mpi_scan.3

MPI stands for the Message Passing Interface. Written by the MPI Forum (a large committee comprising

.\"Copyright 2006, Sun Microsystems, Inc.
.\" 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