code: public long getEndToEndDuration() {
  		SubtaskStateStats subtask = getLatestAcknowledgedSubtaskStats();
  		if (subtask != null) {
  			return Math.max(0, subtask.getAckTimestamp() - triggerTimestamp);
  		} else {
  			return -1;
  		}
  	}
docs: Returns the duration of this checkpoint calculated as the time since
  triggering until the latest acknowledged subtask or <code>-1</code> if
  no subtask was acknowledged yet.
  
  @return Duration of this checkpoint or <code>-1</code> if no subtask was acknowledged yet.