https://github.com/igrigorik/em-synchrony/issues/112

Gemfile

# A sample Gemfile
source "http://rubygems.org"

gem "em-synchrony", git: "https://github.com/igrigorik/em-synchrony.git"
gem "mysql2", ["= 0.3.11"]
gem "activerecord"

Gemfile.lock

GIT
  remote: https://github.com/igrigorik/em-synchrony.git
  revision: 0d176e900c589d1a8a04429bc91594cd842c3ec7
  specs:
    em-synchrony (1.0.0)
      eventmachine (>= 1.0.0.beta.1)

GEM
  remote: http://rubygems.org/
  specs:
    activemodel (3.2.2)
      activesupport (= 3.2.2)
      builder (~> 3.0.0)
    activerecord (3.2.2)
      activemodel (= 3.2.2)
      activesupport (= 3.2.2)
      arel (~> 3.0.2)
      tzinfo (~> 0.3.29)
    activesupport (3.2.2)
      i18n (~> 0.6)
      multi_json (~> 1.0)
    arel (3.0.2)
    builder (3.0.0)
    eventmachine (1.0.0.beta.4)
    i18n (0.6.0)
    multi_json (1.1.0)
    mysql2 (0.3.11)
    tzinfo (0.3.31)

PLATFORMS
  ruby

DEPENDENCIES
  activerecord
  em-synchrony!
  mysql2 (= 0.3.11)

issue112.rb

require 'bundler'

Bundler.setup

require 'em-synchrony'
require 'active_record'
#require 'rack/fiber_pool'


ActiveRecord::Base.establish_connection({:host => 'localhost',
                                         :port => 3306,
                                         :adapter => "em_mysql2",
                                         :database => 'widgets',
                                         :username => 'root',
                                         :password => '',
                                         :encoding => 'utf8',
                                         :pool => 5})


class Widget < ActiveRecord::Base
end

EM.synchrony do
  timer = Time.now

    fibers = []
    1.upto(10) do
      fibers << Fiber.new do
        time = Benchmark.realtime {
          1.upto(10000) do |index|
            Widget.find_by_id(index)
          end
        }
        puts "#{time*1000} milliseconds"
      end
    end
    fibers.each { |t| t.resume }

  Fiber.new do
    loop do
      unless fibers.any?(&:alive?)
        puts ""
        puts "Real mysql connections: #{ObjectSpace.each_object(Mysql2::EM::Client).count}"
        puts "Total time: #{Time.now - timer}"
        EM.stop
      end

      EM::Synchrony.sleep 0.5
    end
  end.resume

end