Profiling RSpec tests

14 Dec 2014

Knowing why something is slow is key to making it faster. Everybody hates slow tests, so I’m sharing my approach to measuring why some RSpec tests were taking ages to run.

The starting point was a model spec that took 22 seconds to run. My immediate assumption was that the tests we are running slowly because they rely heavily on interacting with the database (IO). At this point this is just a theory and it will need to be proven before we take steps to remedy the slowness. Cue profiling ruby code with Stackprof.

Profiling RSpec 1.3 specs with Stackprof

I conjured up the following script to benchmark any RSpec (1.3) test with Stackprof.

ENV['RAILS_ENV'] = 'test'

require 'stackprof'
$:.unshift 'spec'
require 'spec/autorun'
require 'spec_helper'

interval = ENV.fetch('INTERVAL', 1000).to_i
limit = ENV.fetch('LIMIT', 20)
output_file = "tmp/#{ARGV[0].split('/').last}.dump" :cpu, out: output_file, interval: interval) do

system("stackprof #{output_file} --text --limit #{limit}")

You need to add gem 'stackprof' to your Gemfile in order for this to run. Save it to a file and run it like this

bundle exec path/to/script <spec-file>

You can tweak the sampling interval by providing INTERVAL=1000 and limit the results output by providing a LIMIT=20.

The output looks something like this:

Mode: cpu(1000)
Samples: 32177 (6.74% miss rate)
GC: 13931 (43.29%)
3545 (11.0%) 3545 (11.0%) ActiveRecord::ConnectionAdapters::Mysql2Adapter#query
14202 (44.1%) 3358 (10.4%) ActiveSupport::Dependencies::Loadable#require
1653 (5.1%) 1641 (5.1%) block in ActiveSupport::BufferedLogger#flush
794 (2.5%) 515 (1.6%) ActiveSupport::Callbacks::Callback#should_run_callback?
497 (1.5%) 307 (1.0%) ActiveSupport::Memoizable::InstanceMethods#prime_cache
1207 (3.8%) 300 (0.9%) block in ActiveSupport::Dependencies::Loadable#require
399 (1.2%) 263 (0.8%) GetText::PoParser#parse
249 (0.8%) 249 (0.8%) block in ActiveSupport::Dependencies#search_for_file
2208 (6.9%) 108 (0.3%) ActiveRecord::ConnectionAdapters::Mysql2Adapter#select

The results above list the percentage spent in various methods whilst running the spec. From this it is clear that the biggest timesink whilst running the spec was ActiveRecord querying the database. Having this knowledge enabled me to start looking into how to reduce the problem, and also to have a concrete number to benchmark to see whether I’m making any progress.

Another offender that I hadn’t suspected was ActiveSupport::BufferedLogger writing the teste log to a file. This was easily reduced by adding config.log_level = :error to config/environments/test.rb.

Profiling RSpec 3 specs with Stackprof

I’ve ported the script to work with RSpec 3 as that will benefit the majority of setups out there.

#!/usr/bin/env ruby

require 'stackprof'
$:.unshift 'spec'
require 'rails_helper'

spec = ARGV[0]
interval = ENV.fetch('INTERVAL', 1000).to_i
limit = ENV.fetch('LIMIT', 20)
output_file = "tmp/#{spec.split('/').last}.dump" :cpu, out: output_file, interval: interval) do[spec], $stderr, $stdout)

system("stackprof #{output_file} --text --limit #{limit}")


You cannot optimise what you can’t measure. Some tools are a bit arcane, but a bit of digging can quickly lead to tangible results that can then be improved upon.