[petsc-users] Load balancing / redistributing a 1D DM

Åsmund Ervik Asmund.Ervik at sintef.no
Mon Mar 5 08:25:02 CST 2018

My only argument "against" using Plex is that I don't understand how to use it. Is there a simple example anywhere that shows how to set up a 1D simplical (?) mesh, and then just get/return data between vectors associated with the Plex and (local) Fortran arrays on each proc? I don't have any KSP, SNES etc.

I assume such a code would go something like

<Plex mesh init stuff>
<Plex initial (uniform) parallel distribution>
<My code's init stuff>
<loop over time>
    <Plex equivalent of DMDAGetCorners>
    <Plex equivalent of DMDAVecGetArrayF90>
    <My code that computes things with the array, and finds weights for redistribution>
    <Plex equivalent of DMDAVecRestoreArrayF90>
    <Plex redistribution>
<end loop>


> -----Original Message-----
> From: Matthew Knepley [mailto:knepley at gmail.com]
> Sent: Monday, March 5, 2018 3:08 PM
> To: Tobin Isaac <tisaac at cc.gatech.edu>
> Cc: Jed Brown <jed at jedbrown.org>; Dave May
> <dave.mayhem23 at gmail.com>; Åsmund Ervik <Asmund.Ervik at sintef.no>;
> petsc-users at mcs.anl.gov
> Subject: Re: [petsc-users] Load balancing / redistributing a 1D DM
> On Mon, Mar 5, 2018 at 9:01 AM, Tobin Isaac <tisaac at cc.gatech.edu
> <mailto:tisaac at cc.gatech.edu> > wrote:
> 	This is a somewhat incomplete description of the steps in linear
> partitioning.  The rest can be accomplished with PetscSF calls, but I should
> wrap it up in a PetscPartitioner because it's a mistake-prone operation.
> Jed likes to do everything by hand because it is transparent, but then you
> become the maintainer.
> I think this is easy to do in Plex, and we maintain the code. It is less
> transparent, which is the tradeoff.
>    Matt
> 	On March 5, 2018 8:31:42 AM EST, Jed Brown <jed at jedbrown.org
> <mailto:jed at jedbrown.org> > wrote:
> 	>Dave May <dave.mayhem23 at gmail.com
> <mailto:dave.mayhem23 at gmail.com> > writes:
> 	>
> 	>> For a 1D problem such as yours, I would use your favourite graph
> 	>> partitioner (Metis,Parmetis, Scotch) together with your cell based
> 	>> weighting and repartition the data yourself.
> 	>
> 	>That's overkill in 1D.  You can MPI_Allreduce(SUM) and
> MPI_Scan(SUM)
> 	>the
> 	>weights, then find the transition indices in each subdomain.  It'll be
> 	>cheaper, more intuitive/deterministic, and avoid the extra library
> 	>dependency.  Of course if you think you may want to move to
> multiple
> 	>dimensions, it would make sense to consider DMPlex or DMForest.
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
> https://www.cse.buffalo.edu/~knepley/
> <http://www.caam.rice.edu/~mk51/>

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