[petsc-users] question about PC_BJACOBI

[Matthew Knepley]

On Thu, Sep 23, 2010 at 10:41 AM, Leo van Kampenhout <
lvankampenhout at gmail.com> wrote:

>
> Thank you both. The solvers are the same, I double checked that. It could
> be the case that the type of partitioning plays a role here, since i'm
> indeed using a DA. However, why is it that for example a run on 2 processors
> the number of iterations is higher than on 8? Both use DA-partitioning in
> this case. To specify subdomains manually, where do i start?
>

It is an open secret that Krylov methods are incredibly sensitive to
orderings, especially when combined with
incomplete factorization preconditioners. Since the ordering depends on the
division (see tutorials for a picture
of "petsc" orderings which are just contiguous per process), you can get
non-intuitive effects.

   Matt


> Leo
>
>
> 2010/9/23 Jed Brown <jed at 59a2.org>
>
>> 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
>>
>>
>>
>>
>>
>>
>


-- 
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
lead.
-- Norbert Wiener
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