Ruby's concurrency, an example.

concurrency.rb

require "open-uri"
require "json"
require "benchmark"

REPO_URL = "https://api.github.com/users/JEG2/repos"

def github_api(url)
  open(url) { |response| JSON.parse(response.read) }
end

concurrency = (ARGV.shift || 10).to_i

serial = Benchmark.realtime do
  jeg2 = github_api(REPO_URL)
  jeg2.first(concurrency).each do |repo|
    name = github_api(repo["url"])["full_name"]
    puts "Fetched #{name} repository."
  end
end
puts "Serial time:  #{serial}"

concurrent = Benchmark.realtime do
  urls = Queue.new
  names = Queue.new
  threads = Array.new(concurrency) {
    Thread.new do
      loop do
        url = urls.deq
        data = github_api(url)
        if data.is_a?(Array)  # repo list
          data.first(concurrency).each do |repo|
            q.enq(repo["url"])
          end
        else
          names.enq(data["full_name"])
        end
      end
    end
  }

  urls.enq(REPO_URL)
  concurrency.times do
    name = names.deq
    puts "Fetched #{name} repository."
  end
end
puts "Concurrent time:  #{concurrent}"
# >> Fetched JEG2/advent_of_code_2015 repository.
# >> Fetched JEG2/amalgalite repository.
# >> Fetched JEG2/asciimation repository.
# >> Fetched JEG2/attachment_fu repository.
# >> Fetched JEG2/better_bj repository.
# >> Fetched JEG2/bird_of_paradise repository.
# >> Fetched JEG2/bottles_of_beer_song repository.
# >> Fetched JEG2/broadsides repository.
# >> Fetched JEG2/browser_captcha repository.
# >> Fetched JEG2/challenges_for_game_designers repository.
# >> Serial time:  3.4831192519995966
# >> Fetched JEG2/bottles_of_beer_song repository.
# >> Fetched JEG2/bird_of_paradise repository.
# >> Fetched JEG2/better_bj repository.
# >> Fetched JEG2/asciimation repository.
# >> Fetched JEG2/advent_of_code_2015 repository.
# >> Fetched JEG2/broadsides repository.
# >> Fetched JEG2/challenges_for_game_designers repository.
# >> Fetched JEG2/amalgalite repository.
# >> Fetched JEG2/browser_captcha repository.
# >> Fetched JEG2/attachment_fu repository.
# >> Concurrent time:  0.7062488960000337