[petsc-users] ILU preconditioner hangs with some zero elements on the diagonal

[Matthew Knepley]

On Tue, Oct 27, 2015 at 9:10 AM, Matthew Knepley <knepley at gmail.com> wrote:

> On Tue, Oct 27, 2015 at 9:06 AM, Gary Rebt <gary.rebt at gmx.ch> wrote:
>
>> Dear petsc-users,
>>
>> While using the FEniCS package to Solve a simple Stokes' flow problem, I
>> have run into problems with PETSc preconditioners. In particular, I would
>> like to use ILU (no parallel version) along with GMRES to solve my linear
>> system but the solver just hangs indefinitely
>> at MatLUFactorNumeric_SeqAIJ_Inode without outputting anything. CPU usage
>> is at 100% but even for a tiny system (59x59 for minimal test case), the
>> solver does not seem to manage to push through it after 30 mins.
>>
>> PETSc version is 3.6 and the matrix for the minimal test case is as
>> follows :
>> http://pastebin.com/t3fvdkaS
>>
>
> Hanging is a bug. We will check it out.
>

I do not have any way to read in this ASCII. Can you output a binary version

  -mat_view binary:mat.bin

  Thanks,

     Matt


> It contains zero diagonal entries, has a condition number of around 1e3
>> but is definitely non-singular. Direct solvers manage to solve the system
>> as well as GMRES without preconditioner (although after many iterations for
>> a 59x59 system..).
>>
>
> This will never work. Direct solvers work because they pivot away the
> zeros, but ILU is defined by having no pivoting.
>
>   Thanks,
>
>      Matt
>
>
>> Playing with the available options here
>> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCILU.html
>> did not seem to solve the issue (even after activating diagonal_fill and/or
>> nonzeros_along_diagonal) although sometimes error 71 is returned which
>> stands for zero pivot detected. Are there yet other options that I have not
>> considered? The default ILU factorization in MATLAB returns satisfactory
>> problems without errors so surely it must be possible with PETSc?
>>
>> As for the choice of ILU, I agree it might be suboptimal in this setting
>> but I do need it for benchmarking purposes.
>>
>> Best regards,
>>
>> Gary
>>
>
>
>
> --
> 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
>



-- 
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
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