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+h2. Performance Testing Rails Applications
+
+This guide covers the various ways of performance testing a Ruby on Rails application. By referring to this guide, you will be able to:
+
+* Understand the various types of benchmarking and profiling metrics
+* Generate performance and benchmarking tests
+* Install and use a GC-patched Ruby binary to measure memory usage and object allocation
+* Understand the benchmarking information provided by Rails inside the log files
+* Learn about 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.
+
+endprologue.
+
+h3. Performance Test Cases
+
+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.
+
+In a freshly generated Rails application, +test/performance/browsing_test.rb+ contains an example of a performance test:
+
+<ruby>
+require 'test_helper'
+require 'rails/performance_test_help'
+
+# Profiling results for each test method are written to tmp/performance.
+class BrowsingTest < ActionDispatch::PerformanceTest
+  def test_homepage
+    get '/'
+  end
+end
+</ruby>
+
+This example is a simple performance test case for profiling a GET request to the application's homepage.
+
+h4. Generating Performance Tests
+
+Rails provides a generator called +performance_test+ for creating new performance tests:
+
+<shell>
+$ rails generate performance_test homepage
+</shell>
+
+This generates +homepage_test.rb+ in the +test/performance+ directory:
+
+<ruby>
+require 'test_helper'
+require 'rails/performance_test_help'
+
+class HomepageTest < ActionDispatch::PerformanceTest
+  # Replace this with your real tests.
+  def test_homepage
+    get '/'
+  end
+end
+</ruby>
+
+h4. Examples
+
+Let's assume your application has the following controller and model:
+
+<ruby>
+# routes.rb
+root :to => 'home#index'
+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
+</ruby>
+
+h5. Controller Example
+
+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+:
+
+<ruby>
+require 'test_helper'
+require 'rails/performance_test_help'
+
+class PostPerformanceTest < ActionDispatch::PerformanceTest
+  def setup
+    # Application requires logged-in user
+    login_as(:lifo)
+  end
+
+  def test_homepage
+    get '/dashboard'
+  end
+
+  def test_creating_new_post
+    post '/posts', :post => { :body => 'lifo is fooling you' }
+  end
+end
+</ruby>
+
+You can find more details about the +get+ and +post+ methods in the "Testing Rails Applications":testing.html guide.
+
+h5. Model Example
+
+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:
+
+<ruby>
+require 'test_helper'
+require 'rails/performance_test_help'
+
+class PostModelTest < ActionDispatch::PerformanceTest
+  def test_creation
+    Post.create :body => 'still fooling you', :cost => '100'
+  end
+
+  def test_slow_method
+    # Using posts(:awesome) fixture
+    posts(:awesome).slow_method
+  end
+end
+</ruby>
+
+h4. Modes
+
+Performance tests can be run in two modes: Benchmarking and Profiling.
+
+h5. Benchmarking
+
+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:
+
+<shell>
+$ rake test:benchmark
+</shell>
+
+h5. Profiling
+
+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:
+
+<shell>
+$ rake test:profile
+</shell>
+
+h4. Metrics
+
+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.
+
+h5. Wall Time
+
+Wall time measures the real world time elapsed during the test run. It is affected by any other processes concurrently running on the system.
+
+h5. Process Time
+
+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.
+
+h5. CPU Time
+
+Similar to process time, but leverages the more accurate CPU clock counter available on the Pentium and PowerPC platforms.
+
+h5. User Time
+
+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.
+
+h5. Memory
+
+Memory measures the amount of memory used for the performance test case.
+
+h5. Objects
+
+Objects measures the number of objects allocated for the performance test case.
+
+h5. GC Runs
+
+GC Runs measures the number of times GC was invoked for the performance test case.
+
+h5. GC Time
+
+GC Time measures the amount of time spent in GC for the performance test case.
