symmetric reordering and incomplete factorization with tolerance?

[Barry Smith]

   If you run with -ksp_view it calls PCView_ICC internally, or you  
can call PCView(). You never call PCView_ICC() directly.

    Barry

On Sep 16, 2008, at 11:32 AM, zhifeng sheng wrote:

> 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
>>>>>
>>>>>
>>>>
>>>
>>
>