config.ru

# ... app code ....

 require 'ruby-prof'

 run(lambda do |env|
  case env['PATH_INFO']
  when '/__START__' then
    RubyProf.start
    Rack::Response.new("Start profile").finish
  when '/__STOP__' then
    out = StringIO.new
    RubyProf::DotPrinter.new(RubyProf.stop).print(out, min_percent: 2)
    out.close
    Rack::Response.new(out.string).finish
  else
    $app.call(env)
    # Cuba.call(env)
  end
 end)

# $ curl http://127.0.0.1:8080/__START__; ab -n 1000 -c 10 http://127.0.0.1:8080/home; curl http://127.0.0.1:8080/__START__ > g.dot; dot -Tpng g.dot > g.png; open g.png