> On Mar 11, 2019, at 9:42 AM, Pietro Benedusi via petsc-users
> <petsc-users@mcs.anl.gov> wrote:
>
> Dear Petsc team,
>
> I have a question about the setting up of a multigrid solver.
>
> I would like yo use a PCG smoother, preconditioned with a mass matrix, just
> on the fine level.
> But when add the line for preconditioning the CG with the mass matrix my MG
> diverges.
>
> I have implemented the same solver in MATLAB and it converges fine. Also the
> operators in PETSc are the same and the PCG applied directly on the problem
> (without MG) works the same in both PETSC and MATLAB.
>
> This is what I do in PETSC for 2 levels:
>
>
> KSP space_solver;
>
> ierr = KSPCreate(PETSC_COMM_WORLD,&space_solver);CHKERRQ(ierr);
> ierr =
> KSPSetTolerances(space_solver,1e-8,PETSC_DEFAULT,PETSC_DEFAULT,PETSC_DEFAULT);CHKERRQ(ierr);
>
> ierr = KSPSetOperators(space_solver, K, K);CHKERRQ(ierr);
> ierr = KSPSetNormType(space_solver, KSP_NORM_UNPRECONDITIONED
> );CHKERRQ(ierr);
>
> ierr = KSPSetType(space_solver,KSPRICHARDSON);CHKERRQ(ierr);
> ierr = KSPSetFromOptions(space_solver);CHKERRQ(ierr);
> ierr = KSPSetUp(space_solver);CHKERRQ(ierr);
>
> PC pcmg;
> ierr = KSPGetPC(space_solver, &pcmg);
> ierr = PCSetType(pcmg, PCMG);
> ierr = PCMGSetLevels(pcmg,levels, NULL);CHKERRQ(ierr);
> ierr = PCMGSetGalerkin(pcmg,PC_MG_GALERKIN_BOTH);CHKERRQ(ierr);
>
> // smoothers
> for (int i = 1; i < levels; ++i)
> {
> KSP smoother;
> ierr = PCMGGetSmoother(pcmg, i, &smoother);CHKERRQ(ierr);
>
> ierr = KSPSetType(smoother, KSPCG);CHKERRQ(ierr);
> ierr = KSPSetOperators(smoother, K, M);CHKERRQ(ierr);
I'm not sure what you mean by "preconditioned with a mass matrix" but I
don't think this line makes sense. The last argument to this call is the matrix
from which the preconditioner is constructed. I don't think it ever makes
sense to precondition a Laplacian with a mass matrix; they are very different
beasts.
Barry
>
>
> // ierr = KSPSetUp(smoother);CHKERRQ(ierr);
>
> ierr =
> KSPSetTolerances(smoother,1e-12,PETSC_DEFAULT,PETSC_DEFAULT,s_p);CHKERRQ(ierr);
>
> ierr = KSPSetNormType(smoother, KSP_NORM_NONE);CHKERRQ(ierr);
>
> PC sm;
> ierr = KSPGetPC(smoother, &sm);CHKERRQ(ierr);
> ierr = PCSetType(sm, PCLU);CHKERRQ(ierr);
>
> ierr = PCMGSetInterpolation(pcmg, i,
> interpolation_operators[i-1]);CHKERRQ(ierr);
> }
>
>
>
> I think there is a problem with the PETSc syntax, because I checked
> everything else and it is fine.
>
> Do you any ideas?
>
> Thank you very much!
>
> Best,
> Pietro
>
> ~~~~~~~~~~~~
> Pietro Benedusi
>
> Numerical Simulation in Science,
> Medicine and Engineering research group
> ICS, Institute of Computational Science
> USI, Università della Svizzera Italiana
> Via Giuseppe Buffi, 13
> CH - 6900 Lugano
> ben...@usi.ch
>
>
