file.rb

require 'stringio'                                                              
require 'rack-cache'                                                            
require 'time'                                                                  
                                                                                
app = lambda {|env| [200, {'Content-Type' => 'text/html'}, ["Hello World!"]]}   
                                                                                
cache = Rack::Cache.new(app, :verbose => true)                                  
                                                                                
threads = []                                                                    
1.upto(4).each do                                                               
  threads << Thread.new do                                                      
    rounds = 500000                                                             
    start = Time.new                                                            
    (1..rounds).each do                                                         
      cache.call({'rack.errors' => StringIO.new})                               
    end                                                                         
    puts ((Time.new - start) / rounds)                                                        
  end                                                                           
end                                                                             
                                                                                
threads.each {|t| t.join }