Performance Testing Rails Applications

This guide covers the various ways of performance testing a Ruby on Rails application.

After reading this guide, you will know:

• The various types of benchmarking and profiling metrics.
• How to generate performance and benchmarking tests.
• How to install and use a GC-patched Ruby binary to measure memory usage and object allocation.
• The benchmarking information provided by Rails inside the log files.
• Various tools facilitating benchmarking and profiling.

Performance testing is an integral part of the development cycle. It is very important that you don't make your end users wait for too long before the page is completely loaded. Ensuring a pleasant browsing experience for end users and cutting the cost of unnecessary hardware is important for any non-trivial web application.

As of rails 4 performance tests are no longer part of the default stack. If you want to use performance tests simply follow these instructions.

Add this line to your application's Gemfile:

gem 'rails-perftest'

If you want to benchmark/profile under MRI or REE, add this line as well:

gem 'ruby-prof'

Now run bundle install and you're ready to go.

Rails performance tests are a special type of integration tests, designed for benchmarking and profiling the test code. With performance tests, you can determine where your application's memory or speed problems are coming from, and get a more in-depth picture of those problems.

Rails provides a generator called performance_test for creating new performance tests:

$ rails generate performance_test homepage

This generates homepage_test.rb in the test/performance directory:

require 'test_helper'
require 'rails/performance_test_help'

class HomepageTest < ActionDispatch::PerformanceTest
  # Refer to the documentation for all available options
  # self.profile_options = { runs: 5, metrics: [:wall_time, :memory],
  #                          output: 'tmp/performance', formats: [:flat] }

  test "homepage" do
    get '/'
  end
end

Let's assume your application has the following controller and model:

# routes.rb
root to: 'home#dashboard'
resources :posts

# home_controller.rb
class HomeController < ApplicationController
  def dashboard
    @users = User.last_ten.includes(:avatars)
    @posts = Post.all_today
  end
end

# posts_controller.rb
class PostsController < ApplicationController
  def create
    @post = Post.create(params[:post])
    redirect_to(@post)
  end
end

# post.rb
class Post < ActiveRecord::Base
  before_save :recalculate_costly_stats

  def slow_method
    # I fire gallzilion queries sleeping all around
  end

  private

  def recalculate_costly_stats
    # CPU heavy calculations
  end
end

Because performance tests are a special kind of integration test, you can use the get and post methods in them.

Here's the performance test for HomeController#dashboard and PostsController#create:

require 'test_helper'
require 'rails/performance_test_help'

class PostPerformanceTest < ActionDispatch::PerformanceTest
  def setup
    # Application requires logged-in user
    login_as(:lifo)
  end

  test "homepage" do
    get '/dashboard'
  end

  test "creating new post" do
    post '/posts', post: { body: 'lifo is fooling you' }
  end
end

You can find more details about the get and post methods in the
Testing Rails Applications guide.

Even though the performance tests are integration tests and hence closer to the request/response cycle by nature, you can still performance test pure model code.

Performance test for Post model:

require 'test_helper'
require 'rails/performance_test_help'

class PostModelTest < ActionDispatch::PerformanceTest
  test "creation" do
    Post.create body: 'still fooling you', cost: '100'
  end

  test "slow method" do
    # Using posts(:awesome) fixture
    posts(:awesome).slow_method
  end
end

Performance tests can be run in two modes: Benchmarking and Profiling.

Benchmarking makes it easy to quickly gather a few metrics about each test run. By default, each test case is run 4 times in benchmarking mode.

To run performance tests in benchmarking mode:

$ rake test:benchmark

To run a single test pass it as TEST:

$ bin/rake test:benchmark TEST=test/performance/your_test.rb

Profiling allows you to make an in-depth analysis of each of your tests by using an external profiler. Depending on your Ruby interpreter, this profiler can be native (Rubinius, JRuby) or not (MRI, which uses RubyProf). By default, each test case is run once in profiling mode.

To run performance tests in profiling mode:

$ rake test:profile

Benchmarking and profiling run performance tests and give you multiple metrics. The availability of each metric is determined by the interpreter being used—none of them support all metrics—and by the mode in use. A brief description of each metric and their availability across interpreters/modes is given below.

