[petsc-users] Trying to write Fortran version of ksp/ex34.c

[Matthew Knepley]

On Thu, Apr 3, 2014 at 2:49 PM, Åsmund Ervik <asmund.ervik at ntnu.no> wrote:

>  Hi Matt,
>
> I'm handling the null space in the same way as ex34, that is, I remove the
> null space (containing just the constant functions, I think) in both
> ComputeRHS and ComputeMatrix.
>

That should be fine.


> However, I just tried commenting out those lines (MatNullSpaceCreate,
> MatSetNullSpace or MatNullSpaceRemove, MatNullSpaceDestroy) and that has no
> effect. The same happens in ex34.c, commenting out the lines fixing the
> null space makes no difference. Does that make sense?
>

It appears that the RHS in ex34 already has zero mean. Its possible for
some Krylov iteration to preserve this property, but
I think if you played around with solvers you could find one that screws it
up without null space removal.


> Are these examples intended to be run with some special options? I see in
> the makefile (e.g. runex34) that there are a lot of multigrid options, but
> I don't think that should matter?
>
> Also, could you confirm that ex34.c does zero Neumann BCs? The header was
> a bit confusing, it says u=0 but I assume it should be du/dn = 0 (zero
> normal derivative)?
>

Yes, its pure Neumann. Note that this is an easy analytical solution, so
you can check your example.

   Matt


> This is with 3.4.4, by the way.
>
> Thanks,
> Åsmund
>
>  ------------------------------
> *Fra:* Matthew Knepley [knepley at gmail.com]
> *Sendt:* 3. april 2014 15:56
> *Til:* Åsmund Ervik
> *Kopi:* petsc-users at mcs.anl.gov
> *Emne:* Re: [petsc-users] Trying to write Fortran version of ksp/ex34.c
>
>    On Thu, Apr 3, 2014 at 8:43 AM, Åsmund Ervik <asmund.ervik at ntnu.no>wrote:
>
>> -----BEGIN PGP SIGNED MESSAGE-----
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>> Dear PETSc users,
>>
>> I'm trying to write a Poisson solver in Fortran using the
>> KSPSetComputeOperators etc. framework. When debugging this, I ended up
>> modifying ksp/ex22.f so that it matches ksp/ex34.c. The difference
>> between these is that ex34.c has a non-constant RHS and Neumann BCs,
>> which is closer to what I want.
>>
>
>  If it has Neumann conditions, then it has a null space. Have you
> included this in
> your solver? That can cause a residual offset.
>
>    Matt
>
>
>> Now, when I run these two programs I get the following:
>>
>> ./ex22f_mod
>>    Residual, L2 norm         1.007312
>>    Error, sup norm         0.020941
>>    Error, L1 norm        10.687882
>>    Error, L2 norm         0.340425
>>
>> ./ex34
>>    Residual, L2 norm 1.07124e-05
>>    Error, sup norm 0.0209405
>>    Error, L1 norm 0.00618512
>>    Error, L2 norm 0.000197005
>>
>> but when I MatView/VecView the matrix and RHS for each example (write
>> them to file), there is no difference. I have attached ex22f_mod.F90,
>> any suggestions on what the error is?
>>
>> Once this example works, you can of course include it with PETSc.
>>
>> Regards,
>> Åsmund
>>
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>
>
>
>  --
> 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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