# File lib/rack/perftools_profiler/profiler.rb, line 74
    def start(mode = nil)
      ensure_mode_is_changeable(mode) if mode
      PerfTools::CpuProfiler.stop
      if (mode)
        @mode_for_request = mode
      end
      unset_env_vars
      set_env_vars
      PerfTools::CpuProfiler.start(PROFILING_DATA_FILE)
      self.profiling = true
    ensure
      @mode_for_request = nil
    end