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)
- 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.
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