code: def _submit(self, pool, args, callback):
          
          if self.config.args.single_threaded:
              callback(self.call_runner(*args))
          else:
              pool.apply_async(self.call_runner, args=args, callback=callback)
docs: If the caller has passed the magic 'single-threaded' flag, call the
          function directly instead of pool.apply_async. The single-threaded flag
           is intended for gathering more useful performance information about
          what appens beneath `call_runner`, since python's default profiling
          tools ignore child threads.
  
          This does still go through the callback path for result collection.