NAME

       MPI_Reduce -  Reduces values on all processes to a single value

SYNOPSIS

       #include <mpi.h>
       int MPI_Reduce(void *sbuf, void* rbuf, int count,
                      MPI_Datatype dtype, MPI_Op op, int root,
                      MPI_Comm comm)

INPUT PARAMETERS

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

OUTPUT PARAMETER

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

USAGE WITH IMPI EXTENSIONS

       This  function  has  had  the  IMPI  extensions  implemented.   It is legal to call this function on IMPI
       communicators.

ALGORITHM

       If there are 4 or less ranks involved, the root loops over receiving from each rank,  and  then  performs
       the final reduction locally.

       If  there  are  more than 4 ranks involved, a tree-based algorithm is used to collate the reduced data at
       the root (the data is reduced at each parent in the tree so that the reduction  operations  are  actaully
       distributed).

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 (e.g., MPI_ERRORS_RETURN ), then no  error  may
       be indicated.

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

ERRORS

       If an error occurs in an MPI function, the current MPI error handler is called to handle it.  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 (in C and
       Fortran; this error handler is less useful in with the C++ MPI bindings.  The  predefined  error  handler
       MPI::ERRORS_THROW_EXCEPTIONS  should be used in C++ if the error value needs to be recovered).  Note that
       MPI does not guarantee that an MPI program can continue past an error.

       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.  The C++ bindings for MPI do not return error values;
       instead,  error  values  are  communicated  by  throwing  exceptions  of  type MPI::Exception (but not by
       default).  Exceptions are only thrown if the error value is not MPI::SUCCESS .

       Note that if the MPI::ERRORS_RETURN handler is set in C++, while MPI functions will return upon an error,
       there will be no way to recover what the actual error value was.
       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_OTHER
              - A collective implementation was not able to be located at run-time for this communicator.
       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).
       MPI_ERR_ROOT
              -  Invalid root.  The root must be specified as a rank in the communicator.  Ranks must be between
              zero and the size of the communicator minus one.

MORE INFORMATION

       For more information, please see the official MPI Forum web site, which contains the  text  of  both  the
       MPI-1 and MPI-2 standards.  These documents contain detailed information about each MPI function (most of
       which is not duplicated in these man pages).

       http://www.mpi-forum.org/

ACKNOWLEDGEMENTS

       The  LAM  Team would like the thank the MPICH Team for the handy program to generate man pages ("doctext"
       from ftp://ftp.mcs.anl.gov/pub/sowing/sowing.tar.gz ), the initial formatting, and some initial text  for
       most of the MPI-1 man pages.

LOCATION

       reduce.c

LAM/MPI 7.1.4                                       6/24/2006                                      MPI_Reduce(3)