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

[Tabrez Ali]

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 
> <mailto: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?

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 <mailto: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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