[petsc-users] MatPtAP

[Matthew Knepley]

On Fri, Jun 1, 2018 at 9:21 AM, Samuel Lanthaler <s.lanthaler at gmail.com>
wrote:

> Hi,
>
> I was wondering what the most efficient way to use MatPtAP would be in the
> following situation: I am discretizing a PDE system. The discretization
> yields a matrix A that has a band structure (with k upper and lower bands,
> say). In order to implement the boundary conditions, I use a transformation
> matrix P which is essentially the unit matrix, except for the entries
> P_{ij} where i,j<k and n-i,n-j<k, so
>
> P =  [ B, 0, 0, 0, ..., 0, 0 ]
>        [  0, 1, 0, 0, ..., 0, 0 ]
>        [                              ]
>        [                              ]
>        [                  ..., 1, 0 ]
>        [  0, 0, 0, 0, ..., 0, C ]
>
> with B,C are (k-by-k) matrices.
> Right now, I'm simply constructing A, P and calling
>
>     CALL MatPtAP(petsc_matA,petsc_matP,MAT_INITIAL_MATRIX,PETSC_DEFAU
> LT_REAL,petsc_matPtAP,ierr)
>
> where I haven't done anything to pestc_matPtAP, prior to this call. Is
> this the way to do it?
>
> I'm asking because, currently, setting up the matrices A and P takes very
> little time, whereas the operation MatPtAP is taking quite long, which
> seems very odd... The matrices are of type MPIAIJ. In my problem, the total
> matrix dimension is around 10'000 and the matrix blocks (B,C) are of size
> ~100.
>

Are you sure this is what you want to do? Usually BC are local, since by
definition PDE are local, and
are applied pointwise. What kind of BC do you have here?

  Thanks,

    Matt


> Thanks in advance for any ideas.
>
> Cheers,
> Samuel
>



-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20180601/19e448d6/attachment.html>