[petsc-users] Patching in generalized eigen value problems

[Matthew Knepley]

On Sun, Apr 9, 2017 at 2:47 PM, Daralagodu Dattatreya Jois, Sathwik Bharadw
<sdaralagodudatta at wpi.edu> wrote:

> Dear Barry,
>
>
> These operations comes into picture when I have to map a
>
> plane surface to a closed surface (lets say cylinder). As you
>
> can imagine nodes (hence nodal values) at 2 opposite sides
>
> of the plane have to add up to give a closed geometry.
>
> Instead of assembling both, why not just make one row into constraints,
namely
1 for the primal node and -1 for the host node. Then you do not need
communication,
and its sparser. IMHO, these "extra" nodes should just be eliminated from
the system.

   Matt

> Matrices can be as large as 30,000*30,000 or more depending
>
> on the density of the mesh. Since, in effect different elements sits in
>
> different processes doing this in assembly level will be tricky.
>
>
> Sathwik Bharadwaj
> ------------------------------
> *From:* Barry Smith <bsmith at mcs.anl.gov>
> *Sent:* Sunday, April 9, 2017 3:34:13 PM
> *To:* Daralagodu Dattatreya Jois, Sathwik Bharadw
> *Cc:* petsc-users at mcs.anl.gov
> *Subject:* Re: [petsc-users] Patching in generalized eigen value problems
>
>
> > On Apr 9, 2017, at 2:21 PM, Daralagodu Dattatreya Jois, Sathwik Bharadw <
> sdaralagodudatta at wpi.edu> wrote:
> >
> > Dear petsc users,
> >
> > I am solving for generalized eigen value problems using petsc and slepc.
> > Our equation will be of the form,
> >
> > A X=λ B X.
> >
> > I am constructing the A and B matrix of type MATMPIAIJ. Let us consider
> that
> > both of my matrices are of dimension 10*10. When we are solving for a
> closed
> > geometry, we require to add all the entries of the last (9th) row and
> column to
> > the first (0th) row and column respectively for both matrices. In a high
> density mesh,
> > I will have a large number of such row to row and column to column
> additions.
> > For example, I may have to add last 200 rows and columns to first 200
> rows and columns
> > respectively. We will then zero the copied row and column expect the
> diagonal
> > element (9th row/column in the former case).
>
>    Where is this "strange" operation coming from?
>
>    Boundary conditions?
>
>   Is there any way to assemble matrices initially with these sums instead
> of doing it after the fact?
>
>  Why is it always the "last rows" and the "first rows"?
>
>   What happens when you run in parallel where first and last rows are on
> different processes?
>
>   How large will the matrices get?
>
>    Are the matrices symmetric?
>
>
>
>
> >
> > I understand that MatGetRow, MatGetColumnVector, MatGetValues or any
> other
> > MatGet- or VecGet- functions are not collective. Can you suggest any
> > efficient algorithm or function to achieve this way of patching?
> >
> > One way I can think of is to obtain the column vector using
> MatGetColumnVector and
> > row vector by MatZeroRows and then scatter these vectors to all
> processes. Once we have
> > entire row/column vector entries in each process, we can add the values
> to the matrix by
> > their global index. Of course, care should be taken to add the values of
> diagonal element
> > only once. But this will be a quite slow process.
> > Any ideas are appreciated.
> >
> > Thanks,
> > Sathwik Bharadwaj
>
>


-- 
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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