203 строки
5.6 KiB
Groff
203 строки
5.6 KiB
Groff
.\"Copyright 2006, Sun Microsystems, Inc. All rights reserved. Use is subject to license terms.
|
|
.\" Copyright (c) 1996 Thinking Machines Corporation
|
|
.TH MPI_Scan 3OpenMPI "September 2006" "Open MPI 1.2" " "
|
|
|
|
.SH NAME
|
|
\fBMPI_Scan\fP \- Computes an inclusive scan (partial reduction)
|
|
|
|
.SH SYNTAX
|
|
.ft R
|
|
|
|
.SH C Syntax
|
|
.nf
|
|
#include <mpi.h>
|
|
int MPI_Scan(void *\fIsendbuf\fP, void *\fIrecvbuf\fP, int \fIcount\fP,
|
|
MPI_Datatype \fIdatatype\fP, MPI_Op \fIop\fP, MPI_Comm \fIcomm\fP)
|
|
|
|
.SH Fortran Syntax
|
|
.nf
|
|
INCLUDE 'mpif.h'
|
|
MPI_SCAN(\fISENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR\fP)
|
|
<type> \fISENDBUF(*), RECVBUF(*)\fP
|
|
INTEGER \fICOUNT, DATATYPE, OP, COMM, IERROR\fP
|
|
|
|
.SH C++ Syntax
|
|
.nf
|
|
#include <mpi.h>
|
|
void MPI::Intracomm::Scan(const void* \fIsendbuf\fP, void* \fIrecvbuf\fP,
|
|
int \fIcount\fP, const MPI::Datatype& \fIdatatype\fP,
|
|
const MPI::Op& \fIop\fP) const
|
|
|
|
.SH INPUT PARAMETERS
|
|
.ft R
|
|
.TP 1i
|
|
sendbuf
|
|
Send buffer (choice).
|
|
.TP 1i
|
|
count
|
|
Number of elements in input buffer (integer).
|
|
.TP 1i
|
|
datatype
|
|
Data type of elements of input buffer (handle).
|
|
.TP 1i
|
|
op
|
|
Operation (handle).
|
|
.TP 1i
|
|
comm
|
|
Communicator (handle).
|
|
|
|
.SH OUTPUT PARAMETERS
|
|
.ft R
|
|
.TP 1i
|
|
recvbuf
|
|
Receive buffer (choice).
|
|
.ft R
|
|
.TP 1i
|
|
IERROR
|
|
Fortran only: Error status (integer).
|
|
|
|
.SH DESCRIPTION
|
|
.ft R
|
|
MPI_Scan is used to perform an inclusive prefix reduction on data
|
|
distributed across the calling processes. The operation returns, in
|
|
the \fIrecvbuf\fP of the process with rank i, the reduction
|
|
(calculated according to the function \fIop\fP) of the values in the
|
|
\fIsendbuf\fPs of processes with ranks 0, ..., i (inclusive). The type
|
|
of operations supported, their semantics, and the constraints on send
|
|
and receive buffers are as for MPI_Reduce.
|
|
|
|
.SH EXAMPLE
|
|
.ft R
|
|
This example uses a user-defined operation to produce a segmented
|
|
scan. A segmented scan takes, as input, a set of values and a set of
|
|
logicals, where the logicals delineate the various segments of the
|
|
scan. For example,
|
|
.sp
|
|
.nf
|
|
values v1 v2 v3 v4 v5 v6 v7 v8
|
|
logicals 0 0 1 1 1 0 0 1
|
|
result v1 v1+v2 v3 v3+v4 v3+v4+v5 v6 v6+v7 v8
|
|
.fi
|
|
.sp
|
|
The result for rank j is thus the sum v(i) + ... + v(j), where i is
|
|
the lowest rank such that for all ranks n, i <= n <= j, logical(n) =
|
|
logical(j). The operator that produces this effect is
|
|
.sp
|
|
.nf
|
|
[ u ] [ v ] [ w ]
|
|
[ ] o [ ] = [ ]
|
|
[ i ] [ j ] [ j ]
|
|
.sp
|
|
where
|
|
.sp
|
|
( u + v if i = j
|
|
w = (
|
|
( v if i != j
|
|
.fi
|
|
.sp
|
|
Note that this is a noncommutative operator. C code that implements it is
|
|
given below.
