[petsc-users] question about PC_BJACOBI

[Jed Brown]

The latter is using the partition provided by the DA (or user) which looks
to be better than the one computed in the serial run. If you have Parmetis,
then it will be used by PCBJACOBI, otherwise the partition is naive. You can
specify subdomains manually if you want.

Jed

On Sep 23, 2010 1:51 PM, "Leo van Kampenhout" <lvankampenhout at gmail.com>
wrote:

Hi all,

With p number of processors in the communicator, the block preconditioner
PC_BJACOBI will by default use p blocks. So far, so good. However, in order
to compare this algorithmic efficiency decrease (since the bigger p, the
less efficient the preconditioner), i ran the commands

mpirun -n 1 ./program -pc_bjacobi_blocks 8
mpirun -n 8 ./program -pc_bjacobi_blocks 8

I expected the preconditioning to be equally efficient in this case.
However, GMRES makes more iterations in the first case (30 against 28) which
I cannot explain. Are there more subtle differences about the preconditioner
or the KSP that i'm overlooking here?

regards,

Leo
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