tajo-core/tajo-core-backend/src/main/java/org/apache/tajo/master/DefaultTaskScheduler.java [98:128]:
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          } catch (InterruptedException e) {
            break;
          }
          schedule();
        }
        LOG.info("TaskScheduler schedulingThread stopped");
      }
    };

    this.schedulingThread.start();
    super.start();
  }

  private static final QueryUnitAttemptId NULL_ATTEMPT_ID;
  public static final TajoWorkerProtocol.QueryUnitRequestProto stopTaskRunnerReq;
  static {
    ExecutionBlockId nullSubQuery = QueryIdFactory.newExecutionBlockId(QueryIdFactory.NULL_QUERY_ID, 0);
    NULL_ATTEMPT_ID = QueryIdFactory.newQueryUnitAttemptId(QueryIdFactory.newQueryUnitId(nullSubQuery, 0), 0);

    TajoWorkerProtocol.QueryUnitRequestProto.Builder builder =
        TajoWorkerProtocol.QueryUnitRequestProto.newBuilder();
    builder.setId(NULL_ATTEMPT_ID.getProto());
    builder.setShouldDie(true);
    builder.setOutputTable("");
    builder.setSerializedData("");
    builder.setClusteredOutput(false);
    stopTaskRunnerReq = builder.build();
  }

  @Override
  public void stop() {
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tajo-core/tajo-core-backend/src/main/java/org/apache/tajo/master/LazyTaskScheduler.java [111:142]:
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          } catch (InterruptedException e) {
            break;
          }

          schedule();
        }
        LOG.info("TaskScheduler schedulingThread stopped");
      }
    };

    this.schedulingThread.start();
    super.start();
  }

  private static final QueryUnitAttemptId NULL_ATTEMPT_ID;
  public static final TajoWorkerProtocol.QueryUnitRequestProto stopTaskRunnerReq;
  static {
    ExecutionBlockId nullSubQuery = QueryIdFactory.newExecutionBlockId(QueryIdFactory.NULL_QUERY_ID, 0);
    NULL_ATTEMPT_ID = QueryIdFactory.newQueryUnitAttemptId(QueryIdFactory.newQueryUnitId(nullSubQuery, 0), 0);

    TajoWorkerProtocol.QueryUnitRequestProto.Builder builder =
        TajoWorkerProtocol.QueryUnitRequestProto.newBuilder();
    builder.setId(NULL_ATTEMPT_ID.getProto());
    builder.setShouldDie(true);
    builder.setOutputTable("");
    builder.setSerializedData("");
    builder.setClusteredOutput(false);
    stopTaskRunnerReq = builder.build();
  }

  @Override
  public void stop() {
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