[petsc-users] How to speed up geometric multigrid

[Michele Rosso]

Thank you very much!
I will make some tests and let you know how it goes.

Michele

On 09/23/2013 01:24 PM, Mark F. Adams wrote:
> Ah yes, I agree, geometric MG with Galerkin is worth trying.
>
>
> On Sep 23, 2013, at 3:36 PM, Dave May <dave.mayhem23 at gmail.com 
> <mailto:dave.mayhem23 at gmail.com>> wrote:
>
>> Hey Michele,
>>
>> I think Galerkin MG, even with a jump in the coefficient of 10^3, can 
>> be made more robust and faster for your problem.
>> I'd start by trying different smoothers. Try cheby/(bjacobi or asm), 
>> or gmres/(bjacobi or asm). When using cheby, make sure you ask for 
>> the spectrum estimate.
>> Definitely do more than one application of KSP on each level.
>>
>> I don't think you should switch to AMG without first trying more 
>> robust smoothers and profiling how much time is spent in the coarse 
>> grid solve.
>> I solve similar problems (geometry and size of the jump) in 
>> connection with variable visocsity stokes problems on structured 
>> grids, and I'm yet to see a case with AMG is more robust and or 
>> faster than Galerkin MG.
>>
>>
>> Cheers,
>>   Dave
>>
>>
>>
>> On 23 September 2013 20:52, Mark F. Adams <mfadams at lbl.gov 
>> <mailto:mfadams at lbl.gov>> wrote:
>>
>>
>>     >>
>>     > The simulation does start with both phases and the geometry is
>>     supposed to become more complex as the simulation progresses.
>>     > But so far the run is stopped before there are significant
>>     changes in the shape of the droplet.
>>
>>     Humm,  not sure why you are seeing degradation then.  I imagine
>>     the initial geometry is not grid aligned so I'm not sure why you
>>     are seeing degradation in convergence rate.
>>
>>     > I can give a shot to AMG: which options would you suggest to use.
>>
>>     -pc_type gamg -pc_gamg_type agg -pc_gamg_agg_nsmooths 1
>>
>>     > Also, how can I project out the constant from the rhs? Thanks a
>>     lot!
>>
>>     http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetNullSpace.html#KSPSetNullSpace
>>
>>
>

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