[petsc-users] Convergence of AMG

[Manav Bhatia]

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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