[petsc-dev] programming model for PETSc

[Dmitry Karpeev]

On Thu, Nov 24, 2011 at 6:56 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Thu, Nov 24, 2011 at 6:41 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>
>> On Thu, Nov 24, 2011 at 18:30, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote:
>>
>>> In any event, this business of storing explicit ranks/offsets for the
>>> ghost indices is sort of antithetical to the current
>>> PetscLayout-based approach of indices having parallel semantics from the
>>> outset.  A lot of conceptual difficulties
>>> in Sieve and MOAB (and the need for the extra objects such as Overlap
>>> and ParallelComm (sic!)) stem from a lack of
>>> a global address space.
>>>
>>
>> Global addresses and (rank, offset) are equivalent modulo MPI_Scan()
>> which costs essentially the same as MPI_Reduce(). Not free, but not huge
>> when done in setup. I think global indices are usually easier for users to
>> play with (if they need anything that has global semantics, local indices
>> combined with a local-to-global map is even better when sufficient), but
>> they eventually need to be converted to (rank, offset) for communication
>> (MPI does not know about a "global index" and I don't think it should.)
>>
>
> I think the idea of a local point of view is fundamental, not just for
> parallelism. We use LocalToGlobal mappings in FS so that yu do not have
> to know about other fields, just the same as you should not have to know
> about other processes.
>

Something has to know about the global layout.  In the example about it's
the PetscLayout in the Vec or Mat being split.
If the PetscLayout takes on a different form, then everything that relies
on range in it has to be redesigned.
One way to approach this is to make every PETSc object interact with
PetscLayout through an API that hides its details,
not but looking at the "private" struct members directly. I think that's
the right place to start.

Dmitry.


>    Matt
>
>
> --
> 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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