[petsc-dev] MatNest and FieldSplit

[Mark Adams]

I think he is saying that this line seems to have no effect (and the
comment is hence wrong):

KSPSetOperators(subksp[nsplits - 1], S, S);

// J2 = [[4, 0] ; [0, 0.00001]]


J2 is a 2x2 but this block has been changed into two single equation
fields. Does this KSPSetOperators supposed to copy this 1x1 S matrix into
the (1,1) block of the "J2", or do some sort of correct mixing internally,
to get what he wants?


BTW, this line does not seem necessary to me so maybe I'm missing something.


KSPSetOperators(sub, J2, J2);



On Sun, Mar 24, 2019 at 4:33 PM Matthew Knepley via petsc-dev <
petsc-dev at mcs.anl.gov> wrote:

> On Sun, Mar 24, 2019 at 10:21 AM Pierre Jolivet <
> pierre.jolivet at enseeiht.fr> wrote:
>
>> It’s a 4x4 matrix.
>> The first 2x2 diagonal matrix is a field.
>> The second 2x2 diagonal matrix is another field.
>> In the second field, the first diagonal coefficient is a subfield.
>> In the second field, the second diagonal coefficient is another subfield.
>> I’m changing the operators from the second subfield (last diagonal
>> coefficient of the matrix).
>> When I solve a system with the complete matrix (2 fields), I get a
>> different “partial solution" than when I solve the “partial system” on just
>> the second field (with the two subfields in which I modified the operators
>> from the second one).
>>
>
> I may understand waht you are doing.
> Fieldsplit calls MatGetSubMatrix() which can copy values, depending on the
> implementation,
> so changing values in the original matrix may or may not change it in the
> PC.
>
>    Matt
>
> I don’t know if this makes more or less sense… sorry :\
>> Thanks,
>> Pierre
>>
>> On 24 Mar 2019, at 8:42 PM, Matthew Knepley <knepley at gmail.com> wrote:
>>
>> On Sat, Mar 23, 2019 at 9:12 PM Pierre Jolivet via petsc-dev <
>> petsc-dev at mcs.anl.gov> wrote:
>>
>>> I’m trying to figure out why both solutions are not consistent in the
>>> following example.
>>> Is what I’m doing complete nonsense?
>>>
>>
>> The code does not make clear what you are asking. I can see its a nested
>> fieldsplit.
>>
>>   Thanks,
>>
>>      Matt
>>
>>
>>> Thanks in advance for your help,
>>> 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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