+
+h5. Metric Availability
+
+h6(#benchmarking_1). Benchmarking
+
+|_.Interpreter|_.Wall Time|_.Process Time|_.CPU Time|_.User Time|_.Memory|_.Objects|_.GC Runs|_.GC Time|
+|_.MRI        | yes       | yes          | yes      | no        | yes    | yes     | yes     | yes     |
+|_.REE        | yes       | yes          | yes      | no        | yes    | yes     | yes     | yes     |
+|_.Rubinius   | yes       | no           | no       | no        | yes    | yes     | yes     | yes     |
+|_.JRuby      | yes       | no           | no       | yes       | yes    | yes     | yes     | yes     |
+
+h6(#profiling_1). Profiling
+
+|_.Interpreter|_.Wall Time|_.Process Time|_.CPU Time|_.User Time|_.Memory|_.Objects|_.GC Runs|_.GC Time|
+|_.MRI        | yes       | yes          | no       | no        | yes    | yes     | yes     | yes     |
+|_.REE        | yes       | yes          | no       | no        | yes    | yes     | yes     | yes     |
+|_.Rubinius   | yes       | no           | no       | no        | no     | no      | no      | no      |
+|_.JRuby      | yes       | no           | no       | no        | no     | no      | no      | no      |
+
+NOTE: To profile under JRuby you'll need to run +export JRUBY_OPTS="-Xlaunch.inproc=false --profile.api"+ *before* the performance tests.
+
+h4. Understanding the Output
+
+Performance tests generate different outputs inside +tmp/performance+ directory depending on their mode and metric.
+
+h5(#output-benchmarking). Benchmarking
+
+In benchmarking mode, performance tests generate two types of outputs.
+
+h6(#output-command-line). Command Line
+
+This is the primary form of output in benchmarking mode. Example:
+
+<shell>
+BrowsingTest#test_homepage (31 ms warmup)
+           wall_time: 6 ms
+              memory: 437.27 KB
+             objects: 5,514
+             gc_runs: 0
+             gc_time: 19 ms
+</shell>
+
+h6. CSV Files
+
+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+:
+
+<shell>
+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
+</shell>
+
+h5(#output-profiling). Profiling
+
+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.
+
+h6. Command Line
+
+This is a very basic form of output in profiling mode:
+
+<shell>
+BrowsingTest#test_homepage (58 ms warmup)
+        process_time: 63 ms
+              memory: 832.13 KB
+             objects: 7,882
+</shell>
+
+h6. Flat
+
+Flat output shows the metric—time, memory, etc—measure in each method. "Check Ruby-Prof documentation for a better explanation":http://ruby-prof.rubyforge.org/files/examples/flat_txt.html.
+
+h6. Graph
+
+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":http://ruby-prof.rubyforge.org/files/examples/graph_txt.html.
+
+h6. Tree
+
+Tree output is profiling information in calltree format for use by "kcachegrind":http://kcachegrind.sourceforge.net/html/Home.html and similar tools.
+
+h6. Output Availability
+
+|_.         |_.Flat|_.Graph|_.Tree|
+|_.MRI      | yes  | yes   | yes  |
+|_.REE      | yes  | yes   | yes  |
+|_.Rubinius | yes  | yes   | no   |
+|_.JRuby    | yes  | yes   | no   |
+
+h4. Tuning Test Runs
+
+Test runs can be tuned by setting the +profile_options+ class variable on your test class.
+
+<ruby>
+require 'test_helper'
+require 'rails/performance_test_help'
+
+# Profiling results for each test method are written to tmp/performance.
+class BrowsingTest < ActionDispatch::PerformanceTest
+  self.profile_options = { :runs => 5,
+                           :metrics => [:wall_time, :memory] }
+
+  def test_homepage
+    get '/'
+  end
+end
+</ruby>
+
+In this example, the test would run 5 times and measure wall time and memory. There are a few configurable options:
+
+|_.Option   |_.Description|_.Default|_.Mode|
+|+:runs+    |Number of runs.|Benchmarking: 4, Profiling: 1|Both|
+|+:output+  |Directory to use when writing the results.|+tmp/performance+|Both|
+|+:metrics+ |Metrics to use.|See below.|Both|
+|+:formats+ |Formats to output to.|See below.|Profiling|
+
+Metrics and formats have different defaults depending on the interpreter in use.
