MPI_Scan(3)                        Open MPI                        MPI_Scan(3)



NAME
       MPI_Scan - Computes an inclusive scan (partial reduction)


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


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


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


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).


OUTPUT PARAMETERS
       recvbuf   Receive buffer (choice).

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       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.


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 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
       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.


SEE ALSO
       MPI_Exscan
       MPI_Op_create
       MPI_Reduce




1.4.1                            Jan 14, 2010                      MPI_Scan(3)
