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