[petsc-dev] DMGetMatrix --> DMGetMatrices?

[Dmitry Karpeev]

On Fri, Feb 10, 2012 at 3:38 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Fri, Feb 10, 2012 at 3:24 PM, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote:
>
>>
>>
>> On Fri, Feb 10, 2012 at 12:13 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>>
>>> On Fri, Feb 10, 2012 at 00:05, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote:
>>>
>>>> I don't think so.  Preallocating doesn't set any values and seems to
>>>> leave the matrix marked !assembled.
>>>> MatDuplicate for such a matrix will fail.  Assemblying it before
>>>> setting values (just to force an assembled flag)
>>>> will squeeze out the extra values, won't it?  I think it would just be
>>>> reasonable to allow to duplicate unassembled
>>>> matrices, or, better yet, have a matrix be "assembled" by default until
>>>> MatSetValues has been called.
>>>> But I'm not sure whether either solution will break something else.
>>>>
>>>
>>> Are you calling DMSetMatrixPreallocateOnly()?
>>>
>>
>> So, as I understand, this behavior is reasonable for the use with SNES,
>> since the matrix is assembled with a call to SNES(DM)ComputeJacobian.
>> DMGetMatrix is also used with KSP, when  it is also expected to assemble
>> the matrix.
>> This seems to make the behavior of DMGetMatrix a bit schizophrenic and
>> dependent on whether it is used from a SNES or a KSP.  Should we try to
>> unify this instead?  One option is to assume that DMGetMatrix (or
>> DMGetOperators) merely preallocates the matrix of a DM-preferred type,
>> while a call to SNESDMComputeJacobian or (the  new method)
>> KSPDMFormOperators, respectively, would actually compute the values and
>> assemble or do the equivalent thing for MF.  In fact, one could unify this
>> behavior already by treating linear problems as nonlinear.
>>
>
> It responds the same whether you use it from KSP or SNES. Are actually
> running an example which has
> the behavior you are talking about?
>

Yes, DMGetMatrix responds the same way wherever you call it from, but
what will happen if you call DMSetMatrixPreallocateOnly() and use that DM
with a KSP?
It seems to me that once DMSetMatrixPreallocateOnly() has been called, that
DM is only usable from SNES.

Dmitry.

>
>    Matt
>
>
>> Dmitry.
>>
>> Dmitry.
>>
>>
>
>
> --
> 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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