CommonMergeJoinOperator calls WritableComparator.get() in the inner loop

The CommonMergeJoinOperator wastes CPU looking up the correct comparator for each WritableComparable in each row.

@SuppressWarnings("rawtypes")
  private int compareKeys(List<Object> k1, List<Object> k2) {
    int ret = 0;
....   
      ret = WritableComparator.get(key_1.getClass()).compare(key_1, key_2);
      if (ret != 0) {
        return ret;
      }
    }

The slow part of that get() is deep within ReflectionUtils.setConf, where it tries to use reflection to set the Comparator config for each row being compared.