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.. _mpi_scan:
MPI_Scan
========
.. include_body
:ref:`MPI_Scan`, :ref:`MPI_Iscan`, :ref:`MPI_Scan_init` - Computes an inclusive scan
(partial reduction)
SYNTAX
------
C Syntax
^^^^^^^^
.. code-block:: c
#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
^^^^^^^^^^^^^^
.. code-block:: fortran
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
^^^^^^^^^^^^^^^^^^^
.. code-block:: fortran
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 *sendbuf*\ s 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.
.. code-block:: c
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:
.. code-block:: c
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
------
.. include:: ./ERRORS.rst
.. seealso::
* :ref:`MPI_Exscan`
* :ref:`MPI_Op_create`
* :ref:`MPI_Reduce`
|
