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Reproductibility issue with nn_comm=1 + flux form momentum advection schemes using small mpp domain sizes

ORCA2_ICE_PISCES configuration (with default namelists) is not reproducible when using nn_comm = 1 and ln_dynadv_cen2 or ln_dynadv_ubs advection schemes when using ~200 processors. Switching to nn_comm = 2 solves the issue.

Tested so far:

  • mpp splitting: jpni = 8, jpnj = 4, jpnij = 32 vs jpni=14, jpnj=19, jpnij=223
  • ORCA2_ICE_PISCES is reproducible when using ln_dynadv_vec or less processors
  • Land suppression is not to blame
  • Issue is also present with key_linssh
  • no change with ln_nnogather = .false.
  • no more repro problem with nn_comm = 2
  • repro issue is present on multiple clusters (belenos & kara)
Edited by Guillaume Samson
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