[petsc-users] MG on fieldsplit

[gouarin]

Hi Jed,

thanks for your reply.

On 01/02/2012 16:32, Jed Brown wrote:
> On Wed, Feb 1, 2012 at 04:47, gouarin <loic.gouarin at math.u-psud.fr 
> <mailto:loic.gouarin at math.u-psud.fr>> wrote:
>
>     Hi,
>
>     I sent some email on Stokes solver using DMComposite grid (one for
>     the velocity and an other one for the pressure).
>
>     Now, I try to use PCMG to precondition the velocity part with
>     fieldsplit and PETSc command line options only.
>
>     This is an example of my options:
>
>     ./test -stokes_ksp_type fgmres
>             -stokes_pc_type fieldsplit
>             -stokes_fieldsplit_0_ksp_type preonly
>             -stokes_fieldsplit_0_pc_type mg
>             -stokes_fieldsplit_0_pc_mg_ levels 2
>             -stokes_fieldsplit_0_pc_mg_ galerkin
>            ...
>             -stokes_fieldsplit_1_ksp_type preonly
>             -stokes_fieldsplit_1_pc_type jacobi
>
>     It doesn't work because we need to call PCFieldSplitGetSubKSP to
>     get the subpc[0] to create the interpolation matrices.  So, we
>     have to call KSPSetUp before PCFieldSplitGetSubKSP. But, if we
>     call KSPSetUp, we have an error because the interpolation matrices
>     are not created !
>
>
> The interpolation matrices for PCMG? Those should be constructed by 
> the DM. It looks from your code like you have a DMComposite, in which 
> case you shouldn't have to do anything special.
>
> Try "make runex28_4". (It's broken with a change that went into 
> petsc-dev a few days ago. I'll be able to push the fix whenever I find 
> a wireless network in this airport that doesn't block ssh.)
It seems that I have forgotten to initialize DMSetInitialGuess, 
DMSetFunction, DMSetJacobian.

I don't understand what is the best way to use 4Q1-Q1 elements for the 
Stokes problem:

- Construct the matrices for each DM in the DMComposite
- Construct the global matrix and the global preconditioner
- Use MatNest

There are a lot of possibilities with PETSc with new add-ons and it is 
difficult to find our way with only the documentation.

Best regards,
Loic

>
>     But If I do:
>
>     ------
>     PCSetDM(pc, dom->get_pack());
>     PCSetType(pc, PCFIELDSPLIT);
>     KSPSetUp(solver);
>     PCFieldSplitGetSubKSP(pc, &nsplits, &subksp);
>     PetscMalloc(sizeof(PC)* nsplits, &subpc);
>
>     KSPSetType(subksp[0], KSPPREONLY);
>     KSPGetPC(subksp[0], &subpc[0]);
>     PCSetType(subpc[0], PCMG);
>
>     create the interpolation matrices
>
>     KSPSetFromOptions(solver);
>     ------
>
>     It works but we can't set the mg_levels with the command line
>     options anymore. Moreover, I have some warning of this type:
>
>     Option left: name:-stokes_fieldsplit_0_pc_
>     mg_multiplicative_cycles value: 3
>
>     Is there a way to do that only with the command line option ?
>
>     Thanks,
>     Loic
>
>     -- 
>     Loic Gouarin
>     Laboratoire de Mathématiques
>     Université Paris-Sud
>     Bâtiment 425
>     91405 Orsay Cedex
>     France
>     Tel: (+33) 1 69 15 60 14 <tel:%28%2B33%29%201%2069%2015%2060%2014>
>     Fax: (+33) 1 69 15 67 18 <tel:%28%2B33%29%201%2069%2015%2067%2018>
>
>


-- 
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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