# File lib/sup/modes/thread-index-mode.rb, line 613
  def load_n_threads_background n=LOAD_MORE_THREAD_NUM, opts={}
    return if @load_thread # todo: wrap in mutex
    @load_thread = Redwood::reporting_thread("load threads for thread-index-mode") do
      num = load_n_threads n, opts
      opts[:when_done].call(num) if opts[:when_done]
      @load_thread = nil
    end
  end