Zero Pivot Row in LU Factorization

[Tim Stitt]

Barry,

I will be using these routines from Fortran..so I am assuming that 
Fortran interfaces are available for each routine?

Also, how do I know how many sub ksp's there will be? I am assuming I 
need to dynamically allocate the subksp array in Fortran but do I know 
the size in advance? Is this related to the value 'n' ? If so, how do I 
calculate 'n'.

What is the significance of subksp[0]? Is it just the sub ksp at this 
position I should be interested in?

Finally, which of the PCFactorSetxxxxxx routines should I be using?

Sorry for the twenty questions (well nearly) but I am just a bit 
confused with this approach.

Thanks,

Tim.

 Barry Smith wrote:
>
>   KSP *subksp;
>
>    KSPGetPC(ksp,pc)
>    PCBJacobiGetSubKSP(pc,&n,PETSC_NULL,&subksp)
>    KSPGetPC(subksp[0],&subpc);
>    PCFactorSetxxxxxx(subpc, ....
>
>   Barry
>
>
> On Nov 25, 2007, at 1:10 PM, Tim Stitt wrote:
>
>> I should also add that the code executes without this error when 
>> using 1 processor...but then displays the error when running in 
>> parallel with more than one process.
>>
>> Tim Stitt wrote:
>>> Hi all,
>>>
>>> Can anyone suggest ways of overcoming the following pivot error I 
>>> keep receiving in my PETSc code during a KSPSolve().
>>>
>>> [1]PETSC ERROR: Detected zero pivot in LU factorization
>>> see 
>>> http://www.mcs.anl.gov/petsc/petsc-as/documentation/troubleshooting.html#ZeroPivot! 
>>>
>>> [1]PETSC ERROR: Zero pivot row 1801 value 0.00102826 tolerance 
>>> 0.00165189 * rowsum 1.65189e+09!
>>>
>>> From checking the documentation....the error is in row 1801, which 
>>> means it is most likely not a matrix assembly issue?
>>>
>>> I tried the following prior to the solve with no luck either.....
>>>
>>> call KSPGetPC(ksp,pc,error)
>>> call PCFactorSetShiftNonzero(pc,PETSC_DECIDE,error)
>>>
>>> Is there anything else I can try?
>>>
>>> Thanks,
>>>
>>> Tim.
>>>
>>
>>
>> --Dr. Timothy Stitt <timothy_dot_stitt_at_ichec.ie>
>> HPC Application Consultant - ICHEC (www.ichec.ie)
>>
>> Dublin Institute for Advanced Studies
>> 5 Merrion Square - Dublin 2 - Ireland
>>
>> +353-1-6621333 (tel) / +353-1-6621477 (fax)
>>
>


-- 
Dr. Timothy Stitt <timothy_dot_stitt_at_ichec.ie>
HPC Application Consultant - ICHEC (www.ichec.ie)

Dublin Institute for Advanced Studies
5 Merrion Square - Dublin 2 - Ireland

+353-1-6621333 (tel) / +353-1-6621477 (fax)