[mpiwg-rma] Fence + threads

[Jed Brown]

William Gropp <wgropp at illinois.edu> writes:

> There are lots of choices.  Passive target can be used.  Or a single
> thread can call MPI_Win_fence - all threads can call the RMA
> communication routines.  

The point of my example was that the threads are doing divergent tasks,
so wrapping a user mutex around MPI_Win_fence makes the algorithm more
synchronous and may result in idle/spinning threads.

The argument that passive target can be used isn't compelling to me
since it also applies in other cases that we use to justify active
target.
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