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MPI_Cancel - Cancels a communication request.
#include <mpi.h>
int MPI_Cancel(MPI_Request *request)
INCLUDE ’mpif.h’
MPI_CANCEL(REQUEST, IERROR)
INTEGER REQUEST, IERROR
#include <mpi.h>
void Request::Cancel() const
- request
- Communication request (handle).
- IERROR
- Fortran only: Error status (integer).
The MPI_Cancel operation
allows pending communications to be canceled. This is required for cleanup.
Posting a send or a receive ties up user resources (send or receive buffers),
and a cancel may be needed to free these resources gracefully.
A call to
MPI_Cancel marks for cancellation a pending, nonblocking communication
operation (send or receive). The cancel call is local. It returns immediately,
possibly before the communication is actually canceled. It is still necessary
to complete a communication that has been marked for cancellation, using
a call to MPI_Request_free, MPI_Wait, or MPI_Test (or any of the derived
operations).
If a communication is marked for cancellation, then an MPI_Wait
call for that communication is guaranteed to return, irrespective of the
activities of other processes (i.e., MPI_Wait behaves as a local function);
similarly if MPI_Test is repeatedly called in a busy wait loop for a canceled
communication, then MPI_Test will eventually be successful.
MPI_Cancel can
be used to cancel a communication that uses a persistent request (see Section
3.9 in the MPI-1 Standard, "Persistent Communication Requests") in the same
way it is used for nonpersistent requests. A successful cancellation cancels
the active communication, but not the request itself. After the call to
MPI_Cancel and the subsequent call to MPI_Wait or MPI_Test, the request
becomes inactive and can be activated for a new communication.
The successful
cancellation of a buffered send frees the buffer space occupied by the
pending message.
Either the cancellation succeeds or the communication succeeds,
but not both. If a send is marked for cancellation, then it must be the
case that either the send completes normally, in which case the message
sent is received at the destination process, or that the send is successfully
canceled, in which case no part of the message is received at the destination.
Then, any matching receive has to be satisfied by another send. If a receive
is marked for cancellation, then it must be the case that either the receive
completes normally, or that the receive is successfully canceled, in which
case no part of the receive buffer is altered. Then, any matching send has
to be satisfied by another receive.
If the operation has been canceled,
then information to that effect will be returned in the status argument
of the operation that completes the communication.
The primary expected
use of MPI_Cancel is in multi-buffering schemes, where speculative MPI_Irecvs
are made. When the computation completes, some of these requests may remain;
using MPI_Cancel allows the user to cancel these unsatisfied requests.
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.
MPI_Probe
MPI_Iprobe
MPI_Test_cancelled
MPI_Cart_coords
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