em_http_request.rb

require 'em-http'

def download_em_http(urls, concurrency)
  EventMachine.run do
    multi = EventMachine::MultiRequest.new

    EM::Iterator.new(urls, concurrency).each do |url, iterator|
      req = EventMachine::HttpRequest.new(url).get
      req.callback do
        write_file url, req.response
        iterator.next
      end
      multi.add url, req
      multi.callback { EventMachine.stop } if url == urls.last
    end
  end
end