[97] | 1 | .\"Copyright 2006-2008 Sun Microsystems, Inc. |
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| 2 | .\" Copyright (c) 1996 Thinking Machines Corporation |
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| 3 | .TH MPI_Exscan 3 "Dec 08, 2009" "1.4" "Open MPI" |
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| 4 | |
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| 5 | .SH NAME |
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| 6 | \fBMPI_Exscan\fP \- Computes an exclusive scan (partial reduction) |
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| 7 | |
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| 8 | .SH SYNTAX |
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| 9 | .ft R |
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| 10 | |
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| 11 | .SH C Syntax |
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| 12 | .nf |
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| 13 | #include <mpi.h> |
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| 14 | int MPI_Exscan(void *\fIsendbuf\fP, void *\fIrecvbuf\fP, int \fIcount\fP, |
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| 15 | MPI_Datatype \fIdatatype\fP, MPI_Op \fIop\fP, MPI_Comm \fIcomm\fP) |
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| 16 | |
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| 17 | .SH Fortran Syntax |
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| 18 | .nf |
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| 19 | INCLUDE 'mpif.h' |
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| 20 | MPI_SCAN(\fISENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR\fP) |
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| 21 | <type> \fISENDBUF(*), RECVBUF(*)\fP |
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| 22 | INTEGER \fICOUNT, DATATYPE, OP, COMM, IERROR\fP |
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| 23 | |
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| 24 | .SH C++ Syntax |
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| 25 | .nf |
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| 26 | #include <mpi.h> |
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| 27 | void MPI::Intracomm::Exscan(const void* \fIsendbuf\fP, void* \fIrecvbuf\fP, |
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| 28 | int \fIcount\fP, const MPI::Datatype& \fIdatatype\fP, |
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| 29 | const MPI::Op& \fIop\fP) const |
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| 30 | |
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| 31 | .SH INPUT PARAMETERS |
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| 32 | .ft R |
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| 33 | .TP 1i |
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| 34 | sendbuf |
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| 35 | Send buffer (choice). |
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| 36 | .TP 1i |
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| 37 | count |
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| 38 | Number of elements in input buffer (integer). |
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| 39 | .TP 1i |
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| 40 | datatype |
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| 41 | Data type of elements of input buffer (handle). |
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| 42 | .TP 1i |
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| 43 | op |
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| 44 | Operation (handle). |
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| 45 | .TP 1i |
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| 46 | comm |
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| 47 | Communicator (handle). |
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| 48 | |
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| 49 | .SH OUTPUT PARAMETERS |
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| 50 | .ft R |
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| 51 | .TP 1i |
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| 52 | recvbuf |
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| 53 | Receive buffer (choice). |
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| 54 | .ft R |
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| 55 | .TP 1i |
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| 56 | IERROR |
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| 57 | Fortran only: Error status (integer). |
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| 58 | |
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| 59 | .SH DESCRIPTION |
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| 60 | .ft R |
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| 61 | MPI_Exscan is used to perform an exclusive prefix reduction on data |
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| 62 | distributed across the calling processes. The operation returns, in |
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| 63 | the \fIrecvbuf\fP of the process with rank i, the reduction |
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| 64 | (calculated according to the function \fIop\fP) of the values in the |
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| 65 | \fIsendbuf\fPs of processes with ranks 0, ..., i-1. Compare this with |
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| 66 | the functionality of MPI_Scan, which calculates over the range 0, ..., |
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| 67 | i (inclusive). The type of operations supported, their semantics, and |
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| 68 | the constraints on send and receive buffers are as for MPI_Reduce. |
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| 69 | .sp |
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| 70 | The value in \fIrecvbuf\fP on process 0 is undefined and unreliable |
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| 71 | as \fIrecvbuf\fP is not significant for process 0. The value of |
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| 72 | \fIrecvbuf\fP on process 1 is always the value in \fIsendbuf\fP on |
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| 73 | process 0. |
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| 74 | .sp |
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| 75 | No MPI_IN_PLACE operation is supported. |
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| 76 | |
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| 77 | .SH NOTES |
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| 78 | .ft R |
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| 79 | MPI does not specify which process computes which operation. In |
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| 80 | particular, both processes 0 and 1 may participate in the computation |
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| 81 | even though the results for both processes' \fIrecvbuf\fP are |
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| 82 | degenerate. Therefore, all processes, including 0 and 1, must provide |
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| 83 | the same \fIop\fP. |
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| 84 | .sp |
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| 85 | It can be argued, from a mathematical perspective, that the definition |
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| 86 | of MPI_Exscan is unsatisfactory because the output at process 0 is |
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| 87 | undefined. The "mathematically correct" output for process 0 would be |
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| 88 | the unit element of the reduction operation. However, such a |
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| 89 | definition of an exclusive scan would not work with user-defined |
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| 90 | \fIop\fP functions as there is no way for MPI to "know" the unit value |
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| 91 | for these custom operations. |
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| 92 | |
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| 93 | .SH NOTES ON COLLECTIVE OPERATIONS |
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| 94 | .ft R |
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| 95 | The reduction functions of type MPI_Op do not return an error value. |
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| 96 | As a result, if the functions detect an error, all they can do is |
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| 97 | either call MPI_Abort or silently skip the problem. Thus, if the |
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| 98 | error handler is changed from MPI_ERRORS_ARE_FATAL to something else |
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| 99 | (e.g., MPI_ERRORS_RETURN), then no error may be indicated. |
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| 100 | .sp |
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| 101 | The reason for this is the performance problems in ensuring that |
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| 102 | all collective routines return the same error value. |
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| 103 | |
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| 104 | .SH ERRORS |
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| 105 | .ft R |
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| 106 | Almost all MPI routines return an error value; C routines as |
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| 107 | the value of the function and Fortran routines in the last argument. C++ |
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| 108 | functions do not return errors. If the default error handler is set to |
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| 109 | MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism |
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| 110 | will be used to throw an MPI:Exception object. |
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| 111 | .sp |
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| 112 | Before the error value is returned, the current MPI error handler is |
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| 113 | called. By default, this error handler aborts the MPI job, except for |
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| 114 | I/O function errors. The error handler may be changed with |
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| 115 | MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN |
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| 116 | may be used to cause error values to be returned. Note that MPI does not |
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| 117 | guarantee that an MPI program can continue past an error. |
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| 118 | .sp |
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| 119 | See the MPI man page for a full list of MPI error codes. |
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| 120 | |
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| 121 | .SH SEE ALSO |
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| 122 | .ft R |
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| 123 | .nf |
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| 124 | MPI_Op_create |
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| 125 | MPI_Reduce |
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| 126 | MPI_Scan |
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| 127 | |
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