[petsc-users] Domain partitioning using DAs

[Jed Brown]

On Sat, Oct 8, 2011 at 07:27, Mohamad M. Nasr-Azadani <mmnasr at gmail.com>wrote:

> Thanks Jed. I guess the whole point I am missing is in that "squarish".
> I, for instance, use two processors for a domain of the global size:
> nx=50
> ny=40
> nz=4
>
> To my surprise, the way it was decomposed was in y-direction, i.e.
> processor 0:
> 0-49
> 0-19
> 0-3
>
> processor 1:
> 0-49
> 20-39
> 0-3
>
> and not x-direction which minimizes the surface area between the two
> processors.
> Obviously "2" is a prime number. In that case, is PETSc biased to choose
> any of the directions?
>

There is an order to which directions get tried, but it will never pick a
split that is "too bad". If you run on four processes, it will split in both
the X and Y directions. The complete code is here.

http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-dev/src/dm/impls/da/da3.c.html#line231

I'm a little surprised it doesn't find the "best" split in this simple case.
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