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MPI_Scan.3.rst

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MPI_Scan

:ref:`MPI_Scan`, :ref:`MPI_Iscan`, :ref:`MPI_Scan_init` - Computes an inclusive scan (partial reduction)

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Scan(const void *sendbuf, void *recvbuf, int count,
             MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

int MPI_Iscan(const void *sendbuf, void *recvbuf, int count,
              MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
              MPI_Request *request)

int MPI_Scan_init(const void *sendbuf, void *recvbuf, int count,
              MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
              MPI_Info info, MPI_Request *request)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_SCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER COUNT, DATATYPE, OP, COMM, IERROR

MPI_ISCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER COUNT, DATATYPE, OP, COMM, REQUEST, IERROR

MPI_SCAN_INIT(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, INFO, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER COUNT, DATATYPE, OP, COMM, INFO, REQUEST, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Scan(sendbuf, recvbuf, count, datatype, op, comm, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Op), INTENT(IN) :: op
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Iscan(sendbuf, recvbuf, count, datatype, op, comm, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Op), INTENT(IN) :: op
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Scan_init(sendbuf, recvbuf, count, datatype, op, comm, info, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Op), INTENT(IN) :: op
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS

  • 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).
  • info: Info (handle, persistent only)

OUTPUT PARAMETERS

  • recvbuf: Receive buffer (choice).
  • request: Request (handle, non-blocking only).
  • ierror: Fortran only: Error status (integer).

DESCRIPTION

:ref:`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 :ref:`MPI_Reduce`.

EXAMPLE

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);

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

NOTES ON COLLECTIVE OPERATIONS

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 :ref:`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.

ERRORS

.. seealso::
   * :ref:`MPI_Exscan`
   * :ref:`MPI_Op_create`
   * :ref:`MPI_Reduce`