private int getServerWithBestCacheRatioForRegion(int region) {
      return cluster.getOrComputeServerWithBestRegionCachedRatio()[region];
    }
Collect the amount of region cached for all the regions from all the active region servers.
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/CacheAwareLoadBalancer.java