|
|
.sp
|
|
.nf
|
|
typedef struct {
|
|
double val;
|
|
int log;
|
|
} SegScanPair;
|
|
|
|
/*
|
|
* the user-defined function
|
|
*/
|
|
void segScan(SegScanPair *in, SegScanPair *inout, int *len,
|
|
MPI_Datatype *dptr)
|
|
{
|
|
int i;
|
|
SegScanPair c;
|
|
|
|
for (i = 0; i < *len; ++i) {
|
|
if (in->log == inout->log)
|
|
c.val = in->val + inout->val;
|
|
else
|
|
c.val = inout->val;
|
|
|
|
c.log = inout->log;
|
|
*inout = c;
|
|
in++;
|
|
inout++;
|
|
}
|
|
}
|
|
.fi
|
|
.sp
|
|
Note that the inout argument to the user-defined function corresponds
|
|
to the right-hand operand of the operator. When using this operator,
|
|
we must be careful to specify that it is noncommutative, as in the
|
|
following:
|
|
.sp
|
|
.nf
|
|
int i, base;
|
|
SeqScanPair a, answer;
|
|
MPI_Op myOp;
|
|
MPI_Datatype type[2] = {MPI_DOUBLE, MPI_INT};
|
|
MPI_Aint disp[2];
|
|
int blocklen[2] = {1, 1};
|
|
MPI_Datatype sspair;
|
|
|
|
/*
|
|
* explain to MPI how type SegScanPair is defined
|
|
*/
|
|
MPI_Get_address(a, disp);
|
|
MPI_Get_address(a.log, disp + 1);
|
|
base = disp[0];
|
|
for (i = 0; i < 2; ++i)
|
|
disp[i] -= base;
|
|
MPI_Type_struct(2, blocklen, disp, type, &sspair);
|
|
MPI_Type_commit(&sspair);
|
|
|
|
/*
|
|
* create the segmented-scan user-op
|
|
* noncommutative - set commute (arg 2) to 0
|
|
*/
|
|
MPI_Op_create((MPI_User_function *)segScan, 0, &myOp);
|
|
\&...
|
|
MPI_Scan(a, answer, 1, sspair, myOp, comm);
|
|
.fi
|
|
|
|
.SH USE OF IN-PLACE OPTION
|
|
WHen the communicator is an intracommunicator, you can perform a scanning operation in place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of the \fIsendbuf\fR argument. The input data is taken from the receive buffer and replaced by the output data.
|
|
|
|
.SH NOTES ON COLLECTIVE OPERATIONS
|
|
.ft R
|
|
The reduction functions of type 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 the
|
|
error handler is changed from MPI_ERRORS_ARE_FATAL to something else
|
|
(e.g., MPI_ERRORS_RETURN), then no error may be indicated.
|
|
.sp
|
|
The reason for this is the performance problems in ensuring that
|
|
all collective routines return the same error value.
|
|
|
|
.SH ERRORS
|
|
.ft R
|
|
Almost all MPI routines return an error value; C routines as
|
|
the value of the function and Fortran routines in the last argument. C++
|
|
functions do not return errors. If the default error handler is set to
|
|
MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
|
|
will be used to throw an MPI:Exception object.
|
|
.sp
|
|
Before the error value is returned, the current MPI error handler is
|
|
called. By default, this error handler aborts the MPI job, except for
|
|
I/O function errors. The error handler may be changed with
|
|
MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN
|
|
may be used to cause error values to be returned. Note that MPI does not
|
|
guarantee that an MPI program can continue past an error.
|
|
.sp
|
|
See the MPI man page for a full list of MPI error codes.
|
|
|
|
.SH SEE ALSO
|
|
.ft R
|
|
.nf
|
|
MPI_Exscan
|
|
MPI_Op_create
|
|
MPI_Reduce
|
|
|
|
' @(#)MPI_Scan.3 1.23 06/03/09
|