public ReplicaKey getReplicaKey(RegionInfo regionInfo) {
    return replicaKeyCache == null
      ? new ReplicaKey(regionInfo)
      : replicaKeyCache.getUnchecked(regionInfo);
  }
If memory is available, then set this to a value greater than your region count to maximize
replica distribution performance.
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/replicas/ReplicaKeyCache.java