[Users] About SYNCs with staggered variables

[Roland Haas]

Hello Luciano,

> Thanks for the tip Roland.
> 
> I have tried it and it seems I'm getting the same efficiency nevertheless.

Yeah, it was a bit of a long shot.

> I was wondering whether, perhaps, the prolongations of the staggered
> variables are somehow more expensive/inefficient?

I could not think of anything that would make it more expensive. It is
just a slightly different interpolation stencil, but has the same size
overall.

Could in principle be less efficient if the functions are less well
optimized.

My guess would normally be that its related to the communication time
and not so much to the actual computation time of the interpolation.

Maybe Zach Etienne, whose IllinoisGRMHD code initially required this
sort of interpolation, recalls any issues?

If not you could try and run with all verbose options on 

Carpet::verbose = yes
Carpet::veryverbose = yes

CarpetLib::verbose = yes
CarpetLib::commstate_verbose = yes

and possibly

Carpet::sync_barriers = yes

to get more consistent (but possibly slower than normal) timing results.

Note that these will produce a lot of log output so you only want to
run a very few timesteps.

@Erik: just to confirm, if one has a:

SYNC: Ax, Ay

then those two synchronizations happen in parallel and not first Ax then
a MPI_WaitAll then Ay then a MPI_WaitAll (I had thought they'd be one
after the other, but looking at CarpetLib's code in commstate and
gdata it seems they all happen at once)?

Yours,
Roland

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