+
+|_.Interpreter|_.Mode|_.Default metrics|_.Default formats|
+|/2.MRI/REE |Benchmarking|+[:wall_time, :memory, :objects, :gc_runs, :gc_time]+|N/A|
+|Profiling   |+[:process_time, :memory, :objects]+|+[:flat, :graph_html, :call_tree, :call_stack]+|
+|/2.Rubinius|Benchmarking|+[:wall_time, :memory, :objects, :gc_runs, :gc_time]+|N/A|
+|Profiling   |+[:wall_time]+|+[:flat, :graph]+|
+|/2.JRuby   |Benchmarking|+[:wall_time, :user_time, :memory, :gc_runs, :gc_time]+|N/A|
+|Profiling   |+[:wall_time]+|+[:flat, :graph]+|
+
+As you've probably noticed by now, metrics and formats are specified using a symbol array with each name "underscored.":http://api.rubyonrails.org/classes/String.html#method-i-underscore
+
+h4. Performance Test Environment
+
+Performance tests are run in the +test+ environment. But running performance tests will set the following configuration parameters:
+
+<shell>
+ActionController::Base.perform_caching = true
+ActiveSupport::Dependencies.mechanism = :require
+Rails.logger.level = ActiveSupport::BufferedLogger::INFO
+</shell>
+
+As +ActionController::Base.perform_caching+ is set to +true+, performance tests will behave much as they do in the +production+ environment.
+
+h4. Installing GC-Patched MRI
+
+To get the best from Rails' performance tests under MRI, you'll need to build a special Ruby binary with some super powers.
+
+The recommended patches for each MRI version are:
+
+|_.Version|_.Patch|
+|1.8.6|ruby186gc|
+|1.8.7|ruby187gc|
+|1.9.2 and above|gcdata|
+
+All of these can be found on "RVM's _patches_ directory":https://github.com/wayneeseguin/rvm/tree/master/patches/ruby under each specific interpreter version.
+
+Concerning the installation itself, you can either do this easily by using "RVM":http://rvm.beginrescueend.com or you can build everything from source, which is a little bit harder.
+
+h5. Install Using RVM
+
+The process of installing a patched Ruby interpreter is very easy if you let RVM do the hard work. All of the following RVM commands will provide you with a patched Ruby interpreter:
+
+<shell>
+$ rvm install 1.9.2-p180 --patch gcdata
+$ rvm install 1.8.7 --patch ruby187gc
+$ rvm install 1.9.2-p180 --patch ~/Downloads/downloaded_gcdata_patch.patch
+</shell>
+
+You can even keep your regular interpreter by assigning a name to the patched one:
+
+<shell>
+$ rvm install 1.9.2-p180 --patch gcdata --name gcdata
+$ rvm use 1.9.2-p180 # your regular ruby
+$ rvm use 1.9.2-p180-gcdata # your patched ruby
+</shell>
+
+And it's done! You have installed a patched Ruby interpreter.
+
+h5. Install From Source
+
+This process is a bit more complicated, but straightforward nonetheless. If you've never compiled a Ruby binary before, follow these steps to build a Ruby binary inside your home directory.
+
+h6. Download and Extract
+
+<shell>
+$ mkdir rubygc
+$ wget <the version you want from ftp://ftp.ruby-lang.org/pub/ruby>
+$ tar -xzvf <ruby-version.tar.gz>
+$ cd <ruby-version>
+</shell>
+
+h6. Apply the Patch
+
+<shell>
+$ curl http://github.com/wayneeseguin/rvm/raw/master/patches/ruby/1.9.2/p180/gcdata.patch | patch -p0 # if you're on 1.9.2!
+$ curl http://github.com/wayneeseguin/rvm/raw/master/patches/ruby/1.8.7/ruby187gc.patch | patch -p0 # if you're on 1.8.7!
+</shell>
+
+h6. Configure and Install
+
+The following will install Ruby in your home directory's +/rubygc+ directory. Make sure to replace +&lt;homedir&gt;+ with a full patch to your actual home directory.
+
+<shell>
+$ ./configure --prefix=/<homedir>/rubygc
+$ make && make install
+</shell>
+
+h6. Prepare Aliases
+
+For convenience, add the following lines in your +~/.profile+:
+
+<shell>
+alias gcruby='~/rubygc/bin/ruby'
+alias gcrake='~/rubygc/bin/rake'
+alias gcgem='~/rubygc/bin/gem'
+alias gcirb='~/rubygc/bin/irb'
+alias gcrails='~/rubygc/bin/rails'
+</shell>
+
+Don't forget to use your aliases from now on.
+
+h6. Install RubyGems (1.8 only!)
+
+Download "RubyGems":http://rubyforge.org/projects/rubygems and install it from source. Rubygem's README file should have necessary installation instructions. Please note that this step isn't necessary if you've installed Ruby 1.9 and above.
