[petsc-dev] Unions of IS and MatNest

[Matthew Knepley]

On Fri, Mar 13, 2020 at 3:19 AM Pierre Jolivet <pierre.jolivet at enseeiht.fr>
wrote:

>
>
> On 12 Mar 2020, at 11:40 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
> On Thu, Mar 12, 2020 at 5:59 PM Jed Brown <jed at jedbrown.org> wrote:
>
>> Pierre Jolivet <pierre.jolivet at enseeiht.fr> writes:
>>
>> > Hello,
>> > Has there been any follow-up on this
>> https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/024020.html
>> <https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/024020.html
>> >?
>> > Given a 3x3 MatNest A = [A_00,0,0 ; 0,A_11,0 ; 0,0,A_22], I’d like to
>> setup a two-way fieldsplit coupling [A_00,0 ; 0,A_11] and [A_22] but I
>> can’t figure out the proper options.
>>
>> Are you looking for a Schur split or additive/multiplicative?
>
>
> Don’t know yet which will perform best, do you have a specific solution in
> mind for one scenario or the other?
> I was mostly wondering if it was possible in a general context, not taking
> -pc_fieldsplit_type into account.
>
> -pc_fieldsplit_field_0 0,1 -pc_fieldsplit_field_1 2 -pc_fieldsplit_type
> schur
>
>
> These flags, used with my .cpp, yield:
> [0]PETSC ERROR: Arguments are incompatible
> [0]PETSC ERROR: To use Schur complement preconditioner you must have
> exactly 2 fields
> If I use -pc_fieldsplit_%d_fields <a,b,..> as advocated in the manual
> (instead of -pc_fieldsplit_field_%d as you suggested), I get the same error.
>

Okay it is  -pc_fieldsplit_%d_fields. When you use this, how many fields
does it think you have?

   Matt

> I believe.
>
>    Matt
>
>
>> > Jed, in this answer
>> https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/023993.html
>> <https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/023993.html>,
>> you recommend not to use MatNest. What would you recommend instead?
>>
>> See src/snes/examples/tutorials/ex28.c for my preferred approach.
>
>
> I’m not using DM, so in terms of Mat, I guess this example shows that I
> pretty much need to reimplement the field splitting myself?
>
> Thanks,
> Pierre
>
> --
> 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.cse.buffalo.edu/~knepley/>
>
>
>

-- 
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.cse.buffalo.edu/~knepley/>
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