PETSc developers,
We have a MatZeroRowsColumns() for MPIAIJ and SeqSBAIJ, what about having
one for MPISBAIJ? Anyone want to code it up?
Barry
On Aug 8, 2011, at 9:10 AM, Jed Brown wrote:
> On Mon, Aug 8, 2011 at 08:40, Clemens Domanig <clemens.domanig at uibk.ac.at>
> wrote:
> Hi,
>
> I want my FE-program to be able to use LU- and LDLt-decomp (MUMPS). LU works
> fine and now I want to implement LDLt. My Questions:
> 1.) How can I speed up the assembly of the upper triangle matrix?
> * '-mat_ignore_lower_triangular' makes it terrible slow
> (1000 elements -> 1s explode to 18s
> * MatSetValues with Blocks of 6x6 and single values at the diagonal is even
> worse.
>
> SBAIJ needs different preallocation. If you set it with
> Mat{Seq,MPI}SBAIJSetPreallocation(), assembly will be fast.
>
> 2.) SBAIJ doesn't like MatZeroRows. What are the alternatives to apply
> boundary conditions to my matrix?
>
> That function would make the matrix nonsymmetric because the columns aren't
> also zeroed, so you couldn't use it with a symmetric solver anyway. I believe
> in imposing boundary conditions symmetrically at the local level (either by
> removing them or by imposing them while computing residuals for those nodes),
> but many people like to do it differently.
>
> To do boundary conditions by zeroing rows and columns after assembly, you
> would need to modify the right hand side in a compatible way. We could
> automate this by having the Mat "remember" the entries in the columns that it
> zeroed so that each right hand side could automatically be modified before
> starting the solve, but then solving A*x = b would either yield an x that
> doesn't satisfy this equation or modify b, both of which I think would be
> confusing.
>
> Suggestions for imposing boundary conditions globally without ruining
> symmetry or having confusing semantics would be welcome.
