further about MatGetSubMatrix() Re: distribution of submatrix and subvector Re: how to extract a subvector?

[Yujie]

Dear Matthew:

thanks again. sorry for posting lots of equestions.

Regards,

Yujie

On Sat, Jan 3, 2009 at 10:57 AM, Matthew Knepley <knepley at gmail.com> wrote:

> On Sat, Jan 3, 2009 at 12:52 PM, Yujie <recrusader at gmail.com> wrote:
>
>> Dear Matthew:
>>
>> I have checked the description of MatGetSubMatrix() and ISAllGather(). I
>> am wondering if isrow is created by MPI_COMM, does it work when iscol is
>> obtained based on PETSC_COMM_SELF? thanks.
>>
>> MatGetSubMatrix(Mat mat,IS isrow,IS iscol,PetscInt csize,MatReuse cll,Mat
>> *newmat)
>>
>
> 1) We do not check the comm of IS in this function
>
> 2) If we did we would issue an error code which would be checked by CHKERRQ
> and report the mismatch. This
>     allows experimentation with the code to uncover these answers.
>
>    Matt
>
>
>>
>> Regards,
>>
>> Yujie
>>
>> On Fri, Jan 2, 2009 at 6:20 PM, Matthew Knepley <knepley at gmail.com>wrote:
>>
>>> On Fri, Jan 2, 2009 at 7:58 PM, Yujie <recrusader at gmail.com> wrote:
>>>
>>>> Dear Barry:
>>>>
>>>> When using MatGetSubmatrix() to get a submatrix in parallel mode, to my
>>>> knowledge, this function will exact the rows and cols for submatrix. This
>>>> submatrix is not redistributed. For example, if there is zero row and zero
>>>> col in a process, the row and col of this submatrix in this process are
>>>> zero, right?
>>>> However, to get a subvector using Vec scatter, the user needs to create
>>>> the subvector using MPI_COMM, that is new distribution for subvector is
>>>> generated. Just regarding only rows or cols of the submatrix and the
>>>> subvector, their distribution should be different even if using the same
>>>> index to get them, right? thanks a lot.
>>>
>>>
>>> No. They work exactly the same way.
>>>
>>>   Matt
>>>
>>>
>>>>
>>>> Regards,
>>>> Yujie
>>>>
>>>> On Fri, Jan 2, 2009 at 5:15 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>>>
>>>>>
>>>>>   The ix and iy are always global indices based on where the vector
>>>>> lives.
>>>>>
>>>>>   If you are going from a parallel to parallel vector then both indices
>>>>> are "global"
>>>>>
>>>>>  Barry
>>>>>
>>>>> On Jan 2, 2009, at 2:21 PM, Yujie wrote:
>>>>>
>>>>>  Dear Barry:
>>>>>>
>>>>>> I have a new question about the parameter "iy"(new index set of
>>>>>> subvector). To parallel vector, how to provide "ix" and "iy"?
>>>>>>
>>>>>> Just providing the local index subset for "ix" and "iy"? if it is, it
>>>>>> is a little difficult to let the local vector know its global position in
>>>>>> the new subvector? If the user needs to provide the global position of local
>>>>>> vector for "iy". some MPI communication should be needed, it looks like not
>>>>>> a good method. In exacting submatrix, the function hides this problem. could
>>>>>> you give me any comments? thanks a lot.
>>>>>>
>>>>>> Regards,
>>>>>>
>>>>>> Yujie
>>>>>>
>>>>>>
>>>>>> On Fri, Jan 2, 2009 at 11:46 AM, Barry Smith <bsmith at mcs.anl.gov>
>>>>>> wrote:
>>>>>>
>>>>>>  VecScatter is for this purpose.
>>>>>>
>>>>>>  Rational: extracting subparts of vectors for ghost points etc takes
>>>>>> place many times in a simulation; maybe millions.
>>>>>> Thus separating it into a set-up followed by many uses is a
>>>>>>  worthwhile optimization. Extracting submatrices occur
>>>>>> much less often in a simulation, maybe tens, hundreds or thousands of
>>>>>> times so it is not worth the extra complexity
>>>>>> of having separate set-up followed by many uses. One could argue that
>>>>>> uniformity of design means we should have
>>>>>> handled matrices with a MatScatter concept to parallel the Vec
>>>>>> approach, but it is too late now :-).
>>>>>>
>>>>>>  Barry
>>>>>>
>>>>>>
>>>>>> On Jan 2, 2009, at 1:41 PM, Yujie wrote:
>>>>>>
>>>>>> Like MatGetSubMatrix(), whether is there a function to get a subvector
>>>>>> in parallel mode? I have checked some scatter functios, they don't likely
>>>>>> work. thanks a lot.
>>>>>>
>>>>>> Regards,
>>>>>>
>>>>>> Yujie
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>>
>>> --
>>> 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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