[petsc-users] Convergence of AMG

[Mark Adams]

And you can also always use -mg_levels_esteig_ksp_type cg

On Tue, Oct 30, 2018 at 2:51 PM Mark Adams <mfadams at lbl.gov> wrote:

> The last two cases were cut off but that is OK.
>
> Lets just do -pc_gamg_square_graph 0 and please add/do -ksp_view -
> mg_levels_ksp_type richardson -pc_mg_levels 5 -gamg_est_ksp_type cg
>
> Send the grep GAMG again, and the ksp_view output.
>
> And then do this again with: -pc_gamg_agg_nsmooths
>
> I think the eigen estimators are messed up on the coarse grids.
>
>
> On Tue, Oct 30, 2018 at 12:39 PM Karin&NiKo <niko.karin at gmail.com> wrote:
>
>> Ok, sorry for my misunderstanding  and thank you for the clarification.
>>
>> Le mar. 30 oct. 2018 13:55, Mark Adams <mfadams at lbl.gov> a écrit :
>>
>>> Nicolas,
>>>
>>> Smoothed aggregation is fine with shells. see the original SA paper (
>>> https://link.springer.com/article/10.1007/BF02238511).
>>>
>>> The rotational modes, which are the non-trivial modes that must be
>>> supplied, are used in the interpolation.
>>>
>>> Mark
>>>
>>> On Tue, Oct 30, 2018 at 5:22 AM Karin&NiKo <niko.karin at gmail.com> wrote:
>>>
>>>> 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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