Which way to decompose domain/grid

[Wee-Beng Tay]

Hi Jed,

I've a better understanding now. I think I can use it for my poisson eqn
solving which has pressure at the cell center. However, my U and V are
staggered. May I right to say that I can still use DA to help the domain
spilting process even for this scenario? However, can i solve my momentum
eqn with DA still since there's staggering?

Thanks alot!

On Fri, Dec 11, 2009 at 7:00 PM, Jed Brown <jed at 59a2.org> wrote:

> On Fri, 11 Dec 2009 18:24:58 +0800, Wee-Beng Tay <zonexo at gmail.com> wrote:
>
> > But you mention abt latency, so shouldn't minimizing the number of
> neighbor
> > processes reduce latency and improve performance?
>
> Perhaps, depending on your network.  But there are many tricks to hide
> latency of ghost updates, global reductions (in dot products) are
> harder especially since MPI collectives are synchronous.  The higher
> iteration counts are way more painful than marginally higher update
> cost.
>
> > For both do u mean dividing 1 big grid into 4 55x35 grids?
>
> Yes, instead of 4 thin slices.  And so on as you refine.  DA does this
> automatically, just don't choose a prime number of processes (because
> then it would be forced into doing slices).
>
> > so whichever method I use  (horizontal or vertical) doesn't matter? But
> > splitting to 4 55x35 grids will be better?
>
> Trying to send directly from some contiguous array is not a worthwhile
> optimization.  My comment about latency was to guide against another
> "optimization" of sending some components of a vector problem separately
> when not all components "need" updating (it's likely faster to do one
> update of 5 values per ghost node than to do two separate updates of 1
> value per node).
>
> Splitting into 4 subdomains isn't "better" than 2 subdomains, but when
> using many subdomains, they should not be thin slices.
>
> DA manages all of this.  If you have some compelling reason *not* to use
> DA, you won't go far wrong by copying the design decisions in DA.
>
>
> Jed
>
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