[petsc-users] PCMG questions

Tue May 9 16:40:47 CDT 2023

On Tue, May 9, 2023 at 3:01 PM Barry Smith <bsmith at petsc.dev> wrote:

>
>
> On May 9, 2023, at 12:32 PM, Mark Adams <mfadams at lbl.gov> wrote:
>
> I have a MG hierarchy that I construct manually with DMRefine and
> DMPlexExtrude.
>
> * The solver works great with chevy/sor but with chevy/sor it converges
> slowly or I get indefinite PC errors from CG. And the eigen estimates in
> cheby are really high, like 10-15.
>
>
>    So with Cheby/SOR it works great but with the exact same options
> Cheby/SOR it behaves poorly? Are you using  some quantum computer and NERSc?
>

It turned out that I had the sign wrong on my Laplacian point function and
so the matrix was negative definite. I'm not sure what really happened
exactly but it is sort of behaving better.
It looks like my prolongation operator is garbage, the coarse grid
correction does nothing (cg/jacobi converges in a little less that the
number of MG iterations times the sum of pre and post smoothing steps), and
the rows sums of P are not 1.
Not sure what is going on there, but it is probably related to the DM
hierarchy not being constructed correctly....

>
> * I tried turning galerkin=none and I got this error.
>
>
>   This is because without Garkin it needs to restrict the current solution
> and then compute the coarse grid Jacobian. Since you did not provide a DM
> that has the ability to even generate coarse grid vectors the process can
> work. You need a DM that can provide the coarse grid vectors and restrict
> solutions. Did you forget to pass a DM to the solver?
>

The DM does everything. Similar to many examples but I've been checking
with snes/tutorials/ex12.c today.
I do call:
PetscCall(DMSetCoarseDM(dmhierarchy[r], dmhierarchy[r-1]));
But I am missing something else that goes on in DMRefineHierarchy, which I
can't use because I am semi-coarsening.
I probably have to build a section on each DM or something, but I have
bigger fish to fry at this point.

(I construct a 2D coarse grid, refine that a number of times and
DMPlexExtrude each one the same amount (number and distance), the extruded
direction is wrapped around a torus and made periodic.
The fine grid now looks like, and will eventually be, the grids that
tokamak codes use.)

Thanks,
Mark

>
> Any thoughts on either of these issues?
>
> Thanks,
> Mark
>
> [0]PETSC ERROR: --------------------- Error Message
> --------------------------------------------------------------
> [0]PETSC ERROR: Must call DMShellSetGlobalVector() or
> DMShellSetCreateGlobalVector()
> [0]PETSC ERROR: WARNING! There are option(s) set that were not used! Could
> be the program crashed before they were used or a spelling mistake, etc!
> [0]PETSC ERROR:   Option left: name:-ksp_converged_reason (no value)
> source: command line
> [0]PETSC ERROR:   Option left: name:-mg_levels_esteig_ksp_type value: cg
> source: command line
> [0]PETSC ERROR:   Option left: name:-mg_levels_pc_type value: sor source:
> command line
> [0]PETSC ERROR:   Option left: name:-options_left (no value) source:
> command line
> [0]PETSC ERROR: See https://petsc.org/release/faq/ for trouble shooting.
> [0]PETSC ERROR: Petsc Development GIT revision: v3.19.1-224-g9ed82936d20
>  GIT Date: 2023-05-07 12:33:48 -0400
> [0]PETSC ERROR: ./ex96 on a arch-macosx-gnu-O named MarksMac-302.local by
> markadams Tue May  9 12:26:52 2023
> [0]PETSC ERROR: Configure options CFLAGS="-g  -Wall" CXXFLAGS="-g  -Wall"
> COPTFLAGS=-O CXXOPTFLAGS=-O --with-cc=/usr/local/opt/llvm/bin/clang
> --with-cxx=/usr/local/opt/llvm/bin/clang++ --download-mpich
> --with-strict-petscerrorcode --download-triangle=1 --with-x=0
> --with-debugging=0 --download-hdf5=1 PETSC_ARCH=arch-macosx-gnu-O
> [0]PETSC ERROR: #1 DMCreateGlobalVector_Shell() at
> /Users/markadams/Codes/petsc/src/dm/impls/shell/dmshell.c:210
> [0]PETSC ERROR: #2 DMCreateGlobalVector() at
> /Users/markadams/Codes/petsc/src/dm/interface/dm.c:1022
> [0]PETSC ERROR: #3 DMGetNamedGlobalVector() at
> /Users/markadams/Codes/petsc/src/dm/interface/dmget.c:377
> [0]PETSC ERROR: #4 DMRestrictHook_SNESVecSol() at
> /Users/markadams/Codes/petsc/src/snes/interface/snes.c:649
> [0]PETSC ERROR: #5 DMRestrict() at
> /Users/markadams/Codes/petsc/src/dm/interface/dm.c:3407
> [0]PETSC ERROR: #6 PCSetUp_MG() at
> /Users/markadams/Codes/petsc/src/ksp/pc/impls/mg/mg.c:1074
> [0]PETSC ERROR: #7 PCSetUp() at
> /Users/markadams/Codes/petsc/src/ksp/pc/interface/precon.c:994
> [0]PETSC ERROR: #8 KSPSetUp() at
> /Users/markadams/Codes/petsc/src/ksp/ksp/interface/itfunc.c:406
> [0]PETSC ERROR: #9 KSPSolve_Private() at
> /Users/markadams/Codes/petsc/src/ksp/ksp/interface/itfunc.c:824
> [0]PETSC ERROR: #10 KSPSolve() at
> /Users/markadams/Codes/petsc/src/ksp/ksp/interface/itfunc.c:1070
> [0]PETSC ERROR: #11 SNESSolve_KSPONLY() at
> /Users/markadams/Codes/petsc/src/snes/impls/ksponly/ksponly.c:48
> [0]PETSC ERROR: #12 SNESSolve() at
> /Users/markadams/Codes/petsc/src/snes/interface/snes.c:4663
> [0]PETSC ERROR: #13 main() at ex96.c:433
>
>
>
>
>
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