[Users] Toward an Extensible Volume Integration Thorn

[Frank Loeffler]

On Sat, May 23, 2015 at 09:10:37PM -0400, Zach Etienne wrote:
> Thanks for your tips. After many hours of looking into this problem, and
> even trying (unsuccessfully) to use Carpet macros, I conclude that I am
> being held back by a bug in Cactus scheduling.

Yes and no. The Cactus scheduling is fine, but the interaction with the
Carpet modes is often surprising, and so it is here.

This is (please correct me), how the interaction between Cactus and
Carpet works:

Cactus -> Carpet: please schedule stuff in Analysis
for all modes in Carpet (and levels and components if applicable):
  Carpet -> Cactus: call all functions in Analysis
  for all functions in Analysis:
    Cactus -> Carpet: Call function
    Carpet: if function should be called in current mode: call
            otherwise: ignore

Given this, can you see why your thorn hangs?

You schedule the initialization of the while-variable in 'global'.
'global-early' comes before that, so Cactus gets the task to loop while
'counter' in global-early, on an uninitialized variable, and inside that
loop nothing touches that variable, so the loop never stops.

Yes, this is kind of backward, and feels a bit strange. And it is. We do
have plans to make this simpler.

What you have to do to make this work:
- hold the loop variable at 0 whenever you don't want to loop,
  especially: initialize it right at the start to 0 and make sure
  it doesn't go below 0.
( also, but you already do this:
- make sure you initialize the loop variable (to the number of loops) in
  the same mode as the function that decrements it
)

Btw: probably not connected to this problem: I am surprised to see that
"SCHEDULE InitializeCounter before InitializeCounter" actually works. Is
there any reason for that? :)

Frank

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