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MPI_Exscan - Computes an exclusive scan (partial reduction)
#include <mpi.h>
int MPI_Exscan(void *sendbuf, void *recvbuf, int count,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
INCLUDE ’mpif.h’
MPI_SCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER COUNT, DATATYPE, OP, COMM, IERROR
#include <mpi.h>
void MPI::Intracomm::Exscan(const void* sendbuf, void* recvbuf,
int count, const MPI::Datatype& datatype,
const MPI::Op& op) const
- 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).
- recvbuf
- Receive buffer (choice).
- IERROR
- Fortran only: Error status (integer).
MPI_Exscan
is used to perform an exclusive 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-1. Compare this
with the functionality of MPI_Scan, which calculates over the range 0,
..., i (inclusive). The type of operations supported, their semantics, and
the constraints on send and receive buffers are as for MPI_Reduce.
The value
in recvbuf on process 0 is undefined and unreliable as recvbuf is not significant
for process 0. The value of recvbuf on process 1 is always the value in
sendbuf on process 0.
No MPI_IN_PLACE operation is supported.
MPI does
not specify which process computes which operation. In particular, both
processes 0 and 1 may participate in the computation even though the results
for both processes’ recvbuf are degenerate. Therefore, all processes, including
0 and 1, must provide the same op.
It can be argued, from a mathematical
perspective, that the definition of MPI_Exscan is unsatisfactory because
the output at process 0 is undefined. The "mathematically correct" output
for process 0 would be the unit element of the reduction operation. However,
such a definition of an exclusive scan would not work with user-defined
op functions as there is no way for MPI to "know" the unit value for these
custom 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.
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.
MPI_Op_create
MPI_Reduce
MPI_Scan
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