURN, then no error may be indicated.

The reason for this is the performance problems in ensuring thatall collective routines return the same error value.

Errors

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the lastargument. Before the value is returned, the current MPI error handler iscalled. By default, this error handler aborts the MPI job. The error handlermay be changed with MPI_Errhandler_set; the predefined error handlerMPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue pastan error.

MPI_ERR_BUFFER: Invalid buffer pointer. Usually a null buffer where one is not valid.

MPI_ERR_COUNT: Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.

MPI_ERR_TYPE: Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).

MPI_ERR_OP: Invalid operation. MPI operations (objects of type MPI_Op) must either be one of the predefined operations (e.g., MPI_SUM) or created with MPI_Op_create.

MPI_ERR_COMM: Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).

Location:allreduce.c