[petsc-users] error using CG with matrix-free operator and MG preconditioner

[Stefano Zampini]

If the problem is that the vector is not of nest type, you can overload the
createVecs for the python matrix type, see e.g.
https://urldefense.us/v3/__https://petsc.org/release/petsc4py/petsc_python_types.html__;!!G_uCfscf7eWS!aZjDCYTy1PCHmDJYxODxFR7QrOBVMVDa5NcHQ12xgd56_59kgzghGzTR42NjUnNXhIPGEMTWdWtzqqPdtK9UtT_taE4lvMw$ 

Stefano

On Thu, Aug 7, 2025, 19:52 Moral Sanchez, Elena <
Elena.Moral.Sanchez at ipp.mpg.de> wrote:

> Hello,
> I want to solve a linear system with conjugate gradient and multigrid
> preconditioner. I am using petsc4py. The solution is a Vector of type NEST
> and the operator is a matrix-free operator created using
> PETSc.Mat().createPython. The restriction and interpolation operators are
> all type NEST and their VecType is set to NEST. As smoother I am using CG
> without preconditioner. For the moment I am just using a classical V-cycle
> with 2 levels.
>
> I am able to solve the system with CG without the MG preconditioner.
> However, when I try to solve it with the MG preconditioner, it throws this
> error:
>
> Traceback (most recent call last):
>   File "/home/elena-hp/code/Poisson_MG_Petsc.py", line 321, in <module>
>     solver.solve(b, u)
>   File "petsc4py/PETSc/KSP.pyx", line 1782, in petsc4py.PETSc.KSP.solve
> petsc4py.PETSc.Error: error code 62
> [0] KSPSolve() at /home/elena-hp/petsc/src/ksp/ksp/interface/itfunc.c:1089
> [0] KSPSolve_Private() at /home/elena-hp/petsc/src/ksp/ksp/interface/
> itfunc.c:912
> [0] KSPSolve_CG() at /home/elena-hp/petsc/src/ksp/ksp/impls/cg/cg.c:213
> [0] KSP_PCApply() at /home/elena-hp/petsc/include/petsc/private/
> kspimpl.h:414
> [0] PCApply() at /home/elena-hp/petsc/src/ksp/pc/interface/precon.c:528
> [0] PCApply_MG() at /home/elena-hp/petsc/src/ksp/pc/impls/mg/mg.c:651
> [0] PCApply_MG_Internal() at /home/elena-hp/petsc/src/ksp/pc/impls/mg/
> mg.c:629
> [0] PCMGMCycle_Private() at /home/elena-hp/petsc/src/ksp/pc/impls/mg/
> mg.c:27
> [0] KSPSolve() at /home/elena-hp/petsc/src/ksp/ksp/interface/itfunc.c:1089
> [0] KSPSolve_Private() at /home/elena-hp/petsc/src/ksp/ksp/interface/
> itfunc.c:912
> [0] KSPSolve_CG() at /home/elena-hp/petsc/src/ksp/ksp/impls/cg/cg.c:162
> [0] VecCopy() at /home/elena-hp/petsc/src/vec/vec/interface/vector.c:1796
> [0] VecCopyAsync_Private() at /home/elena-hp/petsc/src/vec/vec/interface/
> vector.c:1755
> [0] VecCopy_Nest() at /home/elena-hp/petsc/src/vec/vec/impls/nest/
> vecnest.c:56
> [0] Invalid argument
> [0] Nest vector argument 2 not setup.
>
> The error does not appear when the operator is also of type NEST and the
> VecType is set to NEST. Unfortunately, for my application I cannot write
> the operator as type NEST.
>
> I think that the problem is related to the fact that the method
> createVecs() of the matrix-free operator does not return NEST vectors. I
> tried to do setVecType(PETSc.Vec.Type.NEST) on the matrix-free operator but
> when I call createVecs() it throws this error:
>
> Traceback (most recent call last):
>   File "petsc4py/PETSc/libpetsc4py.pyx", line 965, in
> petsc4py.PETSc.MatCreateVecs_Python
> petsc4py.PETSc.Error: error code 86
> [0] MatCreateVecs() at /home/elena-hp/petsc/src/mat/interface/
> matrix.c:9557
> [0] VecSetType() at /home/elena-hp/petsc/src/vec/vec/interface/
> vecreg.c:140
> [0] Unknown type. Check for miss-spelling or missing package:
> https://urldefense.us/v3/__https://petsc.org/release/install/install/*external-packages__;Iw!!G_uCfscf7eWS!aZjDCYTy1PCHmDJYxODxFR7QrOBVMVDa5NcHQ12xgd56_59kgzghGzTR42NjUnNXhIPGEMTWdWtzqqPdtK9UtT_tTSSXMDQ$ 
> <https://urldefense.us/v3/__https://petsc.org/release/install/install/*external-packages__;Iw!!G_uCfscf7eWS!dEBOuSXxIqEDpI33Xza0TlRPC5n6sN3fXNslohXzfITEmMwZLwQWJUTyNE-mMJHiXZjxQ-OlWu5625crKTR81qTlPMyhvkR09mUB$>
> [0] Unknown vector type: nest
>
> The above exception was the direct cause of the following exception:
>
> Traceback (most recent call last):
>   File "<string>", line 1, in <module>
>   File "petsc4py/PETSc/Mat.pyx", line 3411, in
> petsc4py.PETSc.Mat.createVecs
> petsc4py.PETSc.Error: error code 101
> [0] MatCreateVecs() at /home/elena-hp/petsc/src/mat/interface/
> matrix.c:9552
>
> Do you know how to fix this?
>
> Thank you in advance.
>
> Regards,
> Elena
>
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