[petsc-users] Convergence problem in nonlinear problem, with PCBDDC.

[Matthew Knepley]

On Mon, Oct 13, 2014 at 11:24 AM, De Groof, Vincent Frans Maria <
Vincent.De-Groof at uibk.ac.at> wrote:

>
>  But if it is indefinite, shouldn't the factorization algorithm complain
> about a zero pivot?
>

Not necessarily. There are plenty of factorizable indefinite systems. Here
is one: an upper triangular
matrix with both 1 and -1 on the diagonal.

  Matt


>
>  Vincent
>  ------------------------------
> *Von:* Matthew Knepley [knepley at gmail.com]
> *Gesendet:* Montag, 13. Oktober 2014 17:23
> *An:* De Groof, Vincent Frans Maria
> *Cc:* petsc-users at mcs.anl.gov
> *Betreff:* Re: [petsc-users] Convergence problem in nonlinear problem,
> with PCBDDC.
>
>    On Mon, Oct 13, 2014 at 10:18 AM, De Groof, Vincent Frans Maria <
> Vincent.De-Groof at uibk.ac.at> wrote:
>
>>  Hi,
>>
>>
>>  I am having a convergence problem in a nonlinear FE problem (we only
>> use ksp from petsc, not snes). We don't have any problems using a direct
>> solver with a cholesky factorization (Petsc factor, cholmod, mumps). Now we
>> want to solve it using the bddc preconditioner. No problems on some
>> problems. So far so good.
>>
>>
>>  Unfortunately, in some problems, the first increment in the newton
>> iteration does not converge. It exits due to divergence (-4) after the
>> first solve (0 iterations). This happens with CG, using the natural or
>> preconditioned norm. Using the unpreconditioned norm, or minres, the
>> problem converges with no problems. Checking the Dirichlet and Neumann
>> errors of the subdomains (using -pc_bddc_check_level 1)  delivers small
>> errors for all subdomains (~1e-16 - 1e-11).
>>
>>
>>  Because minres works, and cg doesn't, it seems like I am missing a null
>> space. But since the subdomain solvers work without problems and do not
>> complain
>>
>
>  MINRES works for symmetric indefinite problems, whereas CG needs SPD
> problems. Could your problem be indefinite? If not, and it is close,
> its possible that the preconditioned problem is indefinite.
>
>    Matt
>
>
>>   about a null pivot this seems unlikely. Also when I check the number
>> of constraints on the subdomains, they should be fully-determined...
>>
>>
>>
>>  any suggestions on what might be going wrong here? Am I really missing
>> a nullspace? Or might there be something else going wrong?
>>
>>
>>
>>  kind regards,
>> Vincent
>>
>>
>
>
>  --
> 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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