Wall time measures the real world time elapsed during the test run. It is affected by any other processes concurrently running on the system.

Process time measures the time taken by the process. It is unaffected by any other processes running concurrently on the same system. Hence, process time is likely to be constant for any given performance test, irrespective of the machine load.

Similar to process time, but leverages the more accurate CPU clock counter available on the Pentium and PowerPC platforms.

User time measures the amount of time the CPU spent in user-mode, i.e. within the process. This is not affected by other processes and by the time it possibly spends blocked.

Memory measures the amount of memory used for the performance test case.

Objects measures the number of objects allocated for the performance test case.

GC Runs measures the number of times GC was invoked for the performance test case.

GC Time measures the amount of time spent in GC for the performance test case.

NOTE: To profile under JRuby you'll need to run export JRUBY_OPTS="-Xlaunch.inproc=false --profile.api" before the performance tests.

Performance tests generate different outputs inside tmp/performance directory depending on their mode and metric.

In benchmarking mode, performance tests generate two types of outputs.

This is the primary form of output in benchmarking mode. Example:

BrowsingTest#test_homepage (31 ms warmup)
           wall_time: 6 ms
              memory: 437.27 KB
             objects: 5,514
             gc_runs: 0
             gc_time: 19 ms

Performance test results are also appended to .csv files inside tmp/performance. For example, running the default BrowsingTest#test_homepage will generate following five files:

• BrowsingTest#test_homepage_gc_runs.csv
• BrowsingTest#test_homepage_gc_time.csv
• BrowsingTest#test_homepage_memory.csv
• BrowsingTest#test_homepage_objects.csv
• BrowsingTest#test_homepage_wall_time.csv

As the results are appended to these files each time the performance tests are run in benchmarking mode, you can collect data over a period of time. This can be very helpful in analyzing the effects of code changes.

Sample output of BrowsingTest#test_homepage_wall_time.csv:

measurement,created_at,app,rails,ruby,platform
0.00738224999999992,2009-01-08T03:40:29Z,,3.0.0,ruby-1.8.7.249,x86_64-linux
0.00755874999999984,2009-01-08T03:46:18Z,,3.0.0,ruby-1.8.7.249,x86_64-linux
0.00762099999999993,2009-01-08T03:49:25Z,,3.0.0,ruby-1.8.7.249,x86_64-linux
0.00603075000000008,2009-01-08T04:03:29Z,,3.0.0,ruby-1.8.7.249,x86_64-linux
0.00619899999999995,2009-01-08T04:03:53Z,,3.0.0,ruby-1.8.7.249,x86_64-linux
0.00755449999999991,2009-01-08T04:04:55Z,,3.0.0,ruby-1.8.7.249,x86_64-linux
0.00595999999999997,2009-01-08T04:05:06Z,,3.0.0,ruby-1.8.7.249,x86_64-linux
0.00740450000000004,2009-01-09T03:54:47Z,,3.0.0,ruby-1.8.7.249,x86_64-linux
0.00603150000000008,2009-01-09T03:54:57Z,,3.0.0,ruby-1.8.7.249,x86_64-linux
0.00771250000000012,2009-01-09T15:46:03Z,,3.0.0,ruby-1.8.7.249,x86_64-linux

In profiling mode, performance tests can generate multiple types of outputs. The command line output is always presented but support for the others is dependent on the interpreter in use. A brief description of each type and their availability across interpreters is given below.

This is a very basic form of output in profiling mode:

BrowsingTest#test_homepage (58 ms warmup)
        process_time: 63 ms
              memory: 832.13 KB
             objects: 7,882

Flat output shows the metric—time, memory, etc—measure in each method. Check Ruby-Prof documentation for a better explanation.

Graph output shows the metric measure in each method, which methods call it and which methods it calls. Check Ruby-Prof documentation for a better explanation.

Tree output is profiling information in calltree format for use by kcachegrind and similar tools.

Test runs can be tuned by setting the profile_options class variable on your test class.

require 'test_helper'
require 'rails/performance_test_help'

class BrowsingTest < ActionDispatch::PerformanceTest
  self.profile_options = { runs: 5, metrics: [:wall_time, :memory] }

  test "homepage"
    get '/'
  end
end

In this example, the test would run 5 times and measure wall time and memory. There are a few configurable options:

Metrics and formats have different defaults depending on the interpreter in use.

