[petsc-users] Convergence of AMG

[Karin&NiKo]

Manav,

How are interpolated the rotational degrees of freedom? AFAIK, when using
smoothed aggregation, the interpolation process tries to mimic linear
interpolation, which can be OK for the displacement DOF but is not for the
rotational DOF using some plate and shell formulations.
This can explain poor convergence of a multilevel approach, which needs to
restrict and extrapolate the unknowns. In order to check this hypothesis,
you can try a test case with zero rotations.

Nicolas

Le lun. 29 oct. 2018 à 22:13, Mark Adams via petsc-users <
petsc-users at mcs.anl.gov> a écrit :

> * the two level results tell us that MG is not doing well on the coarse
> grids. So the coarse grids are the problem.
>
> * Do not worry about timing now. Get the math correct. The two level solve
> is not meant to be a solution just a diagnostic so don't try to optimize it
> by squaring the graph. Use -pc_gamg_square_graph 0.
>
> * It looks like you don't need 4 smoothing steps but lets keep it and we
> can dial it back later.
>
> * This table is interesting. First, you had about 12 iterations earlier
> and I think your rtol was tighter than the default (so the iteration could
> should go down not up). Do you know what change here?
>
> Note, even though -mg_levels_ksp_max_it is not in the ksp_view it does
> work. It is syntactic sugar to just add it to all levels like you did
> manually.
>
> Anyway, these number look reasonable. It is interesting that 3 levels ran
> well but the 4th level ran poorly. This implies we want to slow down
> coarsening on these levels, but ...
>
> First can you please rerun this experiment with -pc_gamg_square_graph 0.
>
> Also, please run with -info. This is very noisy but you can grep on "GAMG"
> and send that output to us (about 15 lines).
>
> Thanks,
> Mark
>
>
>
> On Mon, Oct 29, 2018 at 3:34 PM Manav Bhatia <bhatiamanav at gmail.com>
> wrote:
>
>> Barry,
>>
>>    Here are some quick numbers with the following options on 4 CPUs and
>> 543,606 dofs:
>>
>> -mg_levels_ksp_max_it 4 -pc_gamg_square_graph 1 -pc_gamg_threshold 0.
>>
>>  #levels   |    #KSP Iters
>> ———————————
>>      2        |       18
>>      3        |       18
>>      4        |       40
>>      5        |       59
>>
>> -Manav
>>
>>
>> On Oct 29, 2018, at 2:06 PM, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
>>
>>
>>  Exactly how much does it increase with number of levels? Send a chart
>> number of levels and number of iterations. With say -mg_levels_ksp_maxit 4
>>
>>   Thanks
>>
>>   Barry
>>
>>
>>
>>
>> On Oct 29, 2018, at 12:59 PM, Manav Bhatia <bhatiamanav at gmail.com> wrote:
>>
>> Thanks for the clarification.
>>
>> I also observed that the number of KSP iterations increases with an
>> increase in the levels of AMG. Is this true, in general, for all/most
>> applications?
>>
>> -Manav
>>
>> On Oct 29, 2018, at 12:53 PM, Jed Brown <jed at jedbrown.org> wrote:
>>
>> Manav Bhatia <bhatiamanav at gmail.com> writes:
>>
>> Thanks, Jed.
>>
>> The description says: “ Square the graph, ie. compute A'*A before
>> aggregating it"
>>
>> What is A here?
>>
>>
>> The original matrix, or its "graph" (your 6x6 blocks condensed to
>> scalars).
>>
>> What is the impact of setting this to 0, which led to a very significant
>> increase in the CPU time in my case?
>>
>>
>> The aggregates are formed on the connectivity of your original matrix,
>> so root nodes are aggregated only with their first neighbors, resulting
>> in slower coarsening.
>>
>>
>>
>>
>>
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