[Mpi3-rma] MPI3 RMA Design Goals

Tue Sep 1 10:07:41 CDT 2009

If we take the SINGLE_RMA_INTERFACE_DRAFT_PROPOSAL as an example, and combine it with the draft design goal #1: In order to support RMA to arbitrary locations, no constraints on memory, such as symmetric allocation or collective window creation, can be required

We get an interesting view on how difficult it can be to get "close to the metal".  So, for MPI_RMA_xfer, we have to assume that the user has some array of target_mem data items.  That means the sequence of steps in user space is:

target_mem = ranks[dest];
MPI_RMA_xfer(... target_mem, dest...);

If we assume that the message sizes are small and the destinations randomly selected and the machine is large... every access to ranks is a cache miss, and we cannot prevent that by providing fancy hardware.  This actually leads me to believe that we may need to reconsider design goal #1, or at least clarify what it means in a way that makes the access more efficient.

MPI_RMA_xfer itself is no picnic either.  If we take the draft design goal #5: The RMA model must support non-cache-coherent and heterogeneous environments, then MPI is required to maintain a data structure for every rank (ok, it has to do this anyway, but we are trying to get close to the metal) and do a lookup into that data structure with every MPI_RMA_xfer to find out if the target is heterogeneous relative to the target rank - another cache miss.  Now, nominally, since this is inside MPI, a lower layer could absorb that check... or, a given MPI could refuse to support heterogeneity or... but, you get the idea.

So, we've got two cache line loads for every transfer.  One in the application and one in the MPI library.  One is impossible to move to the hardware and the other is simply very difficult to move.

For a contrast, look at SHMEM.  Assume homogeneous, only one communicator context, and hardware mapping of ranks to physical locations.  A shmem_put() of a short item could literally be turned into a few instructions and a processor store (on machines that supported such things).  Personally, I think we will have done well if we can get to the point that a reasonable hardware implementation can get MPI RMA to within 2x of a reasonable SHMEM implementation.  I think we have a long way to go to get there.

Keith

From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-bounces at lists.mpi-forum.org] On Behalf Of Vinod tipparaju
Sent: Tuesday, September 01, 2009 5:23 AM
To: MPI 3.0 Remote Memory Access working group
Subject: Re: [Mpi3-rma] MPI3 RMA Design Goals

Good points! RMA interfaces should do nothing to prevent utilizing a high message rate (or low overhead communication) that the underlying hardware may offer. To ensure this happens, there should always be a unrestricted path (lets call it this for now, people have called it a "thin layer", "direct access") to the network.

This means, despite the fact the the RMA interface has features that abstract out complexity by providing useful characteristics such as ordering and atomicity, it (the RMA interface) should always have this unrestricted, close to the heart of the hardware, path. To achieve this, the unrestricted path should not require any book keeping (from implementation perspective) in relation to the feature-rich path or vice-versa.

I believe this is what we have demonstrated with the example interfaces hence the null set isn't the case here :-). I will distribute an example implementation very soon so people can get a feel.

---
Vinod Tipparaju ^ http://ft.ornl.gov/~vinod ^ 1-865-241-1802

> From: keith.d.underwood at intel.com
> To: mpi3-rma at lists.mpi-forum.org
> Date: Mon, 31 Aug 2009 16:17:28 -0600
> Subject: Re: [Mpi3-rma] MPI3 RMA Design Goals
>
> There has been stunning silence since this email, so I will go ahead and toss out a thought...
>
> In the draft design goals, I don't see two issues that I see as key. The first is "support for high message rate/low overhead communications to random targets". As best I can tell, this is one of the key places were the existing one-sided operations are perceived as falling down for existing customers of SHMEM/PGAS. The second is "elimination of the access epoch requirement". This one may be, um, more controversial, but I believe it is part and parcel with the first one. That is, the first one is not that valuable if the programming model requires an excessive amount of access epoch opens and closes just to force the global visibility of the operations. Unfortunately, the intersection of this solution space with the solution space for the current draft design goal #5 (support non-cache-coherent and heterogeneous environments) may be the null set... I will hold out hope that this isn't the case ;-)
>
> Keith
>
> > -----Original Message-----
> > From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-
> > bounces at lists.mpi-forum.org] On Behalf Of William Gropp
> > Sent: Wednesday, August 05, 2009 12:37 PM
> > To: mpi3-rma at lists.mpi-forum.org
> > Subject: [Mpi3-rma] MPI3 RMA Design Goals
> >
> > I've added versions of the RMA design goals that we discussed at the
> > Forum meeting last week to the wiki page for our group (
> > https://svn.mpi-forum.org/trac/mpi-forum-web/wiki/RmaWikiPage
> > ). This is a draft; lets discuss these. Also, feel free to add to
> > the discussion, particularly in the background section.
> >
> > Bill
> >
> > William Gropp
> > Deputy Director for Research
> > Institute for Advanced Computing Applications and Technologies
> > Paul and Cynthia Saylor Professor of Computer Science
> > University of Illinois Urbana-Champaign
> >
> >
> >
> >
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> > mpi3-rma at lists.mpi-forum.org
> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma
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