MPI_Scan.openmpi (3)

#include <mpi.h>
int MPI_Scan(void *sendbuf, void *recvbuf, int count,
	MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

INCLUDE 'mpif.h'
MPI_SCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
	<type>	SENDBUF(*), RECVBUF(*)
	INTEGER	COUNT, DATATYPE, OP, COMM, IERROR

#include <mpi.h>
void MPI::Intracomm::Scan(const void* sendbuf, void* recvbuf,
	int count, const MPI::Datatype& datatype,
	const MPI::Op& op) const

sendbuf

Send buffer (choice).

count

Number of elements in input buffer (integer).

datatype

Data type of elements of input buffer (handle).

op

Operation (handle).

comm

Communicator (handle).

recvbuf

Receive buffer (choice).

IERROR

Fortran only: Error status (integer).

MPI_Scan is used to perform an inclusive prefix reduction on data distributed across the calling processes. The operation returns, in the recvbuf of the process with rank i, the reduction (calculated according to the function op) of the values in the sendbufs 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.

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,

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

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

      [ u ]     [ v ]     [ w ]
      [   ]  o  [   ]  =  [   ]
      [ i ]     [ j ]     [ j ]

where

            ( u + v if i  = j
      w  =  (
            ( v     if i != j

Note that this is a noncommutative operator. C code that implements it is given below.

	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++;
		}
	}

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:

	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);

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 sendbuf argument. The input data is taken from the receive buffer and replaced by the output data.

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.

The reason for this is the performance problems in ensuring that all collective routines return the same error value.

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.

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.

See the MPI man page for a full list of MPI error codes.

MPI_Exscan
MPI_Op_create
MPI_Reduce