[ET Trac] [Einstein Toolkit] #1375: Slab does not fill in outmost layer of points in mesh refinement levels

[Einstein Toolkit]

#1375: Slab does not fill in outmost layer of points in mesh refinement levels
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  Reporter:  rhaas                  |       Owner:                     
      Type:  defect                 |      Status:  new                
  Priority:  minor                  |   Milestone:                     
 Component:  EinsteinToolkit thorn  |     Version:  development version
Resolution:                         |    Keywords:  Slab               
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Comment (by eschnett):

 Setting useghosts=1 in Slab is dangerous, since Slab then will use (read)
 ghost zones. This means that one has to synchronize before calling Slab.
 In multiple dimensions, points can be e.g. both symmetry and ghost zones,
 so that one also has to apply all symmetry conditions before
 synchronizing, so that these ghost zones are correct. This is in general
 difficult to ensure; one has to write the schedule very carefully to
 ensure that symmetries are applied last.

 In other words: Slab would use ghost points not only to fill in ghost
 points, but also to fill in non-ghost points. If the ghost points have not
 been set (via synchronisation), non-ghost points would remain undefined.

 However, points that are both symmetry and AMR ghost points (as opposed to
 inter-processor ghost points), cannot be set via AMR interpolation since
 they are too close to the boundary. That means that the symmetry condition
 is the only way to set them. Hence the correct solution is here: (1)
 synchronize first, (2) apply symmetry condition with useghosts=1. In
 addition to setting useghosts=1, we would also need to ensure that
 synchronization occurs first. Ideas?

 I do not understand your argument about "technically correct".
 Technically, with RK3 and 3 ghost zones, a refined region is surrounded by
 9 buffer zones, which are in turn surrounded by 3 ghost zones. This is
 Carpet's terminology, although people often call all 12 points "buffer
 zones".

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/1375#comment:2>
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