@Override
  public void clear() {
    lock.lock();
    try {
      list.clear();
      notFull.signalAll();
    } finally {
      lock.unlock();
    }
  }
Atomically removes all of the elements from this deque.
The deque will be empty after this call returns.
llap-server/src/java/org/apache/hadoop/hive/llap/daemon/impl/PriorityBlockingDeque.java