[petsc-users] DMDA questions

[Xiangdong]

On Fri, Feb 28, 2014 at 4:48 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Feb 28, 2014, at 3:27 PM, Xiangdong <epscodes at gmail.com> wrote:
>
> >
> >
> >
> > On Fri, Feb 28, 2014 at 4:21 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
> > On Fri, Feb 28, 2014 at 1:16 PM, Xiangdong <epscodes at gmail.com> wrote:
> > If I assembly the matrix with MatSetValuesStencil and use
> KspSetOpreators for Ksp, do I need to call KspSetDM first?
> >
> > No
> >
> > What is the key difference or advantage of using KspSetComputeOperators
> against KspSetOperators?
> >
> > With the later, you have to manage creating and preallocating the matrix.
> >
> > If I use DMCreateMatrix and MatSetValuesStencil, I do not need to
> preallocate the matrix.
>
>    Correct.
>
>    The SetComputeOperators and KSPSetDM is useful if you wish to use
> multigrid on system since PCMG will call your compute operators function on
> each level for you automatically. See src/ksp/ksp/examples/tutorials/
> ex25.c ex28.c ex29.c ex31.c ex32.c ex34.c ex45.c ex50.c   If you are just
> solving the one system and not using multigrid then there is no particular
> advantage in SetComputeOperators and KSPSetDM
>

If I want to use multigrid as a preconditioner, I have to call
KspSetComputeOperators. Is this true?

Thank you.

Xiangdong




>
>    Barry
>
> >
> > Xiangdong
> >
> >
> >
> >
> >    Matt
> >
> > Thanks you.
> >
> > Xiangdong
> >
> >
> > On Tue, Feb 25, 2014 at 4:24 PM, Jed Brown <jed at jedbrown.org> wrote:
> > Xiangdong <epscodes at gmail.com> writes:
> > > I am not clear about this. In this example, where is the format
> declaration
> > > (e.g.,mpiaij) of Mat jac? Why is this mat jac always distributed in
> > > a compatible way as the DM vectors?
> >
> > src/ksp/ksp/examples/tutorials/ex25.c calls KSPSetDM and sets functions
> > to assemble the matrices and right hand side.  Those objects are created
> > internally (you can use -dm_mat_type sbaij if you like) and the user
> > doesn't have to see them.
> >
> >
> >
> >
> > --
> > What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> > -- Norbert Wiener
> >
>
>
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