[petsc-users] ML and -pc_factor_shift_nonzero

[tribur at vision.ee.ethz.ch]

Dear Barry and Matt,

thanks for your helpful response.
ML works now using, e.g., -mg_coarse_redundant_pc_factor_shift_type  
POSITIVE_DEFINITE. However, it converges very slowly using the default  
REDUNDANT for the coarse solve. On 10 processors, e.g., even bjacobi  
plus -mg_coarse_ksp_max_it 10 works better. What solver do you  
recommend for the coarse solve? Maybe superlu?

Best regards,

Kathrin


Quoting "Barry Smith" <bsmith at mcs.anl.gov>:

>
> -mg_coarse_pc_factor_shift_nonzero since it is the coarse level of   
> the multigrid that is producing the zero pivot.
>
> Barry
>
> On Apr 13, 2010, at 8:51 AM, Matthew Knepley wrote:
>
>> On Tue, Apr 13, 2010 at 2:49 PM, <tribur at vision.ee.ethz.ch> wrote:
>> Hi,
>>
>> using ML I got the error
>>
>> "[0]PETSC ERROR: Detected zero pivot in LU factorization"
>>
>> As recommended at  
>> http://www.mcs.anl.gov/petsc/petsc-as/documentation/troubleshooting.html ,  
>> I tried -pc_factor_shift_nonzero but it doesn't have the desired   
>> effect using ML.
>>
>> How do I have to formulate the command line option? What does -  
>> [level]_pc_factor_shift_nonzero mean? What other parallel   
>> preconditioner could I try besides Hypre/Boomeramg or ML?
>>
>> This means the MG level, like 2. You can see all available options   
>> using -help.
>>
>> Matt
>>
>> Thanks in advance for your precious help,
>>
>> Kathrin
>>
>>
>>
>>
>>
>> -- 
>> 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
>
>