public double evictedPerEviction() {
    double evictionCount = getEvictionCount();

    if (evictionCount == 0) {
      return 0;
    }

    return getEvictedCount() / evictionCount;
  }
Keep running age at eviction time
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CacheStats.java