[petsc-users] MG on fieldsplit

[Jed Brown]

On Thu, Feb 2, 2012 at 17:24, gouarin <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.
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