[mpiwg-rma] FENCE local completion requirements

[Balaji, Pavan]

On Sep 29, 2014, at 6:30 PM, Rajeev Thakur <thakur at mcs.anl.gov> wrote:
>> All RMA operations on win originating at a given process and started before the fence call will complete at that process before the fence call returns.  They will be completed at their target before the fence call returns at the target.
> 
> The intent of this is to avoid an ack at the end of the fence from target to origin. However, since a fence that completes RMA operations is effectively a barrier, when the fence returns on the origin, the origin can be sure that the target has at least called its fence (but may not have exited the fence).
> 
> pg 441, last para: "A fence call usually entails a barrier synchronization: a process completes a call to MPI_WIN_FENCE only after all other processes in the group entered their matching call.  However, a call to MPI_WIN_FENCE that is known not to end any epoch (in particular, a call with assert equal to MPI_MODE_NOPRECEDE) does not necessarily act as a barrier.”

Is the above sentence to be read as: if you don’t have MPI_MODE_NOPRECEDE, it *will* be a barrier?  That’s not how I interpret it.  If it does not *guarantee* a barrier, then we still need to do an MPI_BARRIER in the application in the example I gave.

  — Pavan

--
Pavan Balaji  ✉️
http://www.mcs.anl.gov/~balaji