symmetric reordering and incomplete factorization with tolerance?

[zhifeng sheng]

Hi Barry

Where can I find and use PCView_ICC ?

thanks
Zhifeng

Barry Smith wrote:
>
> On Sep 16, 2008, at 10:57 AM, zhifeng sheng wrote:
>
>> 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.
>
>    I've added this information to PCView_ICC.
>
>>
>>
>> And when I solve it with KSP, the solver always complains about
>>
>> Detected zero pivot in Cholesky factorization
>>
>> How could it be?
>
>     This is very possible, there is nothing that says you take some 
> symmetric matrix and do an ICC on it that you will get a positive 
> definite matrix or
> no zero pivots.
>
>    You can use -pc_factor_shift_positive_definite to force the ICC to 
> generate a positive definite matrix.
>
>    Barry
>
>>
>>
>> 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
>>>>
>>>>
>>>
>>
>