[petsc-users] large number of iteration.

[Mark Adams]

On Thu, Aug 16, 2018 at 12:20 PM Josh L <ysjosh.lo at gmail.com> wrote:

>
>
>
> 2018-08-16 10:39 GMT-05:00 Mark Adams <mfadams at lbl.gov>:
>
> Hi Mark,
>
> The problem I am running is simply 2-D linear elasticity, and I use
> petsc's ex49 of KSP to do benchmark.
>
> I have solved the error by using MatZeroRowsColumns(diagonal =1.0) to
> apply BC instead of MatZeroRows.
> (My code is actually using newton's method to solve nonlinear problem(Now,
> use it to run a linear elasticity problem), and the rhs entries for
> Dirichlet BC is just zero, so either zerorowscolumns or zerorows gives the
> same solution).
>
> With GMRES and gamg, the 120K degrees of freedoms linear elasticity
> problem converges in 7 iterations.
> But with " -ksp_type chebyshev -ksp_chebyshev_esteig_noisy"  I still get
> NAN residual that comes out of nowhere (it is not diverging. all of a
> sudden NAN shows up)
>

You want to use CG as the ksp type if it is symmetric (and positive
definite). I would guess you don't need to zero columns but go ahead and do
it, if it is easy.

I am confused. I thought you were using '-ksp_type chebyshev' to debug
Cheby errors when using GAMG.

Anyway, with -ksp_type chebyshev you want to use CG in the eigen estimator.
CG is much faster than GAMG. So you should use '-ksp_chebyshev_esteig_ksp_type
cg'. And use -options_left to make sure that these parameters are getting
in correctly.

And you should use '-ksp_type cg'.
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