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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 dis-
tributed 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 logi-
cals, where the logicals delineate the various segments of the scan.
For example,
[ 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 follow-
ing:
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);
* 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 scan-
ning 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++ func-
tions 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
Open MPI 1.2 September 2006 MPI_Scan(3OpenMPI)
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