code: private void addReduceTask(TaskTracker taskTracker, List<Task> tasks,
                                    int maxReduceSlots, int currentReduceSlots) 
                      throws IOException {
      if (maxReduceSlots > currentReduceSlots) {
        reduceScheduler.updateCollectionOfQSIs();
        TaskLookupResult tlr = reduceScheduler.assignTasks(taskTracker);
        if (TaskLookupResult.LookUpStatus.TASK_FOUND == tlr.getLookUpStatus()) {
          tasks.add(tlr.getTask());
        }
      }
    }
docs: on the TT to run one.