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

[Matthew Knepley]

On Thu, Oct 23, 2014 at 10:48 AM, Tabrez Ali <stali at geology.wisc.edu> wrote:

>  Matt
>
> On 10/23/2014 09:54 AM, Matthew Knepley wrote:
>
>  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
>
> I simply used -pc_type lu -pc_factor_mat_solver_package mumps.
>
>
>  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.
>
> I will try this with MatNullSpaceCreateRigidBody. Btw does it matter if
> some nodes are pinned?
>

No these are null modes of the operator, not of the particular problem.

  Matt


> Tabrez
>
>
>    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
>
>


-- 
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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