[MOAB-dev] exchange tags over interface entities

[Jed Brown]

On Wed, Mar 7, 2012 at 09:45, Lorenzo Alessio Botti <ihabiamx at yahoo.it>wrote:

> Yes, thanks. I think the option within brackets is achievable with the
> current behavior of exchange_tags...
> Only not owned interface entities would know the ids of the "ghost"
> interfaces.
> I can create a ghost vector, scatter the values and remember that I have
> to assemble local matrices only if the interface is not owned.
> As a result I might have stashed entries in the matrix but I don't
> integrate redundantly over interfaces.
> Is this what you meant?
>

I was thinking of residual evaluation where the second round of
communication is more latency-sensitive.

1. Take state vector X, residual vector F (starts empty)
2. Evaluate X onto local interface quadrature points QX (your ghost Vec).
3. Gather parts of QX to owner of each quadrature point.
4. Solve Riemann problems at owned interface quadrature points to produce
interface fluxes QF.
5. Broadcast QF from owner to ghoster.
6. (perhaps concurrently with above) evaluate interior and use fluxes QF
when they arrive.

Alternatively:
3. Both sides get Q at quadrature points.
4. Riemann problems solved redundantly.
5. No QF broadcast needed because problems were solved redundantly.


In the case of matrices, you had to differentiate QX with respect to X or
you had to move cells anyway. There is still a choice between redundant
computation and more communication, but it's more complicated.
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