[petsc-users] large number of iteration.

[Mark Adams]

Correction: CG converges to the highest eigenvalue much faster than GMRES.

On Thu, Aug 16, 2018 at 1:33 PM Mark Adams <mfadams at lbl.gov> wrote:

>
>
> 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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