[petsc-users] gamg failure with petsc-dev

[Mark Adams]

Stephan, I have pushed a pull request to fix this but for now you can just
use -mg_levels_ksp_type chebyshev -mg_levels_pc_type jacobi.  This used to
be the default be we move to SOR recently.
Mark


On Sat, Mar 29, 2014 at 5:52 PM, Mark Adams <mfadams at lbl.gov> wrote:

> Sorry for getting to this late.  I think you have figured it out basically
> but there are a few things:
>
> 1) You must set the block size of A (bs=2) for the null spaces to work and
> for aggregation MG to work properly. SA-AMG really does not make sense
> unless you work at the vertex level, for which we need the block size.
>
> 2) You must be right that the zero column is because the aggregation
> produced a singleton aggregate.  And so the coarse grid is low rank.  This
> is not catastrophic, it is like a fake BC equations.  The numerics just
> have to work around it.  Jacobi does this.  I will fix SOR.
>
> Mark
>
>
>> Ok, I found out a bit more. The fact that the prolongator has zero
>> columns appears to arise in petsc 3.4 as well. The only reason it wasn't
>> flagged before is that the default for the smoother (not the aggregation
>> smoother but the standard pre and post smoothing) changed from jacobi to
>> sor. I can make the example work with the additional option:
>>
>> $ ./ex49 -elas_pc_type gamg -mx 100 -my 100 -mat_no_inode
>> -elas_mg_levels_1_pc_type jacobi
>>
>> Vice versa, if in petsc 3.4.4 I change ex49 to include the near nullspace
>> (the /* constrain near-null space bit */) at the end, it works with jacobi
>> (the default in 3.4) but it breaks with sor with the same error message as
>> above. I'm not entirely sure why jacobi doesn't give an error with a zero
>> on the diagonal, but the zero column also means that the related coarse dof
>> doesn't actually affect the fine grid solution.
>>
>> I think (but I might be barking up the wrong tree here) that the zero
>> columns appear because the aggregation method typically will have a few
>> small aggregates that are not big enough to support the polynomials of the
>> near null space (i.e. the polynomials restricted to an aggregate are not
>> linearly independent). A solution would be to reduce the number of
>> polynomials for these aggregates (only take the linearly independent).
>> Obviously this has the down-side that the degrees of freedom per aggregate
>> at the coarse level is no longer a constant making the administration more
>> complicated. It would be nice to find a solution though as I've always been
>> taught that jacobi is not a robust smoother for multigrid.
>>
>> Cheers
>> Stephan
>>
>>
>>
>
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