1MPI_Allreduce(3) LAM/MPI MPI_Allreduce(3)
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6 MPI_Allreduce - Combines values from all processes and distribute the
7 result back to all processes
8
10 #include <mpi.h>
11 int MPI_Allreduce(void *sbuf, void *rbuf, int count,
12 MPI_Datatype dtype, MPI_Op op, MPI_Comm comm)
13
15 sbuf - starting 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 - operation (handle)
19 comm - communicator (handle)
20
23 rbuf - starting address of receive buffer (choice)
24
27 This function has had the IMPI extensions implemented. It is legal to
28 call this function on IMPI communicators.
29
32 All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK ) have
33 an additional argument ierr at the end of the argument list. ierr is
34 an integer and has the same meaning as the return value of the routine
35 in C. In Fortran, MPI routines are subroutines, and are invoked with
36 the call statement.
37 All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in
39 Fortran.
40
43 The reduction functions ( MPI_Op ) do not return an error value. As a
44 result, if the functions detect an error, all they can do is either
45 call MPI_Abort or silently skip the problem. Thus, if you change the
46 error handler from MPI_ERRORS_ARE_FATAL to something else (e.g.,
47 MPI_ERRORS_RETURN ), then no error may be indicated.
48 The reason for this is the performance problems that arise in ensuring
50 that all collective routines return the same error value.
51
54 If an error occurs in an MPI function, the current MPI error handler is
55 called to handle it. By default, this error handler aborts the MPI
56 job. The error handler may be changed with MPI_Errhandler_set ; the
57 predefined error handler MPI_ERRORS_RETURN may be used to cause error
58 values to be returned (in C and Fortran; this error handler is less
59 useful in with the C++ MPI bindings. The predefined error handler
60 MPI::ERRORS_THROW_EXCEPTIONS should be used in C++ if the error value
61 needs to be recovered). Note that MPI does not guarantee that an MPI
62 program can continue past an error.
63 All MPI routines (except MPI_Wtime and MPI_Wtick ) return an error
65 value; C routines as the value of the function and Fortran routines in
66 the last argument. The C++ bindings for MPI do not return error val‐
67 ues; instead, error values are communicated by throwing exceptions of
68 type MPI::Exception (but not by default). Exceptions are only thrown
69 if the error value is not MPI::SUCCESS .
70 Note that if the MPI::ERRORS_RETURN handler is set in C++, while MPI
73 functions will return upon an error, there will be no way to recover
74 what the actual error value was.
75 MPI_ERR_COMM
76 - Invalid communicator. A common error is to use a null commu‐
77 nicator in a call (not even allowed in MPI_Comm_rank ).
78 MPI_ERR_OTHER
79 - A collective implementation was not able to be located at run-
80 time for this communicator.
81 MPI_ERR_OTHER
82 - A communicator that contains some non-local IMPI procs was
83 used for some function which has not yet had the IMPI extensions
84 implemented yet. For example, most collectives on IMPI communi‐
85 cators have not been implemented yet.
86 MPI_ERR_BUFFER
87 - Invalid buffer pointer. Usually a null buffer where one is
88 not valid.
89 MPI_ERR_COUNT
90 - Invalid count argument. Count arguments must be non-negative;
91 a count of zero is often valid.
92 MPI_ERR_TYPE
93 - Invalid datatype argument. May be an uncommitted MPI_Datatype
94 (see MPI_Type_commit ).
95 MPI_ERR_OP
96 - Invalid operation. MPI operations (objects of type MPI_Op )
97 must either be one of the predefined operations (e.g., MPI_SUM )
98 or created with MPI_Op_create . Additionally, only certain
99 datatypes are alloed with given predefined operations. See
100 MPI-1, section 4.9.2.
101
104 For more information, please see the official MPI Forum web site, which
105 contains the text of both the MPI-1 and MPI-2 standards. These docu‐
106 ments contain detailed information about each MPI function (most of
107 which is not duplicated in these man pages).
108 http://www.mpi-forum.org/
110
114 The LAM Team would like the thank the MPICH Team for the handy program
115 to generate man pages ("doctext" from ftp://ftp.mcs.anl.gov/pub/sow‐
116 ing/sowing.tar.gz ), the initial formatting, and some initial text for
117 most of the MPI-1 man pages.
118
120 allreduce.c
121LAM/MPI 7.1.2 2/23/2006 MPI_Allreduce(3)