# distribution of submatrix and subvector Re: how to extract a subvector?

Fri Jan 2 22:08:53 CST 2009

```Dear Matthew:

They have the same distribution according to the distribution I have
mentioned about submatrix? Could you give me more details? thanks a lot.

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