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

[Pierre Jolivet]

> 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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