[Carpet] getting dt for the finest grid

[Yosef Zlochower]

Hi,

  What is the best procedure for obtaining the timestep for the finest  grid
currently used in a Carpet driven simulation?

Would something like the code below work for arbitrary numbers
 of processors, or would some processors find different values of dt
 because they do not own a segment of the finest grid? Can ResetDt be
 scheduled in the same timebin as FindDt (I am thinking about the case where
 a new refinement level may be added, or perhaps taken away)? Would
 the correct value of dt be used in MainCode as scheduled?
 Is there a much cleaner way to do this?

schedule.ccl:

schedule ResetDT AT EVOL BEFORE FindDt
{
  LANG: C
  OPTIONS: GLOBAL
} "set dt to some big value"

schedule FindDt AT EVOL BEFORE MainCode
{
  LANG: C
  OPTIONS: GLOBAL LOOP-LOCAL
} "find dt"

schedule MainCode AT EVOL AFTER FindDT
{
   LANG: C
   OPTIONS: GLOBAL
 } "use the smallest value of dt"

source.c:

static CCTK_REAL smallest_dt

void ResetDT(CCTK_ARGUMENTS)
{
  smallest_dt = BIG_NUMBER;
}

void FindDt(CCTK_ARGUMENTS)
{
  DECLARE_CCTK_ARGUMENTS
  if (fabs(smallest_dt)> fabs(CCTK_DELTA_TIME))
  {
     smallest_dt = CCTK_DELTA_TIME;
   }
}

void MainCode(CCTK_ARGUMENTS)
{
  ...
  fdot = INTERPOLATE_SOME_VALUE_FROM_GFS;

  /* f is a cactus grid array */
  f = f_p + smallest_dt * fdot;
}

Thanks

Yosef