code: public void logTaskStarted(TaskID taskId, String taskType,
                               long startTime, String splitLocations) {
      if (disableHistory) {
        return;
      }
  
      JobID id = taskId.getJobID();
      if (!this.jobId.equals(id)) {
        throw new RuntimeException("JobId from task: " + id +
                                   " does not match expected: " + jobId);
      }
  
      if (null != writers) {
        log(writers, RecordTypes.Task,
            new Keys[]{Keys.TASKID, Keys.TASK_TYPE ,
                       Keys.START_TIME, Keys.SPLITS},
            new String[]{taskId.toString(), taskType,
                         String.valueOf(startTime),
                         splitLocations});
      }
    }
docs: Log start time of task (TIP).
  @param taskId task id
  @param taskType MAP or REDUCE
  @param startTime startTime of tip.