[petsc-dev] MATSCATTER Transpose

[Barry Smith]

On Mar 29, 2010, at 4:29 PM, Dima Karpeyev wrote:

> I can work within MATSCATTER, but the question is what is the
> VecScatter returned by
> MatScatterGetVecScatter applied to such a transposed MATSCATTER?
>
> I guess it can be clarified in documentation that extracting this
> scatter from B, which is
> MATSCATTER and a transpose of another MATSCATTER A, then using it
> in another MatCreateScatter call (or its equivalent) will reproduce
> not B, but A.

    You could have it return nothing. Or you could have it generate an  
error.  Or you could on the fly create the flipped VecScatter and  
return that.

    I'd have it generate an error PETSC_ERR_SUP and then we can delay  
dealing with it until someone complains.
>
> Dmitry.
>
> On Mon, Mar 29, 2010 at 3:23 PM, Barry Smith <bsmith at mcs.anl.gov>  
> wrote:
>>
>> On Mar 29, 2010, at 2:42 PM, Dmitry Karpeev wrote:
>>
>>> Good point.  Actually, should this be pushed all the way to the  
>>> level
>>> of VecScatter?
>>> That would necessitate an introduction of something like
>>> VecScatterTranspose
>>> (by analogy with the induced linear operator) or VecScatterInvert.
>>> The implementation of VecScatterTranspose is trivially carried out  
>>> at
>>> the interface level
>>> with a flag, which would cause an appropriate change in the
>>> ScatterMode submitted to
>>> ops->scatterbegin, ops->scatterend.
>>
>>   Yuck, unless there is a good reason for VecScatterInvert/ 
>> Transpose?() I'd
>> much prefer that you just work inside the MATSCATTER matrix  
>> implementation.
>>
>>   Barry
>>
>>>
>>> Then, MatTranspose for MATSCATTER could be implemented by  
>>> transposing
>>> the underlying
>>> VecScatter (incurring extra storage) or transposing the underlying
>>> scatter on MatScatterGetVecScatter
>>> (this is much less clean, though).
>>>
>>> Any thoughts?
>>>
>>> Thanks.
>>> Dmitry.
>>>
>>> On Mon, Mar 29, 2010 at 1:14 PM, Jed Brown <jed at 59a2.org> wrote:
>>>>
>>>> On Mon, 29 Mar 2010 12:57:53 -0600, Dmitry Karpeev <karpeev at mcs.anl.gov 
>>>> >
>>>> wrote:
>>>>>
>>>>> Is there any reason why MATSCATTER doesn't implement MatTranspose?
>>>>> It seems to me that it would be easy to accomplish by reversing
>>>>> MatMultXXX and MatMultTransposeXXX
>>>>> in MATSCATTER's ops vtable.
>>>>
>>>> Makes sense to me, just be careful with MatScatterGetVecScatter().
>>>>
>>>> Jed
>>>>
>>
>>