[Mpi-forum] Cancelling a send matched by a matching probe

Marek Tomáštík tomastik.marek at gmail.com
Tue Nov 3 15:41:10 CST 2015

Thank you for the detailed explanation. The described intent makes sense,
but I am not sure how this statement follows from the standard:

2015-11-02 15:41 GMT+01:00 William Gropp <wgropp at illinois.edu>:

> However, note that if the cancel fails, then the communication is not
> marked for cancellation, and an MPI_Wait could then wait until the message
> is received.

The standard says that "[a] call to MPI_CANCEL marks for cancellation a
pending, nonblocking communication operation". It does not say that the
cancellation has to suceed, but it does say that MPI_Cancel marks the
operation for cancellation -- unconditionally, as far as I can see (unless
an error occurs, of course -- but that wasn't what you meant by the cancel
failing, or was it?). From this and the statement I cited previously ("[i]f
a communication is marked for cancellation, then a 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)") it follows
by modus ponens that MPI_Wait for a request on which MPI_Cancel has been
called must behave as a local function. Am I interpreting this incorrectly?

Once again, thank you for your time.

Marek Tomáštík
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