NAME

MPI_Allreduce - Combines values from all processes and distributes the result back to all processes

SYNOPSIS

int MPI_Allreduce(const 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 PARAMETERS

recvbuf: - starting address of receive buffer (choice)

THREAD AND INTERRUPT SAFETY

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

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_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. 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; however, MPI implementations will attempt to continue whenever possible.

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. Additionally, this error can occur if an uncommitted MPI_Datatype (see MPI_Type_commit ) is used in a communication call.
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 ).