[petsc-dev] Generality of VecScatter

[Dmitry Karpeev]

On Nov 23, 2011, at 18:28, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> I had a useful conversation with the MPICH guys today about the unstructured communication primitives that Mark, Matt, Barry, and I have been discussing. It's looking like we can do a very thin communication library that has the nice semantics I was looking for, namely that the local-to-global is stored only as indices in the local space. To implement the communication primitives, the owner of a global node never needs to know how many or which processes have that point in their local space. There is an MPI-3 feature that we can use to elegantly avoid two-way knowledge for pointwise gather/scatter. (We can always implement the same interface using vanilla MPI-1, but the MPI-2/3 implementation would have less overhead.)
> 
> If I/we write this layer, VecScatter (with arbitrary data types, etc) could be implemented very easily on top of it. I'm in no rush to do this, but I could imagine getting there eventually. The VecScatter interface allows some things that might not be important. For example, any process can specify an edge in the communication graph, even if it does not own the source or the destination vertex. Also, both source and destination vertices can have degree higher than one. We can even put a link between the same points twice (effectively multiplying that contribution by 2 when using ADD_VALUES). I don't know if this ever makes semantic sense, but the interface allows it.
> 
> I wonder if we can express all useful communication with a more restrictive interface similar to a local-to-global map (but mapping between any vectors) where one side (typically called "local", but doesn't need to actually be a local Vec) has at most one edge.
That would disallow accumulating a MAX "locally".
> 
> What weird things do people use VecScatter for that might break with this more restrictive model?