[mpiwg-rma] ambiguity in MPI_WIN_FLUSH_LOCAL and completion of bidirectional operations

[Jim Dinan]

Hi Jeff,

IIRC the intended semantic was that local and remote completion are the
same for gets, and read-modify-write atomics.

 ~Jim.


On Thu, Oct 17, 2013 at 2:09 PM, Jeff Hammond <jeff.science at gmail.com>wrote:

> From Page 450 (MPI_WIN_FLUSH_LOCAL):
>
> "Locally completes at the origin all outstanding RMA operations
> initiated by the calling process to the target process specified by
> rank on the specified window. For example, after this routine
> completes, the user may reuse any buffers provided to put, get, or
> accumulate operations."
>
> The list of operations includes only those present from MPI-2.  This
> leads to ambiguity.  Because all MPI-2 functions were unidirectional,
> the reader is required to assume that the implication that any (hence,
> all) buffers can be reused implies round-trip i.e. remote completion
> of bidirectional operations like MPI_GET_ACCUMULATE,
> MPI_COMPARE_AND_SWAP and MPI_FETCH_AND_OP.
>
> On the other hand, if we meant to say that MPI_WIN_FLUSH_LOCAL only
> implies the reuse of the origin buffer and that MPI_WIN_FLUSH is
> required for (re)use of the result buffer, then that must be specified
> explicitly since it is not obvious and possible contradictory to the
> text that is currently present.
>
> It might be prudent to take advantage of this ambiguity as an
> opportunity to introduce the latter semantics since I can imagine
> cases where it is beneficial to separate outbound (i.e. local) and
> inbound (i.e. round-trip i.e. remote) completion of bidirectional
> operations.
>
> All of my use cases block on round-trip completion but I prefer to
> maximize flexibility of RMA synchronization semantics, which argues
> for the second of the two definitions noted above.
>
> Best,
>
> Jeff
>
> --
> Jeff Hammond
> jeff.science at gmail.com
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