symmetric reordering and incomplete factorization with tolerance?

[zhifeng sheng]

Dear Barry

I tried the RCM+ICC(2) in Petsc.... it seems that the reordering does 
not work well... as I view the preconditioner, it does not say in which 
way the matrix is reordered.

And when I solve it with KSP, the solver always complains about

Detected zero pivot in Cholesky factorization

How could it be?

Thanks
Best regards
Zhifeng

Barry Smith wrote:
>
>    You should use rcm+icc if you want to keep a symmetric preconditioner.
>
>    Depending on your matrix you might want to use KSPCR or KSPMINRES 
> or even KSPSYMMLQ
> instead of bicgstab?
>
>    We don't have a drop tolerance ICC and I do not recommend our drop 
> tolerance ILU.
>
>    Barry
>
>
> On Sep 16, 2008, at 10:23 AM, zhifeng sheng wrote:
>
>> Dear all
>>
>> I used the reordering scheme in petsc, and I would like to know 
>> whether they are symmetric or not.
>>
>> I have a symmetric matrix (for 3D system) and I tried to solve it 
>> with petsc and with matlab.
>>
>> In petsc I used rcm + ILU + bicgstab. while in matlab I used 
>> symrcm+ICC+bicgstab.
>>
>> it seems that with symmetric reordering, the factorization is 
>> somewhat faster... So I am wondering if it is possible to apply a 
>> symmetric reordering?
>>
>> PS: in Matlab, an incomplete factorization can be done with a 
>> tolerance (e.g. ICC(1e-4) ), can I do something like that with Petsc?
>>
>> Thanks a lot
>> Best regards
>> Zhifeng Sheng
>>
>>
>