MPI_Reduce

Reduces values on all processes to a single value

Synopsis


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

Input Parameters

sendbuf - address of send buffer (choice)
count - number of elements in send buffer (integer)
datatype - data type of elements of send buffer (handle)
op - reduce operation (handle)
root - rank of root process (integer)
comm - communicator (handle)

Output Parameter

recvbuf - address of receive buffer (choice, significant only at root)

Algorithm

This implementation currently uses a simple tree algorithm.

Notes for Fortran

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

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in 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_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.

The reason for this is the performance problems in ensuring that all 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 last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Errhandler_set; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error.

MPI_SUCCESS - No error; MPI routine completed successfully.
MPI_ERR_COMM - Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
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_BUFFER - Invalid buffer pointer. Usually a null buffer where one is not valid.
MPI_ERR_BUFFER - This error class is associcated with an error code that indicates that two buffer arguments are aliased; that is, the describe overlapping storage (often the exact same storage). This is prohibited in MPI (because it is prohibited by the Fortran standard, and rather than have a separate case for C and Fortran, the
MPI Forum adopted the more restrictive requirements of Fortran).

Location:reduce.c