[petsc-users] Advice Being Sought

[Mark F. Adams]

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

> 
>   Mark,
> 
>   Are you going to try this out?
> 

Blowing up after some time steps is most likely (e.g., certainly) from the PC getting old.  In particular the largest eigen values probably go up and the cheby smoother becomes unstable, among other things.  This is a known bug and we have fixed it with:

-pc_gamg_reuse_interpolation false

This is a newish feature and will force the whole PC to be recomputed in each solve (this is the default behavior for ML and perhaps hypre). 

Mark

>    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.
> 
>