[petsc-users] Fieldsplit with LSC for constrained elasticity/poroelasticity?

[Matthew Knepley]

On Thu, Oct 23, 2014 at 7:20 AM, Tabrez Ali <stali at geology.wisc.edu> wrote:

>  Hello
>
> I am using the following options (below) for solving linear
> elasticity/poroelasticity problems involving slip between two surfaces
> involving non-trivial geometries, i.e., elements with high aspect ratios,
> large contrasts in material properties etc. The constraints are imposed
> using Lagrange Multipliers.
>
> A picture (shows displacement magnitude) is attached. The boundary nodes,
> i.e., the base and the four side are pinned.
>
> The following options appear to work well for the saddle point problem:
>
> -pc_type fieldsplit -pc_fieldsplit_type schur
> -pc_fieldsplit_detect_saddle_point -fieldsplit_0_pc_type gamg
> -fieldsplit_0_ksp_type preonly -fieldsplit_1_pc_type lsc
> -fieldsplit_1_ksp_type preonly -pc_fieldsplit_schur_fact_type lower
> -ksp_monitor
>
> However, the number of iterations keep on increasing with the problems
> size (see attached plot), e.g.,
>
> 120K Tets        *507* Iterations (KSP Residual norm 8.827362494659e-05)
> in  17 secs on   3 cores
> 1 Million Tets  *1374* Iterations (KSP Residual norm 7.164704416296e-05)
> in 117 secs on  20 cores
> 8 Million Tets  *2495* Iterations (KSP Residual norm 9.101247550026e-05)
> in 225 secs on 160 cores
>
> So what other options should I try to improve solver performance? Any
> tips/insights would be appreciated as preconditioning is black magic to me.
>

For reports, always run with

  -ksp_view -ksp_monitor_true_residual -ksp_converged_reason

so that we can see exactly what you used.

I believe the default is a diagonal factorization. Since your outer
iterates are increasing, I would strengthen this
to either upper or full

  -pc_fieldsplit_schur_factorization_type <upper, full>

  Thanks,

      Matt


> Thanks in advance.
>
> Tabrez
>



-- 
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
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