docs: def start():
      """
      Start recording stats.  Call this from a benchmark script when your setup
      is done.  Call this at most once.
  
      @raise RuntimeError: Raised if the parent process responds with anything
      other than an acknowledgement of this message.
      """
      os.write(BenchmarkProcess.BACKCHANNEL_OUT, BenchmarkProcess.START)
      response = util.untilConcludes(os.read, BenchmarkProcess.BACKCHANNEL_IN, 1)
      if response != BenchmarkProcess.START:
          raise RuntimeError(
              "Parent process responded with %r instead of START " % (response,))