[petsc-users] MG on fieldsplit

[gouarin]

On 02/02/2012 16:41, Jed Brown wrote:
> On Thu, Feb 2, 2012 at 17:24, gouarin <loic.gouarin at math.u-psud.fr 
> <mailto:loic.gouarin at math.u-psud.fr>> wrote:
>
>     No. In those examples, you have the same grid for the velocity and
>     the pressure. If I use 4Q1_Q1 elements, I have not the same grid.
>     I have one for the velocity and an other for the pressure in the
>     DMComposite. And for me, it is the difficulty because as you say
>     after, I have to do my own preallocation step to have the good
>     off-diagonal blocks.
>
>
> Okay, I misinterpreted your notation. For your mixed elements, you 
> would couple with DMComposite.
>
>
>     This is why I asked what is the best way to construct my matrix. I
>     hoped that now it is not necessary to do this preallocation.
>
>
> As long as dynamic preallocation is slow, you need to provide it.
>
>     This is also a difficulty to use a multigrid only on the velocity.
>
>
> What is hard about using multigrid only on velocity?
>
> -pc_type fieldsplit -fieldsplit_velocity_pc_type mg 
> -fieldsplit_velocity_pc_mg_levels 3
>
> will do geometric MG on the velocity block. Note that your elements 
> are slightly non-standard, so depending on how you want to work, you 
> might provide your own coarsening and interpolation.
It is not hard to use multigrid on velocity if I provide the 
interpolation as I said in my first email and as you say here. But it is 
for me too intrusive because I have to do a special case to use a 
multigrid pcmg whereas I can use boomeramg easily. I don't know if what 
I say is clear ...

The idea is to test many solvers on this problem.

Thanks for your replies.

Loic

-- 
Loic Gouarin
Laboratoire de Mathématiques
Université Paris-Sud
Bâtiment 425
91405 Orsay Cedex
France
Tel: (+33) 1 69 15 60 14
Fax: (+33) 1 69 15 67 18

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