[petsc-users] Global numbering obtained via ISPartitioningToNumbering is inconsistent with the partitioning indexset

[Mohammad Mirzadeh]

Aaah! Thanks Barry. Just to make sure though, is my assumption on the
natural ordering of PETSc correct?

Thanks

On Mon, Apr 9, 2012 at 3:10 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Apr 9, 2012, at 5:06 PM, Mohammad Mirzadeh wrote:
>
> > Hi,
> >
> > I am using  'ISPartitioningToNumbering' to generate new global numbering
> from a partitioning indexset and I'm baffled at the following situation.
> I'm debugging my code on a simple grid consisting of 81 grid points
> partitioned among two processes. When I look into the partitioning indexset
> (i.e. looking at the indecies via ISView) I can see that 40 points have
> been assigned to proc 0 and 41 to processor 1. Isn't it true that when 81
> points are distributed among two processors 41 should go to proc 0 and 40
> to proc 1?
> >
> > I have based my whole code on the assumption (verified before through
> mailing list i guess) that natural ordering in PETSc leads to a
> distribution of points such that all processors get the same number of
> points ( [0, n/p) on proc 0, [n/p, 2n/p) on proc 1, ... ) unless n%p != 0,
> in which case the first k (with k = n%p) processors receive 1 point extra.
> Am I wrong to assume this?
> >
> > Thanks,
> > Mohammad
> >
>
> > PS: Is it relevant that the partitioning indexset is obtained via
> ParMetis?partitiong
>
>    Yes, ParMetis provides no guarantee about how many points would get
> assigned to each process.
>
>   Barry
>
>
>
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