[petsc-dev] MatNest and FieldSplit

[Pierre Jolivet]

Thanks, this makes (slightly) more sense to me know.
For some reason my application is still not acting properly but I must be screwing somewhere else the nested FieldSplit…

Thank you,
Pierre

> On 24 Mar 2019, at 11:42 PM, Dave May via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> 
> 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 <mailto: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 <mailto:petsc-dev at mcs.anl.gov>> wrote:
> On Sun, Mar 24, 2019 at 10:21 AM Pierre Jolivet <pierre.jolivet at enseeiht.fr <mailto: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 <mailto: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 <mailto: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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