Changes (by Roland Haas):

status:

wontfix (was new)

I was working on a par file for a quick regridding test and found out an unexpected behaviour regarding regridding. Basically I want a par file with a few iterations such that it adds a refinement level every 4 iterations for example. I set the following parameters then:

CarpetRegrid2::add_levels_automatically      = "yes"
CarpetRegrid2::regrid_every  = 4

for a grid structure as such:

Cactus::cctk_itlast = 10
CoordBase::xmin = -0.5
CoordBase::ymin = -0.5
CoordBase::zmin = -0.5
CoordBase::xmax =  0.5
CoordBase::ymax =  0.5
CoordBase::zmax =  0.5
CoordBase::ncells_x   =  16 
CoordBase::ncells_y   =  16 
CoordBase::ncells_z   =  16
Carpet::max_refinement_levels    = 3
CarpetRegrid2::num_centres      = 1
CarpetRegrid2::active_1      = "yes"
CarpetRegrid2::num_levels_1  = 1
CarpetRegrid2::radius_1[1]   = 0.12
CarpetRegrid2::radius_1[2]   = 0.04

I was expecting then regridding to happen at iterations 4 and 8, however if you run the attached par file, a modification of balsara shocktube test, you see that the level additions actually happen at iterations 5 and 9 instead:

...
INFO (CarpetRegrid2): Increasing number of levels of centre 1 to 2 (it=5)
...
INFO (CarpetRegrid2): Increasing number of levels of centre 1 to 3 (it=9)

which is apparently an off-by-one kind of error. I tracked down the code producing these messages and it comes from function CarpetRegrid2_RegridMaps at CarpetRegrid2/src/regrid.cc, lines 745 to 747. In order for that piece of code to execute we do have to have do_recompose set to true. Strangely the condition to set it, compares the previous iteration to the regrid_every parameter on line 707 of the same file:

(cctk_iteration - 1) % regrid_every == 0

Investigating if this actually makes sense I was led to what I think it migh

Comment (by Roland Haas):

The observed behaviour was as intended, no bug.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/1906/carpetregrid2-possible-off-by-one-error