code: private void notifyTTAboutTaskCompletion() {
      if (oobHeartbeatOnTaskCompletion) {
        synchronized (finishedCount) {
          int value = finishedCount.get();
          finishedCount.set(value+1);
          finishedCount.notifyAll();
        }
      }
    }
docs: Notify the tasktracker to send an out-of-band heartbeat.