1MPI_Reduce(3) LAM/MPI MPI_Reduce(3)
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6 MPI_Reduce - Reduces values on all processes to a single value
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9 #include <mpi.h>
10 int MPI_Reduce(void *sbuf, void* rbuf, int count,
11 MPI_Datatype dtype, MPI_Op op, int root,
12 MPI_Comm comm)
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15 sbuf - address of send buffer (choice)
16 count - number of elements in send buffer (integer)
17 dtype - data type of elements of send buffer (handle)
18 op - reduce operation (handle)
19 root - rank of root process (integer)
20 comm - communicator (handle)
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24 rbuf - address of receive buffer (choice, significant only at root )
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28 This function has had the IMPI extensions implemented. It is legal to
29 call this function on IMPI communicators.
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33 If there are 4 or less ranks involved, the root loops over receiving
34 from each rank, and then performs the final reduction locally.
35 If there are more than 4 ranks involved, a tree-based algorithm is used
37 to collate the reduced data at the root (the data is reduced at each
38 parent in the tree so that the reduction operations are actaully dis‐
39 tributed).
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43 All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK ) have
44 an additional argument ierr at the end of the argument list. ierr is
45 an integer and has the same meaning as the return value of the routine
46 in C. In Fortran, MPI routines are subroutines, and are invoked with
47 the call statement.
48 All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in
50 Fortran.
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54 The reduction functions ( MPI_Op ) do not return an error value. As a
55 result, if the functions detect an error, all they can do is either
56 call MPI_Abort or silently skip the problem. Thus, if you change the
57 error handler from MPI_ERRORS_ARE_FATAL to something else (e.g.,
58 MPI_ERRORS_RETURN ), then no error may be indicated.
59 The reason for this is the performance problems that arise in ensuring
61 that all collective routines return the same error value.
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65 If an error occurs in an MPI function, the current MPI error handler is
66 called to handle it. By default, this error handler aborts the MPI
67 job. The error handler may be changed with MPI_Errhandler_set ; the
68 predefined error handler MPI_ERRORS_RETURN may be used to cause error
69 values to be returned (in C and Fortran; this error handler is less
70 useful in with the C++ MPI bindings. The predefined error handler
71 MPI::ERRORS_THROW_EXCEPTIONS should be used in C++ if the error value
72 needs to be recovered). Note that MPI does not guarantee that an MPI
73 program can continue past an error.
74 All MPI routines (except MPI_Wtime and MPI_Wtick ) return an error
76 value; C routines as the value of the function and Fortran routines in
77 the last argument. The C++ bindings for MPI do not return error val‐
78 ues; instead, error values are communicated by throwing exceptions of
79 type MPI::Exception (but not by default). Exceptions are only thrown
80 if the error value is not MPI::SUCCESS .
81 Note that if the MPI::ERRORS_RETURN handler is set in C++, while MPI
84 functions will return upon an error, there will be no way to recover
85 what the actual error value was.
86 MPI_SUCCESS
87 - No error; MPI routine completed successfully.
88 MPI_ERR_COMM
89 - Invalid communicator. A common error is to use a null commu‐
90 nicator in a call (not even allowed in MPI_Comm_rank ).
91 MPI_ERR_OTHER
92 - A collective implementation was not able to be located at run-
93 time for this communicator.
94 MPI_ERR_COUNT
95 - Invalid count argument. Count arguments must be non-negative;
96 a count of zero is often valid.
97 MPI_ERR_TYPE
98 - Invalid datatype argument. May be an uncommitted MPI_Datatype
99 (see MPI_Type_commit ).
100 MPI_ERR_BUFFER
101 - Invalid buffer pointer. Usually a null buffer where one is
102 not valid.
103 MPI_ERR_BUFFER
104 - This error class is associcated with an error code that indi‐
105 cates that two buffer arguments are aliased ; that is, the
106 describe overlapping storage (often the exact same storage).
107 This is prohibited in MPI (because it is prohibited by the For‐
108 tran standard, and rather than have a separate case for C and
109 Fortran, the MPI Forum adopted the more restrictive requirements
110 of Fortran).
111 MPI_ERR_ROOT
112 - Invalid root. The root must be specified as a rank in the
113 communicator. Ranks must be between zero and the size of the
114 communicator minus one.
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118 For more information, please see the official MPI Forum web site, which
119 contains the text of both the MPI-1 and MPI-2 standards. These docu‐
120 ments contain detailed information about each MPI function (most of
121 which is not duplicated in these man pages).
122 http://www.mpi-forum.org/
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128 The LAM Team would like the thank the MPICH Team for the handy program
129 to generate man pages ("doctext" from ftp://ftp.mcs.anl.gov/pub/sow‐
130 ing/sowing.tar.gz ), the initial formatting, and some initial text for
131 most of the MPI-1 man pages.
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134 reduce.c
135LAM/MPI 7.1.2 2/23/2006 MPI_Reduce(3)