[petsc-users] Usage of AMG as preconditioner

[Matthew Knepley]

On Fri, Sep 28, 2018 at 3:23 AM Michael Werner <michael.werner at dlr.de>
wrote:

> Hello,
>
> I'm having trouble with getting the AMG preconditioners working. I
> tried all of them (gamg, ml, hypre-boomeramg), with varying
> degrees of "success":
>
> - GAMG:
> CMD options: -ksp_rtol 1e-8 -ksp_monitor_true_residual -ksp_max_it
> 20 -ksp_type fgmres -pc_type gamg -pc_gamg_sym_graph TRUE
> -pc_gamg_agg_nsmooths 1 -ksp_view
> always crashes with the following error:
> Error: error code 77
> [0] KSPSolve() line 780 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/src/ksp/ksp/interface/itfunc.c
> [0] KSPSolve_GMRES() line 233 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/src/ksp/ksp/impls/gmres/gmres.c
> [0] KSPInitialResidual() line 67 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/src/ksp/ksp/interface/itres.c
> [0] KSP_PCApply() line 281 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/include/petsc/private/kspimpl.h
> [0] PCApply() line 462 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/src/ksp/pc/interface/precon.c
> [0] PCApply_MG() line 377 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/src/ksp/pc/impls/mg/mg.c
> [0] PCMGMCycle_Private() line 20 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/src/ksp/pc/impls/mg/mg.c
> [0] KSPSolve() line 780 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/src/ksp/ksp/interface/itfunc.c
> [0] KSPSolve_Chebyshev() line 381 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/src/ksp/ksp/impls/cheby/cheby.c
> [0] Petsc has generated inconsistent data
> [0] Eigen estimator failed: DIVERGED_NANORINF at iteration 0
>
> When I'm using a different solver for -mg_levels_ksp_type, such as
> gmres, GAMG no longer crashes, but I don't see convergence of the
> problem (convergence history and ksp_view output are attached
> below).
>

It uses unpreconditioned GMRES to estimate spectral bounds for the operator
before using a Chebychev smoother.
If your matrix does not have a nice, connected, positive spectrum,
Chebychev will not work. However, the fact that
you get DIVERGED_NANORINF in the estimator tells me that you have a problem
in the matrix.


> - Hypre
> With the default settings, BoomerAMG just returns a Vector of all
> zeros after one iteration.
> When I change the relaxation type
> -pc_hypre_boomeramg_relax_type_all to Jacobi, I get similar
> results than with GAMG: the solver works without errors, but
> doesn't converge. The output for Hypre is also attached below.
>
> - ML
> With default settings the result is just like Boomeramg: a vector
> of all zeros after one iteration.
> When I change -mg_levels_ksp_type the behaviour is identical to
> GAMG.
>
>
> Since none of the packages worked, I'm assuming that the error
> lies with me/ my code,


It looks like a value in the matrix might be bad.


> so I'll give a short overview over what I'm
> trying to do.
> The matrix I'm trying to precondition is the Jacobian of a flow
> field originating from an unstructured finite-volume CFD code. It
> has a blocked structure as each node of the original mesh holds
> several variables (pressure, density, velocities). However, I'm
> not using a DM-Plex since I get the assembled Jacobian in binary
> format from the external CFD code.
> When I'm using direct (lu) I get correct results, so the basic
> code should be fine.


So LU works for the same problem that gave the NANORINF in GMRES?

I would recommend using a Manufactured Solution to check the operation. Then
you can start by feeding in the exact solution, and see that nothing fails.
Also, I
would make a very small test case, so that you can send the matrix and/or
code
to us to check.


> However, for larger problems lu is not
> feasible due the very large memory requirements, therefore I
> wanted to switch over to multigrid.
>
> Currently, PETSc has no information about the original geometry. I
> tried using setCoordinates, but for Hypre and ml it didn't make a
> difference, and GAMG crashed with an error:
>

This is only an optimization, and currently GAMG does not know what to
do for multiple fields (it handles single field problems by building the
nullspace of the symbol for the coarse grid). You could provide this when
you want to optimize things.

  Thanks,

    Matt


> [0] PCSetCoordinates() line 1883 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/src/ksp/pc/interface/precon.c
> [0] PCSetCoordinates_AGG() line 199 in
>
> /home/yoda/wern_mc/Programme/Anaconda/envs/GSA_27/weitere_programme/petsc-3.10.0/src/ksp/pc/impls/gamg/agg.c
> [0] Petsc has generated inconsistent data
> [0] Don't know how to create null space for ndm=2, ndf=4.  Use
> MatSetNearNullSpace.
>
> I would be glad if you could give me some advice on how to deal
> with this.
>
> Kind regards,
> Michael
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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