MPI_Scan(3OpenMPI) MPI_Scan(3OpenMPI)
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 pro-cesses. processes.
cesses. 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 (inclu-sive). (inclusive).
sive). 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 send-
buf 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
Open MPI 1.2 September 2006 MPI_Scan(3OpenMPI)
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