[petsc-dev] Unions of IS and MatNest

[Matthew Knepley]

On Fri, Mar 13, 2020 at 11:20 AM Pierre Jolivet <pierre.jolivet at enseeiht.fr>
wrote:

>
>
> On 13 Mar 2020, at 2:36 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
> 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?
>
>
> Three.
> In case it’s not clear, the MWE is at the bottom of my first email
> (nest.cpp).
> $ mpicxx nest.cpp -I$PETSC_DIR/$PETSC_ARCH/include -I$PETSC_DIR/include
> -L$PETSC_DIR/$PETSC_ARCH/lib -lpetsc
> $ ./a.out -pc_type fieldsplit -pc_fieldsplit_0_fields 0,1
> -pc_fieldsplit_1_fields 2 -pc_fieldsplit_type schur
> […]
> [0]PETSC ERROR: Arguments are incompatible
> [0]PETSC ERROR: To use Schur complement preconditioner you must have
> exactly 2 fields
> […]
>     Split info:
>     Split number 0 Defined by IS
>     Split number 1 Defined by IS
>     Split number 2 Defined by IS
> […]
>

I see now. If you call PCFieldsplitSetIS(), this overrides anything else.
We do not even try to discover the split. If you want
a split determined by command line arguments, you have to defer the split.
It looks like it can get splits from the MatNest,
so just do not call SetIS().

  Thanks,

     Matt


> Thanks,
> Pierre
>
>    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/>
>
>
>

-- 
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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