HDFS-4840, TestNNThroughputBenchmark exits in the middle. This is because ReplicationMonitor is being stopped while NN is still running.
This is only valid during testing. In normal cases, ReplicationMonitor thread runs all the time once started. In standby or safemode, it just skips calculating DN work. I think NNThroughputBenchmark needs to use ExitUtil to prevent termination, rather than modifying ReplicationMonitor.