[petsc-dev] Generality of VecScatter

[Matthew Knepley]

On Thu, Nov 24, 2011 at 10:24 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Thu, Nov 24, 2011 at 09:34, Mark F. Adams <mark.adams at columbia.edu>wrote:
>
>> The primitives I can readily provide are
>>
>> broadcast from Y to X
>> reduce values on X into Y (min, max, sum, replace, etc)
>>
>> gather values on X into arrays for each point in Y (in some ordering,
>> rank ordering if you like)
>>
>>
>> this is awful.  you would at least need to have a way of getting the
>> processors that it came from, the system would have to have this info,
>> right?
>>
>
> You don't need it for the implementation. If the user wants it, they can
> get it trivially (gather the ranks over the same communication graph).
> Sometimes it doesn't matter, and these primitives won't store that by
> default. Not storing it also makes graph updates lighter weight because you
> only need to update on one side.
>

No, I agree with Mark here. I originally wrote this way and had to rewrite
EVERYTHING later on to get this information.

   Matt


>> also, i think you are getting to general here, i'm not sure why anyone
>> would want to do this but i'm sure someone would and they can just write
>> directly to to MPI-3.
>>
>
> My point is to have a more user-friendly way to offer this information.
> The point broadcast ("global to local") and reduce ("local to global with
> operation") are all you need for many operations.
>
> My intent is to find an abstraction that is more general than VecScatter,
> higher level than MPI, and that can be used for all non-synchronizing
> ("neighbor collective") communication patterns in a library like PETSc.
> Having pointwise gather/scatter allows operations involving all copies of a
> point (instead of only those operations expressed as a reduction with a
> binary operation). I think you usually don't need it, but we can provide it
> cheaply and without extra complexity using this abstraction.
>

-- 
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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