[Mpi3-rma] RMA proposal 1 update

[Brian Smith]

Sorry for the late response.... 
On BGP, DCMF Put/Get doesn't do any accounting and DCMF doesn't actually 
have a fence operation. There is no hardware to determine when a put/get 
has completed either. We need to send a get along the same 
(deterministically routed) path to "flush" any messages out to claim we 
are synchronized.

When we implemented ARMCI, we introduced accounting in our "glue" on top 
of DCMF because of the ARMCI_Fence() operation. There are similar concerns 
in the MPI one-sided "glue".

Going forward, we need to figure out how we'd implement the new MPI RMA 
operations and determine if there would be accounting required. If there 
would be (and I'm thinking there would), then an allfenceall in MPI would 
be easy enough to do and would provide a significant benefit on BG. We 
could just do an allreduce to look at counts. If the standard procedure is 
fenceall()+barrier(), I could do that much better as an allfenceall call.

On platforms that have some sort of native accounting, this allfenceall 
would only be the overhead of a barrier. So I think an allfenceall has 
significant value to the middleware more than DCMF and therefore would 
strongly encourage it in MPI, especially given the use-cases we heard from 
Jeff H. at the forum meeting.

This scenario is the same in our next super-secret product offering 
everyone knows about but I don't know if *I* can mention.


Brian Smith (smithbr at us.ibm.com)
BlueGene MPI Development/
Communications Team Lead
IBM Rochester
Phone: 507 253 4717



From:
"Underwood, Keith D" <keith.d.underwood at intel.com>
To:
"MPI 3.0 Remote Memory Access working group" 
<mpi3-rma at lists.mpi-forum.org>
Date:
05/16/2010 09:33 PM
Subject:
Re: [Mpi3-rma] RMA proposal 1 update
Sent by:
mpi3-rma-bounces at lists.mpi-forum.org



Before doing that, can someone sketch out the platform/API and the 
implementation that makes that more efficient?  There is no gain for 
Portals (3 or 4).  There is no gain for anything that supports Cray SHMEM 
reasonably well (shmem_quiet() is approximately the same semantics as 
MPI_flush_all).  Hrm, you can probably say the same thing about anything 
that supports UPC well - a strict access is basically a MPI_flush_all(); 
MPI_Put(); MPI_flush_all();... Also, I thought somebody said that IB gave 
you a notification of remote completion...

The question then turns to the "other networks".  If you can't figure out 
remote completion, then the collective is going to be pretty heavy, right?

Keith

> -----Original Message-----
> From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-
> bounces at lists.mpi-forum.org] On Behalf Of Jeff Hammond
> Sent: Sunday, May 16, 2010 7:27 PM
> To: MPI 3.0 Remote Memory Access working group
> Subject: Re: [Mpi3-rma] RMA proposal 1 update
> 
> Tortsten,
> 
> There seemed to be decent agreement on adding MPI_Win_all_flush_all
> (equivalent to MPI_Win_flush_all called from every rank in the
> communicator associated with the window) since this function can be
> implemented far more efficiently as a collective than the equivalent
> point-wise function calls.
> 
> Is there a problem with adding this to your proposal?
> 
> Jeff
> 
> On Sun, May 16, 2010 at 12:48 AM, Torsten Hoefler <htor at illinois.edu>
> wrote:
> > Hello all,
> >
> > After the discussions at the last Forum I updated the group's first
> > proposal.
> >
> > The proposal (one-side-2.pdf) is attached to the wiki page
> > https://svn.mpi-forum.org/trac/mpi-forum-web/wiki/RmaWikiPage
> >
> > The changes with regards to the last version are:
> >
> > 1) added MPI_NOOP to MPI_Get_accumulate and MPI_Accumulate_get
> >
> > 2) (re)added MPI_Win_flush and MPI_Win_flush_all to passive target
> mode
> >
> > Some remarks:
> >
> > 1) We didn't straw-vote on MPI_Accumulate_get, so this function might
> >   go. The removal would be very clean.
> >
> > 2) Should we allow MPI_NOOP in MPI_Accumulate (this does not make
> sense
> >   and is incorrect in my current proposal)
> >
> > 3) Should we allow MPI_REPLACE in
> MPI_Get_accumulate/MPI_Accumulate_get?
> >   (this would make sense and is allowed in the current proposal but
> we
> >   didn't talk about it in the group)
> >
> >
> > All the Best,
> >  Torsten
> >
> > --
> >  bash$ :(){ :|:&};: --------------------- http://www.unixer.de/ -----
> > Torsten Hoefler         | Research Associate
> > Blue Waters Directorate | University of Illinois
> > 1205 W Clark Street     | Urbana, IL, 61801
> > NCSA Building           | +01 (217) 244-7736
> > _______________________________________________
> > mpi3-rma mailing list
> > mpi3-rma at lists.mpi-forum.org
> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma
> >
> 
> 
> 
> --
> Jeff Hammond
> Argonne Leadership Computing Facility
> jhammond at mcs.anl.gov / (630) 252-5381
> http://www.linkedin.com/in/jeffhammond
> 
> _______________________________________________
> mpi3-rma mailing list
> mpi3-rma at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma

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