[petsc-users] [PCGAMG + AGG + GMRES] Non-Exact Dirichlet Boundary Conditions

[Barry Smith]

   Elias,

    How are you zeroing the rows and columns of the matrix, are you using MatZeroRowsColumns()? 

    If so, are you passing in the vectors b and x (with b set and the x locations associated with the Dirichlet rows set to the correct Dirichlet value)?

    If so, are you calling KSPSetInitializeGuessNonzero(ksp,PETSC_TRUE) before the solve?

    Thanks

    Barry


> On Jul 7, 2022, at 3:14 AM, Karabelas, Elias (elias.karabelas at uni-graz.at) <elias.karabelas at uni-graz.at> wrote:
> 
> Hi Mark,
> thanks for the answer, but I'm struggling to translate your suggestion into solver options. 
> From scrolling through the user manual I think this points towards PCCOMPOSITE.
> However, the User Manual is not very precise with composite PCs, so how would I achieve this on top of my existing solver options?
> Best regards
> Elias
> 
> 
> 
> Am 06.07.22 um 16:41 schrieb Mark Adams:
>> And one iteration of undamped Jacobi after the solve should fix this.
>> 
>> On Wed, Jul 6, 2022 at 8:42 AM Karabelas, Elias (elias.karabelas at uni-graz.at <mailto:elias.karabelas at uni-graz.at>) <elias.karabelas at uni-graz.at <mailto:elias.karabelas at uni-graz.at>> wrote:
>> Dear Matt,
>> 
>> thanks for the fast response. That makes perfect sense to me. 
>> 
>> Best regards
>> Elias
>> 
>> Am 06.07.22 um 14:35 schrieb Matthew Knepley:
>>> On Wed, Jul 6, 2022 at 7:46 AM Karabelas, Elias (elias.karabelas at uni-graz.at <mailto:elias.karabelas at uni-graz.at>) <elias.karabelas at uni-graz.at <mailto:elias.karabelas at uni-graz.at>> wrote:
>>> Dear all,
>>> 
>>> I don't know if this is a bug, but I observed that when using GMRES with AGG-PCGAMG as preconditioner Dirichlet boundary conditions don't seem to be exactly fulfilled.
>>> 
>>> My Matrix has zero rows and cols with 1 on the diagonal where I have dirichlet-bcs in my FE-mesh and I would expect that the eqs in this rows can be exactly fulfilled (as u_i = g_i) there.
>>> 
>>> I would not expect aggregation to be exact here, but only within the iteration tolerance. If instead you eliminate those variables, you can maintain algebraic exactness.
>>> This is what we do in examples, like SNES ex56.
>>> 
>>>   Thanks,
>>> 
>>>      Matt
>>> However, when I solve A*x = b with the above solver I only get u_i = g_i + error in that part of the vector. Switching from pc_gamg_type agg to pc_gamg_type classical cures this problem, but the classical is not advertised in the user manual.
>>> 
>>> These are the options I'm currently using:
>>> 
>>> -ksp_type gmres
>>> -ksp_pc_side right
>>> -pc_type gamg
>>> -pc_gamg_type agg [or classical]
>>> -pc_gamg_sym_graph 1
>>> -pc_gamg_square_graph 1
>>> -pc_gamg_agg_nsmooths 1
>>> -pc_gamg_threshold 0.01
>>> -pc_mg_cycles v
>>> 
>>> Iteration counts are basically the same.
>>> 
>>> Best regards
>>> 
>>> Elias
>>> 
>>> -- 
>>> Dr. Elias Karabelas
>>> Research Associate
>>> University of Graz
>>> Institute of Mathematics and Scientific Computing
>>> Heinrichstraße 36
>>> A-8010 Graz
>>> Austria
>>> 
>>> Phone: +43 316 380 8546
>>> Email: elias.karabelas at uni-graz.at <mailto:elias.karabelas at uni-graz.at>
>>> Web:  https://ccl.medunigraz.at/ <https://ccl.medunigraz.at/>
>>> 
>>> -- 
>>> 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/>
>> 
>> -- 
>> Dr. Elias Karabelas
>> Research Associate
>> University of Graz
>> Institute of Mathematics and Scientific Computing
>> Heinrichstraße 36
>> A-8010 Graz
>> Austria
>> 
>> Phone: +43 316 380 8546
>> Email: elias.karabelas at uni-graz.at <mailto:elias.karabelas at uni-graz.at>
>> Web:  https://ccl.medunigraz.at/ <https://ccl.medunigraz.at/>
> -- 
> Dr. Elias Karabelas
> Research Associate
> University of Graz
> Institute of Mathematics and Scientific Computing
> Heinrichstraße 36
> A-8010 Graz
> Austria
> 
> Phone: +43 316 380 8546
> Email: elias.karabelas at uni-graz.at <mailto:elias.karabelas at uni-graz.at>
> Web:  https://ccl.medunigraz.at/ <https://ccl.medunigraz.at/>
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