These large nearly-diagonal blocks pessimal for BAIJ. If you want to split
into a few blocks that are each nearly-dense then a combination of BAIJ and
Nest should be good.
On Jul 27, 2013 10:30 PM, "Derek Gaston" <fried...@gmail.com> wrote:
> Oops accidentally hit send! Look below for the real email :-)
>
> Sent from my iPad
>
> On Jul 27, 2013, at 5:45 AM, "Kirk, Benjamin (JSC-EG311)"
> <benjamin.k...@nasa.gov> wrote:
>
> > So the petsc matrices assume a constant blocking factor for the entire
> matrix, which I think is the bug fundamental driver here. If you solved
> decoupled flow/temperature in 1 matrix with equal order basis functions
> you'd have 4 fully coupled variables in 3D, with 1 tagalog. I don't think
> petsc will optimize for that case, and in fact you *might* be better memory
> wise splitting into two matrices where the flow vars could be coupled.
>
> I gotcha. There might still be a way to do this in one matrix with
> MatNest. Maybe you would nest two block matrices inside the matrix?
>
> > Is there another use case with non-full dof coupling I could be
> overlooking? I'd be happy to try to address it...
>
> Yes. The normal one for us is block-diagonal (just for
> preconditioning). We might have 2000 coupled variables (literally!)
> that are all linear lagrange.... But we use a block-diagonal
> preconditioning matrix. Is there something we can do about that case?
>
> Regardless of the actual coupling present in our problem we build our
> matrices with arbitrary amounts of off-diagonal blocks... it all
> depends on how important that block may or may not be for
> precnditioning.
>
> Derek
>
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