[petsc-users] Using BDDC preconditioner for assembled matrices

[Abdullah Ali Sivas]

Hi Jed,

Thanks for your reply. The assembled matrix I have corresponds to the full
problem on the full mesh. There are no "Neumann" problems (or any sort of
domain decomposition) defined in the code generates the matrix. However, I
think assembling the full problem is equivalent to implicitly assembling
the "Neumann" problems, since the system can be partitioned as;

[A_{LL} | A_{LI}]  [u_L]     [F]
-----------|------------ -------- = -----
[A_{IL}  |A_{II} ]   [u_I]      [G]

and G should correspond to the Neumann problem. I might be thinking wrong
(or maybe I completely misunderstood the idea), if so please correct me.
But I think that the problem is that I am not explicitly telling PCBDDC
which dofs are interface dofs.

Regards,
Abdullah Ali Sivas

On Tue, 23 Oct 2018 at 23:16, Jed Brown <jed at jedbrown.org> wrote:

> Did you assemble "Neumann" problems that are compatible with your
> definition of interior/interface degrees of freedom?
>
> Abdullah Ali Sivas <abdullahasivas at gmail.com> writes:
>
> > Dear all,
> >
> > I have a series of linear systems coming from a PDE for which BDDC is an
> > optimal preconditioner. These linear systems are assembled and I read
> them
> > from a file, then convert into MATIS as required (as in
> >
> https://www.mcs.anl.gov/petsc/petsc-current/src/ksp/ksp/examples/tutorials/ex72.c.html
> > ). I expect each of the systems converge to the solution in almost same
> > number of iterations but I don't observe it. I think it is because I do
> not
> > provide enough information to the preconditioner. I can get a list of
> inner
> > dofs and interface dofs. However, I do not know how to use them. Has
> anyone
> > have any insights about it or done something similar?
> >
> > Best wishes,
> > Abdullah Ali Sivas
>
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