[petsc-users] storage of parallel dense matrices and (anti)symmetric matrices

[Matthew Knepley]

On Thu, Mar 22, 2012 at 3:32 PM, Gao Bin <bin.gao at uit.no> wrote:

>  Hi, Jed
>
> Thank you very much for your quick reply. May I ask two more further
> questions?
>
> (1) Why does not PETSc also partition the columns so that each processor
> could use less memory?
>

2D distributions are not efficient for sparse matrices. They are sometimes
used for dense.


> (2) If the matrix I use is a square matrix, the number of local columns
> "n" should be equal to the number of local rows "m" when calling
> MatCreateMPIDense, am I right?
>

Yes. You can always let PETSc choose by giving PETSC_DETERMINE.

   Matt


> Thank you again for your answer.
>
> Cheers
>
> Gao
>  ------------------------------
> *From:* petsc-users-bounces at mcs.anl.gov [petsc-users-bounces at mcs.anl.gov]
> on behalf of Jed Brown [jedbrown at mcs.anl.gov]
> *Sent:* Thursday, March 22, 2012 9:17 PM
> *To:* PETSc users list
> *Subject:* Re: [petsc-users] storage of parallel dense matrices and
> (anti)symmetric matrices
>
>   2012/3/22 Gao Bin <bin.gao at uit.no>
>
>> "The parallel dense matrices are partitioned by rows across the
>> processors, so that each local rectangular submatrix is stored in the dense
>> format described above."
>>
>> Does it mean each processor will have several continuous rows and all
>> columns of the matrix? If yes, why do we need to specify "n" -- the number
>> of local columns when calling MatCreateMPIDense?
>>
>
>  Interpret the local column size n as the local size of the Vec that the
> Mat will be applied to.
>
>
>> I am sorry to raise this simple question, since I have read the manual
>> and tutorials, but I have not found a clear answer. Moreover, the reason I
>> am asking this question is that I would like to use PETSc for matrix
>> operations, but the elements of matrices need to be calculate via my own
>> code. If I know the distribution of the matrix, I could let each processor
>> only calculate and set local values (the rows and columns possessed on the
>> processor itself) for efficiency.
>>
>> My second question is if PETSc provides symmetric and anti-symmetric
>> matrices. I have read the manual, the answer seems to be no. Am I right?
>
>
> See the SBAIJ format (it is sparse).
>
>  With a parallel dense matrix, there isn't any point using a symmetric
> format unless you use a different distribution of the entries.
>



-- 
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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