title: Ruby Celluloid and resources consumption
body: <p>I'm new to Celluloid and have some questions about pools and futures. I'm building a simple web crawler (see the example at bottom). My URLS array dozen of thousands of URLs, so the example is stripped to some hundred.</p>
  
  <p>What I now want to do is to group to max. 50 req/s using futures, get their callbacks and crawl further 50 urls etc. The problem I have with this code: I would expect that it would maximum 50 threads but it spawns upto 400 and more in my case. If the input data increases, the code snippet finishes because it cannot spawn further requests (OS limits, OSX in my case).</p>
  
  <p>Why are there so many threads spawned and how to avoid this? I need a fast crawler which uses all resources the OS provides but not more than this :) So 2.000 threads seem to be the limit at OSX, all above this value let the code crashes.</p>
  
  <pre><code>#!/usr/bin/env jruby
  
  require 'celluloid'
  require 'open-uri'
  
  URLS = 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 
  class Crawler
    include Celluloid
  
    def fetch(id)
      uri = URI("http://api.instagram.com/oembed?url=#{id}")
      req = open(uri).read
    end
  end
  
  URLS.each_slice(50).map do |idset|
    pool = Crawler.pool(size: 50)
    crawlers = idset.to_a.map do |id|
      begin
        pool.future(:fetch, id)
      rescue
        nil
      end
    end
    crawlers.compact.each do |resp|
      puts resp.value.size rescue nil
    end
  end
  </code></pre>