how to extract a subvector?

[Yujie]

Dear Matthew:

Assuming the parent vector is 0-9 and there are 3 processes (0,1,2; 3,4,5;
6,7,8,9),
Now the subvector contains 2; 4; 8, 9.
I should create "ix" in 3 processores with MPI_COMM using arraies {2}, {4},
{8,9} respectively?
create "iy" using arraies {0},{1},{2,3} respectively?
If it is, I think it is a little difficult to get "iy". Since the user needs
to communicate between processors to confirm the global position of the
local subvector. thanks  a lot.

Regards,
Yujie

On Fri, Jan 2, 2009 at 6:19 PM, Matthew Knepley <knepley at gmail.com> wrote:

> No, you use the global indices ONLY for the values needed on this
> processor. I would
> also note that there is an extensive discussion of this IN the manual.
>
>    Matt
>
>
> On Fri, Jan 2, 2009 at 7:31 PM, Yujie <recrusader at gmail.com> wrote:
>
>> You mean I need to let all processes have a copy of the whole global index
>> and to use this function? thanks
>>
>>
>> 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
>
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