[petsc-dev] parallel direct solvers for MG

Matthew Knepley knepley at gmail.com
Tue Jun 27 07:35:50 CDT 2017

On Tue, Jun 27, 2017 at 6:36 AM, Mark Adams <mfadams at lbl.gov> wrote:

> In talking with Garth, this will not work.
> I/we am now thinking that we should replace the MG object with Telescope.
> Telescope seems to be designed to be a superset of MG. Telescope does the
> processor reduction, and GAMG does as well, so we would have to reconcile
> this.  Does this sound like a good idea? Am I missing anything important?

I don't think "replace" is the right word. Telescope only does process
reduction. It does not do control flow for solvers,
or restriction/prolongation. You can see telescope interacting with MG here


I think more of this should be "default", in that the options are turned on
if you are running GMG on a large number of procs.

I also think GAMG should reuse the telescope code for doing reduction, but
I am not sure how hard this is. Mark?



> Mark
> On Tue, Jun 27, 2017 at 4:48 AM, Mark Adams <mfadams at lbl.gov> wrote:
>> Parallel coarse grid solvers are a bit broken at large scale where you
>> don't want to use all processors on the coarse grid. The ideal thing might
>> be to create a sub communicator, but it's not clear how to integrate this
>> in (eg, check if the sub communicator exists before calling the coarse grid
>> solver and convert if necessary). A bit messy. It would be nice if a
>> parallel direct solver would not redistribute the matrix, but then it would
>> be asking too much for it to reorder also, so we could have a crappy
>> ordering. So maybe the first option would be best long term.
>> I see we have MUMPS and PaStiX. Do either of these not redistribute if
>> asked?

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

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