@Override
      public int compare(Integer left, Integer right) {
        // We will always get instantiated buckets
        return Float.compare(((float) buckets[left].usedCount) / buckets[left].itemCount,
          ((float) buckets[right].usedCount) / buckets[right].itemCount);
      }
Returns a set of indices of the buckets that are least filled excluding the offsets, we also
the fully free buckets for the BucketSizes where everything is empty and they only have one
completely free bucket as a reserved
@param excludedBuckets the buckets that need to be excluded due to currently being in used
@param bucketCount     max Number of buckets to return
@return set of bucket indices which could be used for eviction
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketAllocator.java