[petsc-dev] parallel direct solvers for MG

Mark Adams mfadams at lbl.gov
Tue Jun 27 09:46:27 CDT 2017

On Tue, Jun 27, 2017 at 8:35 AM, Matthew Knepley <knepley at gmail.com> wrote:

> 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

Oh it is not the answer at all!

>   https://arxiv.org/abs/1604.07163
> 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?

There is a little logic in there for selecting the number of processors on
the coarse grid. I think the way we integrate this is to integrate
parameters, if we want.  I take a hint on the number of equations to try to
keep on a process (nnz would be better). I don't take hints on cluster
size, I don't reduce the number of processors by an integer. I could change
these to be more in line with Telescope, but that does not solve our

Does PETSc now support matrices with an LHS and RHS communicator? I think
it does. I could just make sub communicators for each level in the GAMG
setup. I run PtAP and the see what I get and reduce the number of
processors (using something like MatGetSubMtrix as I recall, to aggregate
the matrix), and repartition if desired (one should).

Would it make sense for me to do this in GAMG, and see if it breaks
anything in MG?

>   Thanks,
>     Matt
>> 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
> http://www.caam.rice.edu/~mk51/
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