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NOTE: In python, if you want to exit early you often use a "break" statement.  When running psana-python with mpi parallelization, however, not all cores will see this statement, and the result will be that your job will hang at the end.  To avoid this use the "max_events" keyword argument to DataSource (see example below).  An example of the failure mode is if an EB core communicates with only 1 BD core and that BD core exits, then the EB core gets stuck and doesn't exit (see MPITaskStructureToSupportScaling).  MPI requires all cores to exit gracefully for a job to exit.  The max_events kwarg allows the SMD0 core to shut everything down.

NOTE: The call below to smd.sum() (or min()/max()) only accepts numpy arrays or None.  This requirement allows cores that don't see any events to not cause errors when summing across cores.  All cores must call .sum() or the process will hang (a property of the underlying mpi "Reduce" call).

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