[petsc-users] MG on fieldsplit
gouarin
loic.gouarin at math.u-psud.fr
Thu Feb 2 09:24:30 CST 2012
On 02/02/2012 15:23, Jed Brown wrote:
> On Thu, Feb 2, 2012 at 13:03, gouarin <loic.gouarin at math.u-psud.fr
> <mailto:loic.gouarin at math.u-psud.fr>> wrote:
>
> 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:
>
>
> src/ksp/ksp/examples/tutorials/ex43.c (2D) and ex42.c (3D) use these
> elements for variable-viscosity Stokes.
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.
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. This is also a
difficulty to use a multigrid only on the velocity.
Thanks,
Loic
>
> - Construct the matrices for each DM in the DMComposite
> - Construct the global matrix and the global preconditioner
> - Use MatNest
>
>
> DMComposite can allocate for AIJ or Nest. Just set the matrix type,
> e.g. -dm_mat_type nest, ex28.c does this (see runex28_3).
>
> But preallocation for off-diagonal blocks (of MatNest or AIJ) is still
> messy and I haven't figured out a good API other than to write an
> efficient dynamic preallocation to make it unnecessary. A lot of users
> like Jacobian-free Newton-Krylov with the true Jacobian applied by
> finite differencing (-snes_mf_operator) in which case the current code
> is fine because we only assemble diagonal blocks.
>
>
> There are a lot of possibilities with PETSc with new add-ons and
> it is difficult to find our way with only the documentation.
>
>
--
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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