[petsc-users] DMDA questions

[Matthew Knepley]

On Fri, Feb 28, 2014 at 2:10 PM, Xiangdong <epscodes at gmail.com> wrote:

>
>
>
> 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?
>

No, you can always do everything by hand, but that call simplifies the job.

   Matt


> 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
>> >
>>
>>
>


-- 
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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