[petsc-users] GAMG issue

[Jed Brown]

-mg_coarse_pc_type svd?

(Use redundant for parallel.)
On Mar 30, 2012 9:21 AM, "Mark F. Adams" <mark.adams at columbia.edu> wrote:

>
> On Mar 30, 2012, at 10:52 AM, John Mousel wrote:
>
> Mark,
>
> I've run GAMG twice with different coarse grid sizes of 2 and 8 with 1
> sweep of SOR on the coarse grid. For a size of 8 it converges nicely, but
> for a size of 2, I think the null space is causing too many problems.
>
>
> YEs, the iterative method is seeing the null space because of floating
> point error.
>
> If GAMG were to coarsen to a size of 1, then there would be no hope
> because only the null space would remain, right? This doesn't ever seem to
> occur with ML because there are at least as many rows as processors.
>
>
> Yes that seems like a good assumption.  The right thing to do here would
> probably be to do and SVD and filter out the very low modes explicitly.
>  For now I guess tweaking -pc_gamg_coarse_eq_limit n is al that can be
> done.  Not very satisfying.  We will think about this ... any thoughts
> anyone?
>
> Mark
>
>
> John
>
> On Fri, Mar 30, 2012 at 8:42 AM, Mark F. Adams <mark.adams at columbia.edu>wrote:
>
>>
>> On Mar 29, 2012, at 2:40 PM, John Mousel wrote:
>>
>> I'm attempting to solve a non-symmetric discretization of a 3D Poisson
>> problem. The problem is singular. I've attached the results of KSPView from
>> runs with ML and GAMG. When I run ML, I get convergence in 30 iterations.
>> When I attempt to use the same settings with GAMG, I'm not getting
>> convergence at all. The two things I notice are:
>>
>> 1. GAMG is using KSPType preonly, even though I've set it to be
>> Richardson in my command line options.
>>
>>
>> PETSc seems to switch the coarse grid solver to GMRES in Setup.  This
>> seems to be a bug and I unwisely decide to override this manually. I will
>> undo this in the next checkin.  This should not be the problem however.
>>
>> 2. ML only coarsens down to 4 rows while GAMG coarsens to 2. My problem
>> is singular, and whenever I try to use LU, I get zero pivot problems. To
>> mitigate this, I've been using Richardson with SOR on the coarse matrix.
>> Could the smaller coarse grid size of GAMG be causing problems with SOR. If
>> so, is there a way to put a lower limit on the coarse grid size?
>>
>>
>> I'm thinking that with a 2x2 coarse grid 8 iterations of SOR is picking
>> up the null space.  Maybe try just one SOR iteration on the coarse grid.
>>
>> Also, can you run with options_left so that I can see your arguments.
>>  One known bug is the mat_diagaonal_scale breaks GAMG, but it should also
>> break ML.
>>
>> Mark
>>
>> John
>>
>> On Thu, Mar 29, 2012 at 11:03 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>>
>>> On Thu, Mar 29, 2012 at 09:18, John Mousel <john.mousel at gmail.com>wrote:
>>>
>>>> [0]PETSC ERROR: Error in external library!
>>>> [0]PETSC ERROR: Cannot disable floating point exceptions!
>>>>
>>>
>>> Looks like something is strange with your environment because fesetenv()
>>> is returning an error. I have disabled the call if the trap mode is not
>>> changing.
>>>
>>> http://petsc.cs.iit.edu/petsc/petsc-dev/rev/352b4c19e451
>>>
>>
>> <KSPView_GAMG.txt><KSPView_ML.txt>
>>
>>
>>
> <KSPView_ML.txt><KSPView_GAMG.txt>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120330/f837b5d0/attachment.htm>