Using rainbows! and em-http-request resulted in a 1600% performance increase compared to using unicorn and net_http.
(For the use case described below)

Situation

• You have an app that does a lot of calls to a 3rd party API. This is often the case with facebook apps for example.
• You want to host it on heroku (one dyno preferred, so it's free)

Problem

• You want to call the 3rd party API from your controller and render the response
• Even if you are using unicorn to forks several worker processes, they will be blocked and waiting for a response from Facebook (or any other potentially slow API) most of the time.

Solution: Use EventMachine and Rainbows!

Here is a quick outline on how to get you up and running with Rainbows! and em-http-requests on heroku.

We will be using the following gems:
Gemfile

gem 'rainbows'
gem 'em-http-request'
gem "faraday"

app/controllers/async_controller.rb

class AsyncController < ApplicationController
  def index
    conn = Faraday.new "http://slowapi.com" do |con|
      con.adapter :em_http
    end
    resp = conn.get '/delay/1'
    resp.on_complete {
      @res = resp.body 
      render
      request.env['async.callback'].call(response)
    }
    throw :async
end

app/views/async/index.html.erb

<%= @res %>

config/rainbows.rb

worker_processes 3
timeout 30
preload_app true

Rainbows! do
  use :EventMachine
end

before_fork do |server, worker|
  # Replace with MongoDB or whatever
  if defined?(ActiveRecord::Base)
    ActiveRecord::Base.connection.disconnect!
  end
end

after_fork do |server, worker|
  if defined?(ActiveRecord::Base)
    ActiveRecord::Base.establish_connection
  end
end

config/routes.rb

match 'async_test' =>   'async#index'

Procfile

web: bundle exec rainbows -p $PORT -c ./config/rainbows.rb

it is also a good idea to set config.threadsafe! in your Rails environment configuration

Finalize

• Deploy to heroku

• Run a benchmark, e.g. the following

  ab -n 600 -c 200 http://mycoolasync.herokuapp.com/async_test

• Enjoy how fast it is

Some results

I set up a test app on heroku and ran
ab -n 600 -c 200 http://mycoolasync.herokuapp.com/async_test

Result:

• Old-school method: 183 second
• Evented method: 11 seconds

Detailed results for old-school synchronous requests

Using non-evented http request by settting con.adapter :net_http in app/controllers/async_controller.rb

Document Path:          /async_test
Document Length:        2755 bytes

Concurrency Level:      100
Time taken for tests:   183.373 seconds
Complete requests:      600
Write errors:           0
Non-2xx responses:      516
Total transferred:      652495 bytes
HTML transferred:       541903 bytes
Requests per second:    3.27 [#/sec] (mean)
Time per request:       30562.206 [ms] (mean)
Time per request:       305.622 [ms] (mean, across all concurrent requests)
Transfer rate:          3.47 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:      101  175 260.7    109    1239
Processing:  1270 28308 5571.9  30115   30978
Waiting:     1146 28156 5882.4  30115   30665
Total:       1376 28484 5582.8  30224   32203

Detailed Results for new and fancy evented request method

Ensured by settting con.adapter :em_http in app/controllers/async_controller.rb

Document Path:          /async_test
Document Length:        2758 bytes

Concurrency Level:      100
Time taken for tests:   11.118 seconds
Complete requests:      600
Write errors:           0
Total transferred:      1734330 bytes
HTML transferred:       1655686 bytes
Requests per second:    53.97 [#/sec] (mean)
Time per request:       1853.048 [ms] (mean)
Time per request:       18.530 [ms] (mean, across all concurrent requests)
Transfer rate:          152.33 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:      101  194 283.0    114    1263
Processing:  1136 1485 416.1   1341    3558
Waiting:     1135 1453 346.2   1339    3159
Total:       1250 1679 472.5   1505    3669

Some final words

I wanted to have a link-baity title once. So here we are with a very unscientific 1600%. You'll be the judge - I think with the right test setting this number could be much much higher. If you replicated this and achieved a better result - let me know. I'll update this post and give you credit.

Cheers,
your friend
dommmel

gem 'rack-fiber_pool', :require => 'rack/fiber_pool'
gem 'em-synchrony'

...
use Rack::FiberPool
run MyApp::Application

conn = Faraday::Connection.new(:url => 'http://slowapi.com') do |builder|
   builder.use Faraday::Adapter::EMSynchrony
end

resp = conn.get '/delay/1'
@res = resp.body 
render