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MPI_Exscan, MPI_Iexscan - Computes an exclusive scan (partial
reduction)
#include <mpi.h>
int MPI_Exscan(const void *sendbuf, void *recvbuf, int count,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
int MPI_Iexscan(const void *sendbuf, void *recvbuf, int count,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
MPI_Request *request)
INCLUDE ’mpif.h’
MPI_EXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER COUNT, DATATYPE, OP, COMM, IERROR
MPI_IEXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, REQUEST, IERROR)
<type> SENDBUF(*), RECVBUF(*)
INTEGER COUNT, DATATYPE, OP, COMM, REQUEST, 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).
- request
- Request (handle, non-blocking only).
- 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.
The ‘in place’ option for intracommunicators is specified
by passing MPI_IN_PLACE in the sendbuf argument. In this case, the input
data is taken from the receive buffer, and replaced by the output data.
Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions
on its use as MPI_BOTTOM.
Because the in-place option converts the receive
buffer into a send-and-receive buffer, a Fortran binding that includes INTENT
must mark these as INOUT, not OUT.
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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