public double getHitCachingRatioPastNPeriods() {
    double ratio = ((double) getSumHitCachingCountsPastNPeriods()
      / (double) getSumRequestCachingCountsPastNPeriods());
    return Double.isNaN(ratio) ? 0 : ratio;
  }
Keep running age at eviction time
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CacheStats.java