[petsc-dev] MatNest and FieldSplit

[Dave May]

Matt is right.

When you defined the operator S, you basically invalidate the operator N
(in the sense that they are no longer consistent). Hence when you use KSP
nest to solve your problem your A matrix looks like
  A = diag[1, 2, 4, 0, 8]
but the B matrix you have defined looks like
  B = diag[1, 2, 4, 0.00001]

The only way to obtain the correct answer with your code is thus to use the
option
-ksp_type preonly

Thanks
Dave



On Sun, 24 Mar 2019 at 22:09, Mark Adams via petsc-dev <
petsc-dev at mcs.anl.gov> wrote:

> 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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