[petsc-users] DMDA questions

[Matthew Knepley]

On Fri, Feb 28, 2014 at 2:14 PM, Xiangdong <epscodes at gmail.com> wrote:
>
> 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?
>

It means providing the matrices directly, or using the Galerkin process.
There is a manual section on it.

   Matt


> 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
>>
>
>


-- 
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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