# [petsc-users] Suggestions for code with dof >> 1 ?

Christophe Ortiz christophe.ortiz at ciemat.es
Thu Oct 17 06:56:49 CDT 2013

```On Thu, Oct 17, 2013 at 12:00 PM, Christophe Ortiz <
christophe.ortiz at ciemat.es> wrote:

>
>
> On Wed, Oct 16, 2013 at 4:22 PM, Christophe Ortiz <
> christophe.ortiz at ciemat.es> wrote:
>
>>
>>
>> On Wed, Oct 16, 2013 at 4:10 PM, Christophe Ortiz <
>> christophe.ortiz at ciemat.es> wrote:
>>
>>>
>>>
>>> On Wed, Oct 16, 2013 at 3:36 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>>>
>>>> Christophe Ortiz <christophe.ortiz at ciemat.es> writes:
>>>> > I found where is the problem. I had
>>>> > set TSARKIMEXSetFullyImplicit(ts,PETSC_TRUE);
>>>> > I found it produces wrong solutions, either with ARKIMEX3, 1BEE or A2.
>>>> > Solution becomes negative. Without this option, all ARKIMEX types
>>>> work very
>>>> > well and very fast.
>>>>
>>>> I'm glad they work as IMEX methods (please check that the solutions are
>>>> correct), but I would like to find out why they are not working in fully
>>>> implicit mode.  Is your problem similar to ex25?
>>>
>>>
>>> Yes. The only difference is that I put everything (diffusion and
>>> reaction terms) under IFunction and IJacobian.
>>>
>>> My system is
>>>
>>> u_t - alpha.u_xx + k.u.v = 0
>>> v_t - beta.v_xx + k.u.v = 0
>>>
>>> with Dirichlet boundary conditions
>>> u(x=0,t) = u(x=L,t) = cste
>>> v(x=0,t) = v(x=L,t) = cste
>>>
>>>  (If I remember
>>>> correctly, that example uses a DAE formulation for boundary conditions
>>>> and thus has trouble with some methods with explicit stages.  There may
>>>> be more going on.)
>>>>
>>>
>>> Now you say it, I'm having some troubles. After checking that it worked
>>> very well with only diffusion terms with ARKIMEX, I introduced the reaction
>>> term. With ARKIMEX 1BEE/A2, it goes very fast up to t=0.27 s, then, steps
>>> start being rejected and then it goes very slowly, with small timesteps.
>>> See output below.
>>> Do you think it is due to the fact that I use IFunction to define
>>> boundary conditions, which produces troubles with ARKIMEX methods ?
>>> What would be the solution ?
>>>
>>> Christophe
>>>
>>>
>> I just tried with TSARKIMEXSetFullyImplicit(ts,PETSC_TRUE); It goes much
>> better and reaches final time....but solution is clearly not correct. The
>> initial gaussian distributions do not change. Like if diffusion was null.
>>
>
> Hi,
>
> I've tried something else. Since ARKIMEX is able to converge up to t~0.20s
> and gives the correct solution for this time, and then comes into trouble,
> I thought that it could come from the SNES.
> Then, instead of using SNESNEWTONLS, I tried SNESNEWTONTR. I summarize my
> findings:
>
> - With NEWTONTR, ARKIMEX (3, 1BEE or A2) converges.
>
> -With NEWTONTR, ARKIMEX 3 converges much faster than 1BEE or A2 (much less
> timesteps).
>
> - No conflicts between NEWTONTR and ARKIMEXFullyImplicit as with NEWTONLS.
> Give the same results.
>
>
> Then I came back to SNESNEWTONLS
>
> - With NEWTONLS, ARKIMEX does not converges.
>
> - When ARKIMEXFullyImplicit is used with NEWTONLS, it converges, but
> solution clearly incorrect. It seems it does nothing, initial distributions
> do not change.
>
>
> And I played a bit with the tolerances:
>
> - No influence of rtol (increasing or decreasing it). It stops at same
> point.
>
> - Changing stol from 1e-8 to 1e-7 seems to ease as it goes up to longer
> time (6.98 s instead of 0.20) but then stops with an error:
>
> [0]PETSC ERROR: --------------------- Error Message
> ------------------------------------
> [0]PETSC ERROR: Floating point exception!
> [0]PETSC ERROR: Vec entry at local location 450 is not-a-number or
> infinite at end of function: Parameter number 3!
> ...
> ...
> [0]PETSC ERROR: VecValidValues() line 30 in src/vec/vec/interface/rvector.c
> [0]PETSC ERROR: SNESComputeFunction() line 1998 in
> src/snes/interface/snes.c
> [0]PETSC ERROR: SNESSolve_NEWTONLS() line 162 in src/snes/impls/ls/ls.c
> [0]PETSC ERROR: SNESSolve() line 3636 in src/snes/interface/snes.c
> [0]PETSC ERROR: TSStep_ARKIMEX() line 765 in src/ts/impls/arkimex/arkimex.c
> [0]PETSC ERROR: TSStep() line 2458 in src/ts/interface/ts.c
> [0]PETSC ERROR: TSSolve() line 2583 in src/ts/interface/ts.c
> [0]PETSC ERROR: main() line 392 in src/ts/examples/tutorials/diffusion.c
>
> Maybe all this will give you a hint of what's going on...
> Saludos,
> Christophe
>
>
>

- NEWTONLS does not work with the default LineSearch, ie bt.

- NEWTONLS works very well with LineSearch basic, l2 and cp. Converges fast
and results are correct.

Seems to be due to the LineSearch bt.

Christophe
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