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

[Matthew Knepley]

On Thu, Oct 23, 2014 at 9:27 AM, Tabrez Ali <stali at geology.wisc.edu> wrote:

>  Matt
>
> Sorry about that (I always forget it). The output for the smallest problem
> is now attached (see log.txt). I am also attaching some results that
> compare results obtained using FS/LSC and the direct solver (MUMPS), again
> for the smallest problem. The difference, as you can see is insignificant
> O(1E-6).
>

1) How do you use MUMPS if you have a saddle point

2) You can see from the output that something is seriously wrong with the
preconditioner. It looks like it has a null space.
    Did you add the elastic null modes to GAMG? Without this, it is not
going to work. We have helper functions for this:


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMPlexCreateRigidBody.html

you could just copy that code. And then use


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatSetNearNullSpace.html

I don't see it in the output, so I think this is your problem.

In order to test, I would first use MUMPS as the A00 solver and get the
Schur stuff worked out. Then I would
replace MUMPS with GAMG and tune it until I get back my original
convergence.

  Thanks,

    Matt


> Also, I did pass 'upper' and 'full' to
> '-pc_fieldsplit_schur_factorization_type' but the iteration count doesn't
> improve (in fact, it increases slightly). The attached log is with 'upper'.
>
> Regards,
>
> Tabrez
>
> On 10/23/2014 07:46 AM, Matthew Knepley wrote:
>
>  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
>
>
>


-- 
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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