Dear Stefano, That was exactly what I needed. It works now, thank you!
Regards, Elena ________________________________ From: Stefano Zampini <stefano.zamp...@gmail.com> Sent: 07 August 2025 19:55:12 To: Moral Sanchez, Elena Cc: PETSc users list Subject: Re: [petsc-users] error using CG with matrix-free operator and MG preconditioner 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!ci0JOiv6LHgM3Yfprnw7HmOcqzmdfgfp959aPpTEkIEr74xepQ4MSMrVMnS89dMTBA3e_vujFa3L5OfLfHuYdRsBecF9tispRqmV$ Stefano On Thu, Aug 7, 2025, 19:52 Moral Sanchez, Elena <elena.moral.sanc...@ipp.mpg.de<mailto:elena.moral.sanc...@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!ci0JOiv6LHgM3Yfprnw7HmOcqzmdfgfp959aPpTEkIEr74xepQ4MSMrVMnS89dMTBA3e_vujFa3L5OfLfHuYdRsBecF9tkMvgYZ5$ <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
