bool ThreadService::set_thread_monitoring_contention(bool flag) {

  {- -------------------------------------------
  (1) ThreadService::_thread_monitoring_contention_enabled フィールドを変更する.
      ---------------------------------------- -}

      MutexLocker m(Management_lock);

      bool prev = _thread_monitoring_contention_enabled;
      _thread_monitoring_contention_enabled = flag;

      return prev;
    }