[petsc-users] Parallel Graph Coloring

[Gaetan Kenway]

Hi Peter

Thanks for the info. I actually already have a coloring that is done
through the discretrization that is near optimal. The issue is that I have
a funny interprocessor dependence that is tricky to do my own sequential
coloring. So I was looking at doing it in a global, parallel sense. For
interest, I tried using the matGetColor() on my assembled matrix just to
see how many colors it would take. My discretrization coloring has 35
colors (for a stencil with 33 cells) and the matGetColor() resulted in
approximately 78 colors or about twice as many.

Thanks,

Gaetan


On Tue, Nov 5, 2013 at 11:39 AM, Peter Brune <brune at mcs.anl.gov> wrote:

>
>
>
> On Tue, Nov 5, 2013 at 10:29 AM, Gaetan Kenway <gaetank at gmail.com> wrote:
>
>> Hi
>>
>> I have a quick question regarding the MatGetColoring() function.
>> According to the documentation,
>>
>>  "For parallel matrices currently converts to sequential matrix and uses
>> the sequential coloring on that."
>>
>>
> The meaning of this is that it transfers the parallel matrix to a single
> processor and does the coloring there, not that it only considers
> on-processor entries.  What comes out is therefore a valid column coloring
> of the whole matrix.  Parallel matrix coloring is in development but is not
> quite ready for public consumption.  Anecdotal evidence suggests that the
> serial coloring does not become a bottleneck on small parallel runs.
>
> A good option is to provide the coloring through your discretization.
>  This will often be more efficient than the matrix colorings, as you know
> the structure of your problem.
>
> - Peter
>
>
>> I am wondering does this give actually give a valid parallel coloring? My
>> application is a cell centered multi-block finite volume code, (with
>> block-based decomposition)  and the communications are done using a two
>> level exchange of halo cells. I would like to use FD (actually forward mode
>> AD to compute the jacbian matrix). So, what I'm wondering is, if a cell
>> with color 0 is perturbed on processor 0, is it guaranteed that a residual
>> on processor 1, that is influenced by the 0-colored cell on proc zero is
>> *only* influenced by the color 0 from proc 0 and not a 0-colored cell on
>> proc 1?
>>
>> I hope that is clear
>>
>> Thank you,
>>
>> Gaetan Kenway
>>
>>
>>
>>
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20131105/24f8783f/attachment.html>