[petsc-users] Modified Taylor-Hood elements with piece-wise constant pressure for Stokes equation

[Matthew Knepley]

On Tue, Jun 2, 2015 at 4:00 PM, Justin Chang <jychang48 at gmail.com> wrote:

> MTH did not converge with the default -ksp_rtol 1e-6 so I had to raise the
> tolerance to 1e-5 in order to get a solution. Attached are the outputs for
> TH and MTH
>

This really looks like it was never solving this system.

  Matt


> Last one with the svd did not work with the way the AMG PC was hard-coded
> into FEniCS. Here's the list of preconditioners my installation of FEniCS
> supports:
>
> Preconditioner    |  Description
> ---------------------------------------------------------------
> default           |  default preconditioner
> ilu               |  Incomplete LU factorization
> icc               |  Incomplete Cholesky factorization
> sor               |  Successive over-relaxation
> petsc_amg         |  PETSc algebraic multigrid
> jacobi            |  Jacobi iteration
> bjacobi           |  Block Jacobi iteration
> additive_schwarz  |  Additive Schwarz
> amg               |  Algebraic multigrid
> hypre_amg         |  Hypre algebraic multigrid (BoomerAMG)
> hypre_euclid      |  Hypre parallel incomplete LU factorization
> hypre_parasails   |  Hypre parallel sparse approximate inverse
> none              |  No preconditioner
>
> I would need to change a few things here and there if we really insisted
> on seeing if svd works.
>
> On Tue, Jun 2, 2015 at 3:00 PM, Jed Brown <jed at jedbrown.org> wrote:
>
>> Justin Chang <jychang48 at gmail.com> writes:
>>
>> > I originally solve that example problem using LU.
>>
>> I'd like to learn why LU didn't notice that the system is singular.
>> (The checks are not reliable, but this case should be pretty obviously
>> bad.)
>>
>> > But when I solve this one:
>> >
>> >
>> http://fenicsproject.org/documentation/dolfin/1.5.0/python/demo/documented/stokes-iterative/python/documentation.html
>> >
>> > By simply running their code as is for TH and adding the one like I
>> > mentioned for MTH, I get the following outputs when I pass in
>> -ksp_monitor
>> > -ksp_view and -log_summary
>>
>> Thanks, Justin.  Could you please run with
>>
>>  -ksp_type gmres -ksp_gmres_restart 1000 -ksp_monitor_true_residual
>> -ksp_monitor_singular_value
>>
>> Also, can you run a tiny problem size (like less than 1000 dofs) with
>>
>>   -pc_type svd -pc_svd_monitor
>>
>> Thanks.
>>
>
>


-- 
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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