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

[Pierre Jolivet]

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.

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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