code: func (api *PrivateMinerAPI) Start(threads *int) error {
  	if threads == nil {
  		return api.e.StartMining(runtime.NumCPU())
  	}
  	return api.e.StartMining(*threads)
  }
docs: // Start starts the miner with the given number of threads. If threads is nil,
  // the number of workers started is equal to the number of logical CPUs that are
  // usable by this process. If mining is already running, this method adjust the
  // number of threads allowed to use and updates the minimum price required by the
  // transaction pool.