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Subject: Re: [OMPI users] dimension mapping of MPI_Cart_create
From: Paul Hilscher (pphilscher.ml_at_[hidden])
Date: 2010-06-30 00:36:19


Dear Jeff, thanks for the information.

>Open MPI currently has very limited cartesian support -- it actually
doesn't remap anything.

I see, OpenMPI doesn't remap anything; this explains probably why my runtime
of my simulation varies sometimes between 30% for the same setup.

>Would you have any interest in writing a partitioning algorithm for your
needs within the context of a plugin? I'd be happy to walk >you through the
process; it's not too complicated (although we should probably move the
discussion off to the Open MPI devel >mailing list).

I guess after using for more than a decade Open Source Software, it's time
to give something back :). ... so yes, I am willing to do that !!

Because I am not yet experienced with OpenMPI internals, I would really
appreciate your advice, if you could tell me where exactly I have to dig
into.. I guess it should be around ompi_topo_create function, but how to
write MPI_Cart_Create as a plugin, I will rely on you information. And do
you know if MPICH, LAM etc. have an efficient implementation of
MPI_Cart_Create ? so I can borrow some ideas from them....

best wishes,

Paul Hilscher

On Tue, Jun 29, 2010 at 8:17 PM, Jeff Squyres <jsquyres_at_[hidden]> wrote:

> Open MPI currently has very limited cartesian support -- it actually
> doesn't remap anything.
>
> That being said, it is *very* easy to extend Open MPI's algorithms for
> cartesian partitioning. As you probably already know, Open MPI is all about
> its plugins -- finding and selecting a good set of plugins to use at
> run-time. Open MPI has many different types of plugins. One of these types
> of plugins performs the cartesian/graph mapping behind MPI_Cart_create (and
> friends) function(s).
>
> Would you have any interest in writing a partitioning algorithm for your
> needs within the context of a plugin? I'd be happy to walk you through the
> process; it's not too complicated (although we should probably move the
> discussion off to the Open MPI devel mailing list).
>
>
> On Jun 29, 2010, at 4:50 AM, Paul Hilscher wrote:
>
> > Dear OpenMPI list,
> >
> > I am using a MPI-parallelized simulation program, with a
> domain-decomposition in 6-Dimensions.
> > In order to improve the scalability of my program I would like to know
> according to what preferences
> > is MPI distributing the ranks when using MPI_Cart_create( reorder
> allowed).
> >
> > To explain my inquiry, imagine a 3-dimensional solver in X-Y-M and 4
> computing
> > nodes, each nodes consist of 4 Quad-Core CPUs (4(Node)x[ 4(CPUs) x
> 4(Cores))] CPUs=64CPUs).
> >
> > Now I decompose all 3 dimensions by 4 (4x4x4 = 64) using
> MPI_Cart_create.
> > MPI has now several possibilities to map the problem e.g. X-M (locally)
> on a node and
> > Y across the nodes, or Y-M (locally) and X across the nodes.
> >
> > Now my question is, how can I tell MPI that I want to distribute X-Y
> locally while
> > M is distributed across nodes. The reason is that X-Y
> > communication ratio is much large (FFT) compared to M where we have only
> > 2 communications per time-step via an Allreduce.
> > An MPI implementation for the BlueGENE for example has an option
> > called mapfile where on can tell MPI how to map the dimensions onto
> > the Nodes. I did not found somethings similar for openmpi.
> >
> > Does anybody know how to achieve this mapping or could anybody
> > tell me where I could find some examples or tutorials ?
> >
> > Thank you very much for your help and best wishes
> >
> > Paul Hilscher
> > _______________________________________________
> >
>