+
+h4. Using Ruby-Prof on MRI and REE
+
+Add Ruby-Prof to your applications' Gemfile if you want to benchmark/profile under MRI or REE:
+
+<ruby>
+gem 'ruby-prof', :git => 'git://github.com/wycats/ruby-prof.git'
+</ruby>
+
+Now run +bundle install+ and you're ready to go.
+
+h3. Command Line Tools
+
+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:
+
+h4. +benchmarker+
+
+Usage:
+
+<shell>
+Usage: rails 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
+</shell>
+
+Example:
+
+<shell>
+$ rails benchmarker 'Item.all' 'CouchItem.all' --runs 3 --metrics wall_time,memory
+</shell>
+
+h4. +profiler+
+
+Usage:
+
+<shell>
+Usage: rails 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
+        --metrics a,b,c              Metrics to use.
+                                     Default: process_time,memory,objects
+    -m, --formats x,y,z              Formats to output to.
+                                     Default: flat,graph_html,call_tree
+</shell>
+
+Example:
+
+<shell>
+$ rails profiler 'Item.all' 'CouchItem.all' --runs 2 --metrics process_time --formats flat
+</shell>
+
+NOTE: Metrics and formats vary from interpreter to interpreter. Pass +--help+ to each tool to see the defaults for your interpreter.
+
+h3. Helper Methods
+
+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.
+
+h4. Model
+
+<ruby>
+Project.benchmark("Creating project") do
+  project = Project.create("name" => "stuff")
+  project.create_manager("name" => "David")
+  project.milestones << Milestone.all
+end
+</ruby>
+
+This benchmarks the code enclosed in the +Project.benchmark("Creating project") do...end+ block and prints the result to the log file:
+
+<ruby>
+Creating project (185.3ms)
+</ruby>
+
+Please refer to the "API docs":http://api.rubyonrails.org/classes/ActiveRecord/Base.html#M001336 for additional options to +benchmark()+
+
+h4. Controller
+
+Similarly, you could use this helper method inside "controllers":http://api.rubyonrails.org/classes/ActionController/Benchmarking/ClassMethods.html#M000715
+
+<ruby>
+def process_projects
+  self.class.benchmark("Processing projects") do
+    Project.process(params[:project_ids])
+    Project.update_cached_projects
+  end
+end
+</ruby>
+
+NOTE: +benchmark+ is a class method inside controllers
+
+h4. View
+
+And in "views":http://api.rubyonrails.org/classes/ActionController/Benchmarking/ClassMethods.html#M000715:
+
+<erb>
+<% benchmark("Showing projects partial") do %>
+  <%= render @projects %>
+<% end %>
+</erb>
+
+h3. Request Logging
+
+Rails log files contain very useful information about the time taken to serve each request. Here's a typical log file entry:
+
+<shell>
+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]
+</shell>
+
+For this section, we're only interested in the last line:
+
+<shell>
+Completed in 5ms (View: 2, DB: 0) | 200 OK [http://0.0.0.0/items]
+</shell>
+
+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.
+
+Michael Koziarski has an "interesting blog post":http://www.therailsway.com/2009/1/6/requests-per-second explaining the importance of using milliseconds as the metric.
+
+h3. Useful Links
+
+h4. Rails Plugins and Gems
+
+* "Rails Analyzer":http://rails-analyzer.rubyforge.org
+* "Palmist":http://www.flyingmachinestudios.com/programming/announcing-palmist
+* "Rails Footnotes":https://github.com/josevalim/rails-footnotes/tree/master
+* "Query Reviewer":https://github.com/dsboulder/query_reviewer/tree/master
+
+h4. Generic Tools
+
+* "httperf":http://www.hpl.hp.com/research/linux/httperf/
+* "ab":http://httpd.apache.org/docs/2.2/programs/ab.html
+* "JMeter":http://jakarta.apache.org/jmeter/
+* "kcachegrind":http://kcachegrind.sourceforge.net/html/Home.html
+
+h4. Tutorials and Documentation
+
+* "ruby-prof API Documentation":http://ruby-prof.rubyforge.org
+* "Request Profiling Railscast":http://railscasts.com/episodes/98-request-profiling - Outdated, but useful for understanding call graphs
+
+h3. Commercial Products
+
+Rails has been lucky to have a few companies dedicated to Rails-specific performance tools. A couple of those are:
+
+* "New Relic":http://www.newrelic.com
+* "Scout":http://scoutapp.com