[petsc-dev] parallel direct solvers for MG

[Hong]

For '-pc_type mg', we use '-mg_levels_0_pc_type
redundant -mg_coarse_pc_redundant_number <n>' to control num of processors
for each coarse-grid solve, n<=np and n=np is the default which solves
coarse-grid sequentially.

See petsc/src/snes/examples/tutorials/runex48_4, for which
you can add option '-mg_coarse_pc_redundant_number 2' to have two
subcommunicators at coarse-grid level.

Can you use pcredundant for gamg?

Hong

On Tue, Jun 27, 2017 at 9:46 AM, Mark Adams <mfadams at lbl.gov> wrote:

>
>
> 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
> problem.
>
> 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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