Replace all 8 blocking MPI_Allreduce error-check calls with MPI_Iallreduce,
overlapping the reduction with subsequent Parallel::Sync/SH->Synch operations.
MPI_Wait is called after Sync completes to retrieve the error result.
This hides the Allreduce latency (46.5% of CPU time) behind the ghost zone
exchange communication that must happen anyway. Safe because Sync only copies
existing data to ghost zones and the error check + abort happens before any
further computation uses the synced data.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
In the two Step() functions that handle both Patch and Shell Patch,
defer the Patch error check until after Shell Patch computation completes,
then perform a single combined MPI_Allreduce instead of two separate ones.
This eliminates 4 MPI_Allreduce calls per timestep (2 per Step function,
Predictor + Corrector phases each). The optimization is mathematically
equivalent: in normal execution (no NaN) behavior is identical; on error,
both Patch and Shell data are dumped before MPI_Abort.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
