[Mpi-forum] Discussion points from the MPI-<next> discussion today

Jed Brown jedbrown at mcs.anl.gov
Sun Sep 23 20:02:21 CDT 2012

MPI-2.2 Example 11.15 is a more direct example of the incorrectness of the
hypothetical code below. Since MPI-2 semantics are like MPI-3
MPI_WIN_SEPARATE, I assumed you were referring to MPI-3 with
MPI_WIN_UNIFIED. Even with MPI_WIN_UNIFIED, there is no portable way for a
target of a passive-mode RMA operation to get the result without an
explicit call (such as MPI_Win_lock, MPI_Win_sync, etc), see MPI-3 Example
11.9 and 11.12. The standard points out that lower-overhead non-portable
solutions may exist:

*"Compiler and hardware specific notations could ensure the load occurs
after the data is updated, or explicit one-sided synchronization calls can
be used to ensure the proper result."*

If you think something is underspecified or improperly specified, please
give an example.

On Sun, Sep 23, 2012 at 4:30 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Obviously the read fence is the relevant issue here. Your example is now
> the following (cf. MPI-3 Example 11.9)
> origin:
> MPI_Win_create
> MPI_Win_lock
> MPI_Put
> MPI_Win_unlock
> notify(side_channel) // e.g., global variable in shared memory,
> memory-mapped serial line, file system
> target:
> double buffer[10] = {0};
> MPI_Win_create(buffer,....)
> wait(side_channel) // e.g., spin
> x = buffer[0]
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