[petsc-users] GAMG

[Mark Adams]

So this is a fully 3D problem, or is it a very flat disc? What is the worst
aspect ratio (or whatever) of an element, approximately. That is, is this a
bad mesh?

You might want to start with a simple problem like a cube. The eigen
estimates (Smooth P0: max eigen=1.011119e+01) are huge and they are a lower
bound.

You might also try -gamg_est_ksp_max_it 50 and see if these eigen
estimates go up much. (and use -gamg_est_ksp_type cg). GAMG's eigen
estimates are working but I use  manufacture a seed vector, which is
slightly different than what Cheby does.

Also, what version of PETSc are you using? It would be best to use git to
clone the repository. This would give you maint or master branch which have
a fix for the cheby eigen estimator that your version might not have (use
-ksp_view and grep for "noisy" to see if you have an up to date version).


On Fri, Oct 28, 2016 at 10:12 AM, jeremy theler <jeremy at seamplex.com> wrote:

> I will try these options in a couple of hours (I have to go out now). I
> forgot to mention that the geometry has revolution symmetry around the z
> axis (just the geometry, not the problem because it has a non-symmetric
> temperature distribution).
> I am solving with only one proc, there are approx 50k nodes so 150k dofs.
> Thanks again.
>
> On Fri, Oct 28, 2016, 11:07 Mark Adams <mfadams at lbl.gov> wrote:
>
>> Also, try solving the problem with a one level iterative method and
>> Chebyshev, like:
>>
>> -ksp_type chebyshev
>> -pc_type jacobi
>>
>> It will take a long time to solve but I just want to see if it has the
>> same error.
>>
>>
>> On Fri, Oct 28, 2016 at 10:04 AM, Mark Adams <mfadams at lbl.gov> wrote:
>>
>> GAMG's eigen estimator worked but the values are very high.  You have
>> very low number of equations per processor, is this a thin body? Are the
>> elements badly stretched?
>>
>> Do this again with these parameters:
>>
>> -mg_levels_ksp_type chebyshev
>> -mg_levels_esteig_ksp_type cg
>> -mg_levels_esteig_ksp_max_it 10
>> ​​
>> -mg_levels_ksp_chebyshev_esteig 0,.1,0,1.05
>> -gamg_est_ksp_type cg
>>
>>
>> On Fri, Oct 28, 2016 at 9:48 AM, Jeremy Theler <jeremy at seamplex.com>
>> wrote:
>>
>> On Fri, 2016-10-28 at 09:46 -0400, Mark Adams wrote:
>> > Please run with -info and grep on GAMG.
>> >
>> [0] PCSetUp_GAMG(): level 0) N=120726, n data rows=3, n data cols=6,
>> nnz/row (ave)=41, np=1
>> [0] PCGAMGFilterGraph():         99.904% nnz after filtering, with
>> threshold 0., 13.7468 nnz ave. (N=40242)
>> [0] PCGAMGCoarsen_AGG(): Square Graph on level 1 of 1 to square
>> [0] PCGAMGProlongator_AGG(): New grid 1894 nodes
>> [0] PCGAMGOptProlongator_AGG(): Smooth P0: max eigen=5.726852e+00
>> min=1.330683e-01 PC=jacobi
>> [0] PCSetUp_GAMG(): 1) N=11364, n data cols=6, nnz/row (ave)=196, 1
>> active pes
>> [0] PCGAMGFilterGraph():         99.9839% nnz after filtering, with
>> threshold 0., 32.7656 nnz ave. (N=1894)
>> [0] PCGAMGProlongator_AGG(): New grid 155 nodes
>> [0] PCGAMGOptProlongator_AGG(): Smooth P0: max eigen=1.011119e+01
>> min=1.832878e-04 PC=jacobi
>> [0] PCSetUp_GAMG(): 2) N=930, n data cols=6, nnz/row (ave)=196, 1 active
>> pes
>> [0] PCGAMGFilterGraph():         100.% nnz after filtering, with
>> threshold 0., 32.7806 nnz ave. (N=155)
>> [0] PCGAMGProlongator_AGG(): New grid 9 nodes
>> [0] PCGAMGOptProlongator_AGG(): Smooth P0: max eigen=2.116373e+00
>> min=6.337173e-03 PC=jacobi
>> [0] PCSetUp_GAMG(): 3) N=54, n data cols=6, nnz/row (ave)=34, 1 active
>> pes
>> [0] PCGAMGFilterGraph():         100.% nnz after filtering, with
>> threshold 0., 5.66667 nnz ave. (N=9)
>> [0] PCGAMGProlongator_AGG(): New grid 2 nodes
>> [0] PCGAMGOptProlongator_AGG(): Smooth P0: max eigen=1.984549e+00
>> min=8.582767e-03 PC=jacobi
>> [0] PCSetUp_GAMG(): 4) N=12, n data cols=6, nnz/row (ave)=12, 1 active
>> pes
>> [0] PCSetUp_GAMG(): 5 levels, grid complexity = 1.48586
>> error: PETSc error 77-0 'Eigen estimator failed: DIVERGED_NANORINF at
>> iteration 0'
>> in /home/gtheler/libs/petsc-3.7.4/src/ksp/ksp/impls/cheby/cheby.c
>> KSPSolve_Chebyshev:440
>>
>>
>>
>> >
>> >
>> >
>> >
>>
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