---
title: "RSpec speed-up (24.6%) by tweaking ruby garbage collection"
kind: article
slug: rspec-speed-up-by-tweaking-ruby-garbage-collection
created_at: 2011-09-24
tags:
  - Ruby
  - Rails
  - rspec
  - speed
  - garbage collection
  - GC
---
[Today I learned][1] that Ruby garbage collection can be of huge importance to performance. More precisely, if Ruby does a lot of garbage collection it may slow down your code. Running garbage collection only every 10 or 20 seconds when running specs may increase performance dramatically.

At this time specs for Ariejan.net take an average of 25.29s to run. This is not bad, but in my opinion faster specs are better. After tweaking ruby garbage collection I got my specs to run in 19.05s, a **24.6% speed increase**!
[1]: http://37signals.com/svn/posts/2742-the-road-to-faster-tests
~
So, how do _you_ speed up _your_ tests?

The key is that you don't want ruby to decide when to do garbage collection for you. Instead, we'll tell Ruby when to do this, and we'll let it do it every 15 seconds.

To set this up for RSpec create a file `spec/support/deferred_garbage_collection.rb`:

    :::ruby
    class DeferredGarbageCollection

      DEFERRED_GC_THRESHOLD = (ENV['DEFER_GC'] || 15.0).to_f

      @@last_gc_run = Time.now

      def self.start
        GC.disable if DEFERRED_GC_THRESHOLD > 0
      end

      def self.reconsider
        if DEFERRED_GC_THRESHOLD > 0 && Time.now - @@last_gc_run >= DEFERRED_GC_THRESHOLD
          GC.enable
          GC.start
          GC.disable
          @@last_gc_run = Time.now
        end
      end
    end

Next, add the following to your `spec/spec_helper.rb` so rspec will use our deferred garbage collection.

    :::ruby
    config.before(:all) do
      DeferredGarbageCollection.start
    end

    config.after(:all) do
      DeferredGarbageCollection.reconsider
    end

Now, when you run `rake spec` you should see a nice speed increase. Try to alter the threshold value a bit to see what gives your best performance:

    DEFER_GC=20 rake spec

Enjoy!