Speed up RSpec by deferring garbage collection

Update: This trick probably isn't very useful anymore in Ruby 2.x Show archive.org snapshot . The Ruby GC has improved a lot over the years.

Joe Van Dyk discovered Show archive.org snapshot that running the Ruby garbage collector only every X seconds can speed up your tests. I found that deferring garbage collection would speed up my RSpec examples by about 15%, but it probably depends on the nature of your tests. I also tried applying it to Cucumber features, but found no performance improvements.

Since Joe is using Test::Unit in his example, here is how to apply the same trick to RSpec.

First, copy this class to spec/support/deferred_garbage_collection.rb:

class DeferredGarbageCollection

  DEFERRED_GC_THRESHOLD = (ENV['DEFER_GC'] || 10.0).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

Then, in your spec_helper.rb, configure RSpec like seen below. Note that you might already have code hooked up to before(:all) and after(:all), so merge it together:

Spec::Runner.configure do |config|
  config.before(:all) do
    DeferredGarbageCollection.start
  end
  config.after(:all) do
    DeferredGarbageCollection.reconsider
  end
end

You can now enjoy faster specs.