[petsc-users] Advice Being Sought

[Mark F. Adams]

Also, another thing to try that should work even with older PETSc versions is:

-mg_levels_ksp_chebyshev_estimate_eigenvalues 0,0.05,0,1.5

The last argument (1.5) is the largest eigenvalue safety factor (1.05 is the default).  I would bet that increasing this will keep the code stable for longer.  But the most robust thing to do is redo the PC every solve as I mentioned before.

Mark

On May 3, 2013, at 5:35 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

> 
>   Mark,
> 
>   Are you going to try this out?
> 
>    Thanks 
> 
>    Barry
> 
> On May 3, 2013, at 9:16 AM, David Scott <d.scott at ed.ac.uk> wrote:
> 
>> Barry,
>> 
>> On 23/04/2013 02:36, Barry Smith wrote:
>>> 
>>> On Apr 22, 2013, at 11:26 AM, David Scott <d.scott at ed.ac.uk> wrote:
>>> 
>>>> Thanks for the suggestion.
>>>> 
>>>> I had tried '-pc_type gamg -pc_gamg_agg_nsmooths 1' with an earlier version of the code without success. I have tried it again but I get NaN's after only 90 time steps whereas with block Jacobi it runs quite happily for 36,000 time steps and produces physically sensible results.
>>> 
>>>  David,
>>> 
>>>    We would be very interested in determining what is "going wrong" with the solver here since we hope to make it robust.  Would it be possible for you to use a MatView() and VecView() on the matrix and the right hand side with a binary viewer when it "goes bad" and send us the resulting file?
>>> 
>>>   Barry
>>> 
>>> We'd run the gamg solver on your matrix and track down what is happening.
>>> 
>>> 
>> 
>> I have inserted
>>   call PetscViewerBinaryOpen(PETSC_COMM_WORLD, 'DifficultSystem', &
>>     FILE_MODE_WRITE, viewer, ierr)
>>   call KSPGetOperators(ksp, A, PETSC_NULL_OBJECT, &
>>     PETSC_NULL_INTEGER, ierr)
>>   call MatView(A, viewer, ierr)
>>   call KSPGetRhs(ksp, b, ierr)
>>   call VecView(b, viewer, ierr)
>>   call PetscViewerDestroy(viewer, ierr)
>> in my code after KSPSolve.
>> 
>> You may download the result for the 29th time step from
>> http://www2.epcc.ed.ac.uk/~dscott6/
>> 
>> I can generate data for other time steps if necessary.
>> 
>> David
>> -- 
>> Dr. D. M. Scott
>> Applications Consultant
>> Edinburgh Parallel Computing Centre
>> Tel. 0131 650 5921
>> 
>> The University of Edinburgh is a charitable body, registered in
>> Scotland, with registration number SC005336.
> 
>