[Mpi3-rma] Alternative Proposal for Shared Memory Support

[Torsten Hoefler]

On Tue, Mar 15, 2011 at 09:22:49PM -0500, Rajeev Thakur wrote:
> All can be used, but none are needed, right?
We might need to talk about synch or flush. Some shared memory systems
are loosely coupled and need membars to guarantee correct ordering. 

> Currently, in the unified model you can do:
> 
> lock
> put(A)
> unlock
> barrier          barrier
>                       x = A
> 
> With shared memory, you could do
> 
> A=10
> barrier         barrier
>                      x = A
>
> Right?
Not really. The barrier and the x=A might be re-ordered. This is also
the biggest issue with our definition of the unified model, some shared
memory systems might actually not be able to support it :-(. We had a
long discussion in Stuttgart about this.

> If the above is true, it might be a bit confusing to users that in the
> RMA chapter you use put, get, and synchronization functions for some
> things and can directly read/write memory for some other things.
Well, the standard is not a user documentation :-). But yes, having this
special kind of window requires some effort to understand it.

Best,
  Torsten

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Torsten Hoefler         | Performance Modeling and Simulation Lead
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