As far as I see, the Forum went into the direction to keep the sentence that (MPI-3.0 page 410 lines 17-19) "A consistent view can be created in the unified memory model (see Section 11.4) by utilizing the window synchronization functions (see Section 11.5) or explicitly completing outstanding store accesses (e.g., by calling MPI_WIN_FLUSH)." For the PSCW synchronization, this definition seems for me to restrictive, because it looks like that wording about local and remote accesses would imply a differentiation between a local part of a shared memory window and remote parts of the shared memory, which is unnatural for a shared Memory. To overcome this unnatural definition of PSCW synchronization (and also for the other one-sided synchronization methods) I wrote the solution in ticket #456 with a generalized definition that - has identical semantics with the existing text (cited above) for the case that one makes such unnatural Differentiation between local and remote shared memory parts, - but additional describes the synchronization without this differentiation, i.e., by defining the semantics of the synchronization functions for accesses to the same shared memory location by two processes. As far as I see, this more general definition does not require any implementation changes in the MPI libraries because all what is needed for the cited sentence implies the semantics defined in ticket #456. By the way, using only C11 functionality means that the user has to program its own process to process synchronization and all needed memory fences. It may make a code faster but not really easier to program. There is a high risk to make bugs or to use more fences than necessary. The definitions in #456 by using general access patterns is easy to program because the synchronization and memory fences are combined inside of the one-sided synchronization routines. Rolf ----- Original Message -----

From: "William Gropp" <[email protected]> To: "MPI WG Remote Memory Access working group" <[email protected]> Sent: Thursday, January 29, 2015 9:01:25 AM Subject: Re: [mpiwg-rma] RMA Errata

I believe that this is not only a pragmatic approach, but the correct one. You should be able to use the C11 atomic memory features to have a portable shared memory code (once we have full C11 compilers).

Bill

On Jan 28, 2015, at 11:22 PM, Jeff Hammond <[email protected]> wrote:

...

Thanks. I understand that this isn't an easy issue. We are trying to use MPI-3 shared memory windows internally with PSCW-like synchronization and I am inclined to just give up and use x86 intrinsics to do what I know is sufficient for our processors and compilers, just to make it easy (and fast).

Jeff

On Wed, Jan 28, 2015 at 1:43 PM, William Gropp <[email protected]> wrote:

...

That’s a harder topic and I wanted to think more about it. Here are the two issues with which I am most concerned:

1) We are defining the interaction of MPI with stuff that happens outside of MPI, in the programming language. Yes, we always sort of did, but this is in an area where many skilled people have made mistakes, and the likelyhood of an error is much higher. The lack of precision in these discussions reinforces my concern.

2) We are not considering the potential overhead that requiring other than a small set of RMA synchronization routines to guarantee some shared memory synchronization. The sense that I get is that the Forum is unconcerned about this, which I think is a serious mistake, and one for which I have yet to see a compelling use case.

Bill

On Jan 28, 2015, at 7:46 PM, Jeff Hammond <[email protected]> wrote:

Fine, but it would be helpful if you would respond to my comments about allowing point-to-point _synchronization_ on shared memory using PSCW, since those were the more germane ones anyways.

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-- Dr. Rolf Rabenseifner . . . . . . . . . .. email [email protected] High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307)