As you've probably noticed by now, metrics and formats are specified using a symbol array with each name underscored.

Performance tests are run in the test environment. But running performance tests will set the following configuration parameters:

ActionController::Base.perform_caching = true
ActiveSupport::Dependencies.mechanism = :require if ActiveSupport::Dependencies.respond_to?(:mechanism=)
Rails.logger.level = ActiveSupport::Logger::INFO

As ActionController::Base.perform_caching is set to true, performance tests will behave much as they do in the production environment.

Since Ruby 2 is now mainstream and handles garbage collection issues these docs have been cut. View older readme explaining how to install optimized Ruby 1 builds.

Writing performance test cases could be an overkill when you are looking for one time tests. Rails ships with two command line tools that enable quick and dirty performance testing:

Usage:

Usage: perftest benchmarker 'Ruby.code' 'Ruby.more_code' ... [OPTS]
    -r, --runs N                     Number of runs.
                                     Default: 4
    -o, --output PATH                Directory to use when writing the results.
                                     Default: tmp/performance
    -m, --metrics a,b,c              Metrics to use.
                                     Default: wall_time,memory,objects,gc_runs,gc_time

Example:

$ perftest benchmarker 'Item.all' 'CouchItem.all' --runs 3 --metrics wall_time,memory

Usage:

Usage: perftest profiler 'Ruby.code' 'Ruby.more_code' ... [OPTS]
    -r, --runs N                     Number of runs.
                                     Default: 1
    -o, --output PATH                Directory to use when writing the results.
                                     Default: tmp/performance
    -m, --metrics a,b,c              Metrics to use.
                                     Default: process_time,memory,objects
    -f, --formats x,y,z              Formats to output to.
                                     Default: flat,graph_html,call_tree

Example:

$ perftest profiler 'Item.all' 'CouchItem.all' --runs 2 --metrics process_time --formats flat

NOTE: Metrics and formats vary from interpreter to interpreter. Pass --help to each tool to see the defaults for your interpreter.

Rails provides various helper methods inside Active Record, Action Controller and Action View to measure the time taken by a given piece of code. The method is called benchmark() in all the three components.

Project.benchmark("Creating project") do
  project = Project.create("name" => "stuff")
  project.create_manager("name" => "David")
  project.milestones << Milestone.all
end

This benchmarks the code enclosed in the Project.benchmark("Creating project") do...end block and prints the result to the log file:

Creating project (185.3ms)

Please refer to the API docs for additional options to benchmark().

Similarly, you could use this helper method inside controllers.

def process_projects
  benchmark("Processing projects") do
    Project.process(params[:project_ids])
    Project.update_cached_projects
  end
end

NOTE: benchmark is a class method inside controllers.

And in views

<% benchmark("Showing projects partial") do %>
  <%= render @projects %>
<% end %>

Rails log files contain very useful information about the time taken to serve each request. Here's a typical log file entry:

Processing ItemsController#index (for 127.0.0.1 at 2009-01-08 03:06:39) [GET]
Rendering template within layouts/items
Rendering items/index
Completed in 5ms (View: 2, DB: 0) | 200 OK [http://0.0.0.0/items]

For this section, we're only interested in the last line:

Completed in 5ms (View: 2, DB: 0) | 200 OK [http://0.0.0.0/items]

This data is fairly straightforward to understand. Rails uses millisecond(ms) as the metric to measure the time taken. The complete request spent 5 ms inside Rails, out of which 2 ms were spent rendering views and none was spent communication with the database. It's safe to assume that the remaining 3 ms were spent inside the controller.

• Rails Analyzer
• Rails Footnotes
• Query Reviewer
• MiniProfiler

• httperf
• ab
• JMeter
• kcachegrind

• ruby-prof API Documentation
• Request Profiling Railscast - Outdated, but useful for understanding call graphs.

Rails has been lucky to have a few companies dedicated to Rails-specific performance tools:

• New Relic
• Scout
• Skylight