[petsc-dev] do all MIS algorithms require a symmetric structure?

[Barry Smith]

  Right, but I want to run with the old options MIS (instead of MIS-2) and get the same times as before. But instead I am getting much larger times.

> On Sep 18, 2022, at 6:58 PM, Mark Adams <mfadams at lbl.gov> wrote:
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> 
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> On Sun, Sep 18, 2022 at 6:19 PM Barry Smith <bsmith at petsc.dev <mailto:bsmith at petsc.dev>> wrote:
> 
>   Mark,
> 
>     Some how your changes to GAMG in June slowed down the time to compute the MIS dramatically. I cannot figure out what options to use to 
> get the exact same performance as an older branch. -mat_coarsen_type mis -pc_gamg_threshold 0 result in longer times than the older code with its default options.
> 
> The new MIS-2 folds in the square graph with the MIS. Before the square graph was in a separate method that created an squared graph explicitly.  So don't use aggressive coarsening (you will see a PtAP if you use aggressive coarsening in the new code)
> And -pc_gamg_threshold 0 will filter (zeros only). Use < 0 for no filtering.
> The old code also had this optimization to not create a graph for bs==1 and no filter,
> 
> Mark
>  
> 
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> 
>> On Sep 18, 2022, at 4:21 PM, Mark Adams <mfadams at lbl.gov <mailto:mfadams at lbl.gov>> wrote:
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>> 
>> 
>> On Sun, Sep 18, 2022 at 4:02 PM Barry Smith <bsmith at petsc.dev <mailto:bsmith at petsc.dev>> wrote:
>> 
>>   Mark,
>> 
>>    Do all MIS algorithms  in PETSc require a symmetric graph structure? And parallel ones can hang if not structurally symmetric?
>> 
>> Yes,
>>  
>> 
>>    When used sequentially I guess it never hangs but it may not produce a "correct" MIS if the matrix structure is not symmetric?
>> 
>> It is fine in serial and it is not necessarily an MIS of the symmetrized graph.
>> If there is a one way edge between two vertices and the order of the greedy MIS process picks the root of the edge it is an MIS of the symmetrized graph, otherwise both vertices could get selected.
>>  
>> But like the MIS is fine for GAMG in this circumstance?
>> 
>> It will be fine for GAMG. The MIS is just a heuristic.
>>  
>> 
>>   Barry
>> 
> 

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