[petsc-users] DMDA questions

[Xiangdong]

On Fri, Feb 28, 2014 at 5:11 PM, Matthew Knepley <knepley at gmail.com> wrote:

> 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.
>

Does "by hand" mean providing the routines for assembling the matrix at
different level/size manually?

Xiangdong


>
>    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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