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

[Matthew Knepley]

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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> ____________________________________________________
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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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