[petsc-users] [PATCH 2/2] Call KSPSetFromOptions before setting fieldsplit-specific options

[Dave May]

Remove the option
   -fieldsplit_1_pc_type icc
It is clobbering the LU option Matt suggested you try first (
-fieldsplit_1_pc_type lu)

When debigging such solvers, you should always run with -ksp_view to check
your solver is configured correctly.



On 8 September 2013 15:32, Umut Tabak <u.tabak at tudelft.nl> wrote:

>  On 09/08/2013 03:00 PM, Matthew Knepley wrote:
>
>
>
>  If you want to solve hard systems, you must learn to debug a solver
> yourself. What does "just hangs" mean? Start with a full
> Schur complement factorization with LU on block 1 and a tiny tolerance on
> block 2. This will converge in 1 iterate. Then
> start slowly making the preconditioner weaker.
>
> Hi Matt,
>
> Thanks for the suggestion but I did not completely understand the options
> to use for this, I am still learning the fieldsplit interface. It was my
> mistake to not to give more in formation on what 'Just hangs' means, sorry.
>
> For the previous message: that meant extremely slow iterations, the
> iteration process was not advancing at all.
>
> After your reply and looking at the fieldsplit options a bit more, I came
> up with these set of options, now iterations are going fast but the
> decrease in relative residual is so slow. But apparently this does not
> converge in 1 iteration, I am still mistaken at some point.
>
> ./ex3 -pc_type fieldsplit -pc_fieldsplit_type schur -fieldsplit_1_pc_type
> lu -fieldsplit_1_ksp_type preonly -fieldsplit_1_pc_type icc
> -fieldsplit_2_ksp_type minres -fieldsplit_2_ksp_rtol 1e-8
> -pc_fieldsplit_schur_factorization_type full -ksp_monitor_true_residual
>
> Maybe you can help me at this point again.
>
> Thanks.
> Umut
>
>
>
>
>     Matt
>
>
>> The system is a symmetric system in this case given as
>>
>> A =
>> A11 C
>> C^T A22
>>
>> C is of size 1490X2432
>>
>> Could you please comment on this point for me, do I still have an error
>> on the problem setup or on the usage of the options?
>>
>> BR,
>> Umut
>>
>>
>>>
>>>  ISCreateBlock(PETSC_COMM_SELF,bs,n1,inputindices,PETSC_COPY_VALUES,&is_row1);
>>>
>>>  ISCreateBlock(PETSC_COMM_SELF,bs,n2,inputindices,PETSC_COPY_VALUES,&is_row2);
>>>
>>> We could check for this case, but it would require a little code and
>>> isn't currently done.  This is probably what you wanted:
>>>
>>> diff --git i/ex2.c w/ex2.c
>>> index 94cea2c..0cce3e6 100644
>>> --- i/ex2.c
>>> +++ w/ex2.c
>>> @@ -45,7 +45,7 @@ int main(int argc,char **args)
>>>     PetscInt bs = 2, n1=1, n2=1, inputindices[]={0,1}; /* block size is
>>> 2 */
>>>
>>> ISCreateBlock(PETSC_COMM_SELF,bs,n1,inputindices,PETSC_COPY_VALUES,&is_row1);
>>> -
>>>  ISCreateBlock(PETSC_COMM_SELF,bs,n2,inputindices,PETSC_COPY_VALUES,&is_row2);
>>> +
>>>  ISCreateBlock(PETSC_COMM_SELF,bs,n2,inputindices+n1,PETSC_COPY_VALUES,&is_row2);
>>>       ISView(is_row1,PETSC_VIEWER_STDOUT_SELF);
>>>     ISView(is_row2,PETSC_VIEWER_STDOUT_SELF);
>>>
>>
>>
>
>
>  --
> 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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