[ET Trac] [Einstein Toolkit] #2182: NewRad's extrapolation methods for Gammas expect ghost zones to be valid

Einstein Toolkit trac-noreply at einsteintoolkit.org
Sun Aug 5 14:33:36 CDT 2018

#2182: NewRad's extrapolation methods for Gammas expect ghost zones to be valid
 Reporter:  Roland Haas          |       Type:  defect
   Status:  new                  |   Priority:  major
Milestone:  ET_2018_08           |  Component:  EinsteinToolkit thorn
  Version:  development version  |   Keywords:  ML_BSSN NewRad
 The current stencil size for NewRad's extrapolation of the Xt (contracted
 Christoffel tensors) variables is 4 and the code checks for a large enough
 grid like so (extrap.cc line 61ff)
 if (dir[d]<0) {
   assert(bmax[d] + 4 <=
          (cctkGH->cctk_bbox[2*d+1] ? imax[d] : cctkGH->cctk_lsh[d]));
 } else if (dir[d]>0) {
   assert(bmin[d] - 4 >= (cctkGH->cctk_bbox[2*d] ? imin[d] : 0));
 which checks (eg for {{{dir[d] > 0}}} ie an upper boundary) that, unless
 the *lower* boundary is a grid boundary, that there are 4 points between
 bmin and the beginning of the patch (0).

 This therefore assumes that ghost points (points 0...cctk_nghostzones) are
 valid since they will be used if the grid is small enough (less than

 Currently this is not ensured by McLachlan and only points in the interior
 are valid.

 This causes the attached parfile to produce poison in the output if run
 with 2 MPI ranks.

 A fix would be to add a {{{SYNC: ML_Gamma}}} to ML_BSSN's
 ML_BSSN_InitialADMBase2Interior routine or to make the check in NewRad
 stricter, requiring that there are at least 4 points in the interior
 (which would also make the check simpler).

 My feeling would be that this also explains NaNs that people have seen
 when running a TOV simulation with many MPI ranks.

 I attach a sample parfile as well as sample output.

Ticket URL: <https://trac.einsteintoolkit.org/ticket/2182>
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