docs: public static boolean tryReserveTaskMem(long m){
      if(!CAN_ALLOC)return false;
      if( m == 0 ) return true;
      assert m >= 0:"m < 0: " + m;
      long current = _taskMem.addAndGet(-m);
      if(current < 0){
        _taskMem.addAndGet(m);
        return false;
      }
      return true;
    }