MPI_Reduce(3) LAM/MPI MPI_Reduce(3)
MPI_Reduce - Reduces values on all processes to a single value
int MPI_Reduce(void *sbuf, void* rbuf, int count,
MPI_Datatype dtype, MPI_Op op, int root,
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)
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.
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 argu-
ment 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.
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 excep-
tions 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.
- No error; MPI routine completed successfully.
- Invalid communicator. A common error is to use a null communicator in a call
(not even allowed in MPI_Comm_rank ).
- Invalid communicator. LAM/MPI does not yet support invoking collectives on
- A communicator that contains some non-local IMPI procs was used for some function
which has not yet had the IMPI extensions implemented yet. For example, most col-
lectives on IMPI communicators have not been implemented yet.
- Invalid count argument. Count arguments must be non-negative; a count of zero is
- Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_com-
- Invalid buffer pointer. Usually a null buffer where one is not valid.
- 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).
- 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.
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).
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 for-
matting, and some initial text for most of the MPI-1 man pages.
LAM/MPI 6.5.8 11/10/2002 MPI_Reduce(3)