[petsc-users] Near null space for a fieldsplit in petsc4py

[Karin&NiKo]

Thank you very much for the information Matt.
Unfortunately, I do not use DM....  :-(

Le jeu. 13 juil. 2023 à 13:44, Matthew Knepley <knepley at gmail.com> a écrit :

> On Thu, Jul 13, 2023 at 5:33 AM Pierre Jolivet <pierre.jolivet at lip6.fr>
> wrote:
>
>> Dear Nicolas,
>>
>> On 13 Jul 2023, at 10:17 AM, TARDIEU Nicolas <nicolas.tardieu at edf.fr>
>> wrote:
>>
>> Dear Pierre,
>>
>> You are absolutely right. I was using a --with-debugging=0 (aka release)
>> install and this is definitely an error.
>> Once I used my debug install, I found the way to fix my problem. The
>> solution is in the attached script: I first need to extract the correct
>> block from the PC operator's MatNest and then append the null space to it.
>> Anyway this is a bit tricky...
>>
>>
>> Yep, it’s the same with all “nested” solvers, fieldsplit, ASM, MG, you
>> name it.
>> You first need the initial PCSetUp() so that the bare minimum is put in
>> place, then you have to fetch things yourself and adapt it to your needs.
>> We had a similar discussion with the MEF++ people last week, there is
>> currently no way around this, AFAIK.
>>
>
> Actually, I hated this as well, so I built a way around it _if_ you are
> using a DM to define the problem. Then
> you can set a "nullspace constructor" to make it if the field you are
> talking about is ever extracted. You use DMSetNullSpaceConstructor(). I do
> this in SNES ex62 and ex69, and other examples.
>
>   Thanks,
>
>      Matt
>
>
>> Thanks,
>> Pierre
>>
>> Regards,
>> Nicolas
>>
>> ------------------------------
>> *De :* pierre.jolivet at lip6.fr <pierre.jolivet at lip6.fr>
>> *Envoyé :* mercredi 12 juillet 2023 19:52
>> *À :* TARDIEU Nicolas <nicolas.tardieu at edf.fr>
>> *Cc :* petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
>> *Objet :* Re: [petsc-users] Near null space for a fieldsplit in petsc4py
>>
>>
>> > On 12 Jul 2023, at 6:04 PM, TARDIEU Nicolas via petsc-users <
>> petsc-users at mcs.anl.gov> wrote:
>> >
>> > Dear PETSc team,
>> >
>> > In the attached example, I set up a block pc for a saddle-point problem
>> in petsc4py. The IS define the unknowns, namely some physical quantity
>> (phys) and a Lagrange multiplier (lags).
>> > I would like to attach a near null space to the physical block, in
>> order to get the best performance from an AMG pc.
>> > I have been trying hard, attaching it to the initial block, to the IS
>> but no matter what I am doing, when it comes to "ksp_view", no near null
>> space is attached to the matrix.
>> >
>> > Could you please help me figure out what I am doing wrong ?
>>
>> Are you using a double-precision 32-bit integers real build of PETSc?
>> Is it --with-debugging=0?
>> Because with my debug build, I get the following error (thus explaining
>> why it’s not attached to the KSP).
>> Traceback (most recent call last):
>>   File "/Volumes/Data/Downloads/test/test_NullSpace.py", line 35, in
>> <module>
>>     ns = NullSpace().create(True, [v], comm=comm)
>>          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>   File "petsc4py/PETSc/Mat.pyx", line 5611, in
>> petsc4py.PETSc.NullSpace.create
>> petsc4py.PETSc.Error: error code 62
>> [0] MatNullSpaceCreate() at
>> /Volumes/Data/repositories/petsc/src/mat/interface/matnull.c:249
>> [0] Invalid argument
>> [0] Vector 0 must have 2-norm of 1.0, it is 22.3159
>>
>> Furthermore, if you set yourself the constant vector in the near
>> null-space, then the first argument of create() must be False, otherwise,
>> you’ll have twice the same vector, and you’ll end up with another error
>> (the vectors in the near null-space must be orthonormal).
>> If things still don’t work after those couple of fixes, please feel free
>> to send an up-to-date reproducer.
>>
>> Thanks,
>> Pierre
>>
>> > Thanks,
>> > Nicolas
>> >
>> >
>> >
>> >
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>> ou partielle, est interdite sauf autorisation expresse.
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>> le copier, de le faire suivre, de le divulguer ou d'en utiliser tout ou
>> partie. Si vous avez reçu ce Message par erreur, merci de le supprimer de
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>> ____________________________________________________
>> This message and any attachments (the 'Message') are intended solely for
>> the addressees. The information contained in this Message is confidential.
>> Any use of information contained in this Message not in accord with its
>> purpose, any dissemination or disclosure, either whole or partial, is
>> prohibited except formal approval.
>> If you are not the addressee, you may not copy, forward, disclose or use
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>> <test_NullSpace.py.gz>
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>>
>>
>
> --
> 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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