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

[Umut Tabak]

> Your splits were both the same, so the preconditioner was applied twice
> to the first block and zero to the second block.  That made the
> preconditioner singular.

Dear Jed,

Thanks a lot for your time and help, attached you can find a very 
challenging ill-conditioned problem:

The first block comes from a structural problem and its size is 1490. 
The second block results from a fluid discretization and its size is 
2432. So in total the coupled system is of size 3922. I created the 
blocks as given in the simple problem. However, with

./ex3 -pc_type fieldsplit -pc_fieldsplit_type schur 
-fieldsplit_1_pc_type icc -fieldsplit_1_ksp_type cg 
-fieldsplit_1_pc_type icc -fieldsplit_1_ksp_type minres 
-pc_fieldsplit_schur_factorization_type upper -ksp_monitor_true_residual

The solver just hangs.

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);

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