Parallel reduce which domains should be read

For RAMSES datasets, each MPI rank may have already indexed fully
part of the domains, leading to different chunks across MPI ranks.

This means that we can do an MPI reduction over which domains
should be read to get the most specific subset of domains, but also
to ensure that all ranks have the same list of domains when chunking.

Author: cphyc

yt/frontends/ramses/data_structures.py

@@ -18,6 +18,7 @@
 from yt.utilities.cython_fortran_utils import FortranFile as fpu
 from yt.utilities.lib.cosmology_time import t_frw, tau_frw
 from yt.utilities.on_demand_imports import _f90nml as f90nml
+from yt.utilities.parallel_tools.parallel_analysis_interface import communication_system
 from yt.utilities.physical_constants import kb, mp
 
 from .definitions import (
@@ -678,6 +679,15 @@ def _identify_base_chunk(self, dobj):
                     mylog.info("Identified %s intersecting domains", len(domains))
             base_region = getattr(dobj, "base_region", dobj)
 
+            # In case of MPI parallelism, we need to keep the intersection
+            # of all domains across all ranks
+            comm = communication_system.communicators[-1]
+            idomains = {dom.domain_id for dom in domains}
+            idomains = comm.comm.allreduce(set(idomains), op=lambda a, b: a & b)
+
+            # Keep domains that every rank has
+            domains = [dom for dom in domains if dom.domain_id in idomains]
+
             subsets = [
                 RAMSESDomainSubset(
                     base_region,