code: void markCompletedTaskAttempt(String taskTracker, TaskAttemptID taskid) {
      // tracker --> taskid
      Set<TaskAttemptID> taskset = trackerToMarkedTasksMap.get(taskTracker);
      if (taskset == null) {
        taskset = new TreeSet<TaskAttemptID>();
        trackerToMarkedTasksMap.put(taskTracker, taskset);
      }
      taskset.add(taskid);
  
      LOG.debug("Marked '" + taskid + "' from '" + taskTracker + "'");
    }
docs: Mark a 'task' for removal later.
  This function assumes that the JobTracker is locked on entry.
  
  @param taskTracker the tasktracker at which the 'task' was running
  @param taskid completed (success/failure/killed) task