Crank Your Specs

Ben Lindsey ·

One of the downsides to test driven development (TDD) on a large project is that over time your suite of tests will ultimately slow down. Whether it’s stale code, bloated tests, or just an increasing number of features to support, the more tests you write, the more time you will spend watching them run. Imagine how many times the oldest test you wrote gets run over the life of the project. Thousands? Tens of thousands?

At the start of a project, when all is ponies and rainbows, the cost of running all your tests is marginal. But after 6 months, 20 ActiveRecord classes, 15,000 lines of code, and 2,000 specs, you might be staring across the chasm of a 5 minute test suite. 5 minutes before you can check in a code change. 5 minutes before your feature passes continuous integration and hits the deployment server. Ugh.

Round 1

A little background first — on this project we are running rails 3.0.3 on ruby 1.9.2 and testing with rspec and factorygirl. After reading the road to faster tests and armed with perftools.rb, I decided to do some investigating.

Finished in 248.22 seconds
1916 examples, 0 failures

Total: 27519 samples
    9565  34.8%  34.8%     9565  34.8% garbage_collector
    3281  11.9%  46.7%     3314  12.0% Mysql2::Client#query
    1055   3.8%  50.5%     1058   3.8% Module#remove_method
    1001   3.6%  54.2%     1500   5.5% Kernel#require
     347   1.3%  55.4%      347   1.3% IO#write

Garbage collection is taking up 35% of the test time! Wow. Here’s the REPL that I used to query the ObjectSpace (in spec/spec_helper.rb):

    config.after(:suite) do
      while true
        '> '.display
          puts eval($stdin.gets)
        rescue Exception => e
          puts e.message

And the results:

>@count = 0; ObjectSpace.each_object(ActiveRecord::Base) { @count += 1 }
{:TOTAL=>2215274, :FREE=>477896, :T_OBJECT=>103363, :T_CLASS=>20276, :T_MODULE=>1550, :T_FLOAT=>3602, :T_STRING=>1078471, :T_REGEXP=>3295, :T_ARRAY=>171818, :T_HASH=>170042, :T_STRUCT=>12041, :T_BIGNUM=>167, :T_FILE=>6, :T_DATA=>122410, :T_MATCH=>497, :T_COMPLEX=>1, :T_RATIONAL=>3161, :T_NODE=>33802, :T_ICLASS=>12876}

17,000 ActiveRecord objects and 1.7 million total objects in use. Looks like rspec is holding onto instance variables and leaking memory just like Test::Unit. No wonder garbage collection is a dog. So I went ahead and applied the fix from Jamis like this:

    RSpec.configure do |config|
      RESERVED_IVARS = %w(@loaded_fixtures)
      last_gc_run =
      config.before(:each) do
      config.after(:each) do
        (instance_variables - RESERVED_IVARS).each do |ivar|
          instance_variable_set(ivar, nil)
        if - last_gc_run > 1.0
          last_gc_run =
Finished in 145.54 seconds
1916 examples, 0 failures

After releasing instance variables and triggering garbage collection every second, we just gained 100 seconds (40%!?!) of our developer lives back. Hell yeah.

Round 2

Let’s profile it again and see if anything else jumps out:

Total: 17445 samples
    2673  15.3%  15.3%     2673  15.3% garbage_collector
    1971  11.3%  26.6%     1984  11.4% Mysql2::Client#query
     962   5.5%  32.1%     1452   8.3% Kernel#require
     950   5.4%  37.6%      953   5.5% Module#remove_method
     336   1.9%  39.5%     1000   5.7% ActiveRecord::AttributeMethods::Read#read_attribute

Mysql2::Client#query is the next biggest contributor. For a while I have been wary of the way factorygirl automatically builds associations for you. For example in a typical blog app a Factory(:comment) will create a comment, its author, its post, its post’s author, etc, in order to build up a valid object graph. It would be nice if we could avoid taking this hit for every test.

The solution is to leverage the speed of fixtures with the ease and explicitness of factories. Instead of building a new object for the association, we replace it with User.first, Comment.first, Post.first, Blog.first, etc and rely on a single object defined in the fixture. This results in a single query to the database. We are basically using transactional fixtures as singletons which are linked together in a valid graph by default and get reset after every test run.

Factory.define(:comment) do |f|
  f.body "This is my first comment"
  f.user { User.first } { Post.first }

The knock against fixtures has always been that they are difficult to maintain since they don’t live in code. And they introduce dependencies between unrelated tests which should be running in isolation. Having one valid object per YAML fixture file seems like a perfectly good trade off and none of the downside. The results:

Finished in 103.22 seconds
1916 examples, 0 failures

Total: 13290 samples
    1881  14.2%  14.2%     1881  14.2% garbage_collector
    1290   9.7%  23.9%     1307   9.8% Mysql2::Client#query
    1019   7.7%  31.5%     1021   7.7% Module#remove_method
     973   7.3%  38.8%     1446  10.9% Kernel#require
     304   2.3%  41.1%      304   2.3% Module#to_s

Another 40 seconds of our developer lives back. Here are the stats from our continuous integration server (TeamCity):

Build Duration


With a little bit of research and some minor enhancements, we were able to cut our test time by more than half. The graph from TeamCity shows a remarkably consistent test suite since the change. As you can see the trend was not tenable. Take the time to investigate why your tests are running slowly. The amount of time and suffering that you save your team will be worth it. After all, what would TDD be without an efficient red-green-refactor cycle?

Further Reading

Ben Lindsey
Ben Lindsey

Runner; Chef; Trancer; Traveler; Technologist; Agile evangelist. I'm a firm believer in that which you measure will surely improve. Currently I am working on how to test drive and scale a node.js application in the cloud.