[petsc-users] Differences between jacobi and bjacobi preconditioner for cg method 1-processor/block

[Jonathan Wong]

Thanks for the input. To clarify, I'm trying to compare GPU algorithms to
Petsc, and they only have cg/jacobi for what I'm comparing at the moment.
This is why I'm not using gmres (which also works well).

I can solve the problem with the GPU (custom code) using CG + jacobi for
all the meshes. On the CPU side, I can solve everything with cg/bjacobi and
almost all of my meshes with cg/jacobi except for my 50k node mesh. I can
solve the problem with my finite element built-in direct solver (just takes
awhile) on one processor. I've been reading that by default the bjacobi pc
uses one block per processor. So I had assumed that for one processor
block-jacobi and jacobi would give similar results. cg+bjacobi works fine.
cg+jacobi does not.

I'll just look into the preconditioner code and use KSPview to try to
figure out what the differences are for one processor. I'm not sure why the
GPU can consistently solve the problem with cg/jacobi. I'm assuming this is
due to the way round-off or the order of operations differences between the
two.


On Mon, May 19, 2014 at 6:35 AM, Jed Brown <jed at jedbrown.org> wrote:

> Matthew Knepley <knepley at gmail.com> writes:
> > No, Block-Jacobi and Jacobi are completely different. If you are not
> > positive definite, you should be using MINRES.
>
> MINRES requires an SPD preconditioner.
>
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