At the Forge - Profiling Rails Applications

by Reuven M. Lerner

I am writing this article in mid-May 2008, several weeks after Twitter was rumored to be moving to a platform other than Ruby on Rails. Twitter, of course, is an extremely popular service that allows users to write updates and notes about their current status—and allows readers to follow any number of people's Twitter feeds. You can think of Twitter as a combination blogging and RSS platform, populated by people who express themselves with only 140 characters at a time.

Like many other runaway Internet successes, Twitter appears to have become too popular for its own good. This has led to some outages, most notably one at the beginning of 2008 that took more than a day to restore. Thus, it was seen as more than a mere coincidence when Twitter's main architect left the company, and that within a few days, the TechCrunch blog was quoting anonymous officials within Twitter about how the service would be transitioning away from Ruby on Rails.

This was followed by a great deal of discussion over whether Rails is a “scalable” architecture. Scalable used to mean that it was possible to scale up applications using a Web site, almost regardless of how many people are using it. But today, a scalable architecture is one that's lean and mean, handling as many users as possible with as few servers as possible. PHP, Java and .NET are pretty universally considered to be scalable in this sense. Although even the most efficient PHP application can handle only a finite number of simultaneous users, it's undeniable that Ruby is a slower language than PHP, and that the Rails framework adds some more overhead.

Of course, it's one thing to say that Rails doesn't scale as quickly as PHP, and another to say that it doesn't scale at all. And, there are other arguments to be made, including the fact that programmers cost more than servers, and that programmer productivity should be at least as significant a factor as scalability.

That said, it's easy for a Rails application to become slow. So, it is nice to know that a variety of utilities can be used to profile Rails applications—meaning, finding out exactly which portion of the program is taking a long time to execute. This month, we look at some techniques for profiling Rails applications. Although such profiling doesn't make the software run any faster, it can help identify the slowest parts of an application.

Profiling Pages

If you aren't happy with the performance of your Web site—and quite frankly, you always should be concerned about the performance, trying to give it a boost wherever possible—the first question to ask is, “Where are people spending their time?” After all, if there are 100 different pages on your site, it doesn't really matter whether page 35 is really slow if no one ever visits it.

The first tool to examine, the production log analyzer, is designed to look at the Rails production log and produce some basic statistics about it. The production log, as well as the development and test logs, are typically stored in the log directory under the Rails project root. Thus, on a production server, the log is in log/production.log. This logfile is not rotated or modified automatically; there are clearly a number of ways to do that using cron and other UNIX command-line tools.

The thing is, there already is a facility on UNIX (and Linux) systems for handling logfiles, including their periodic rotation and disposal. This facility is known as syslog, which makes it possible to send logging information to a variety of different files based on priorities and source materials. The /var/log directory on my Ubuntu server is full of different logfiles, and nearly all of them were created and written to by syslog.

It turns out that we can use syslog for our Rails production logs. Once we have done that—and yes, we must use syslog for this to work—we then can analyze our production logs, learning exactly how much time people have spent in our Rails application.

To move your Rails production log to syslog, you need to do several things. First, you must install the Ruby gem that provides this behavior:

gem install --remote SyslogLogger

This installs the gem in the appropriate place on your system; on mine, it was put into /usr/lib/ruby/gems/1.8/gem. Next, you need to add the following to one or more of your environment configuration files (either environment.rb or one or more files in environments/*.rb) for your Rails system:

require 'syslog_logger'
RAILS_DEFAULT_LOGGER = SyslogLogger.new

This, of course, loads the syslog_logger gem and sets the default logger to a new instance of SyslogLogger.

Now that you have told Rails to use syslog, you must tell syslog what to do with the files that come from Rails. I opened /etc/syslog.conf and added the following lines to the bottom:

*.info             /var/log/production.log

And yes, the documentation system says that you can use a !rails tag before this line, or one like it, to restrict logging to messages coming from Rails. Unfortunately, this syntax does not appear to be supported by Linux. So, this means production.log will include messages from other programs and facilities, not just Rails. That shouldn't concern us right now, although it might be an issue on a busy machine with many services in active use.

Once you have modified syslog.conf in this way, you can restart syslog.conf. Almost immediately, your production log should be stored to /var/log/production.log. You can check this, of course, with the following:

tail -f /var/log/production.log

Now, this logfile is similar in many ways to the logfile you just eliminated from the log directory in your application root. However, it is formatted in such a way that the production log analyzer will be able to find and perform calculations based on its output. To analyze the logfile, type:

pl_analyze /var/log/production_log

If you prefer to have the results sent to you via e-mail, rather than stored to a disk file, use the -e option:

pl_analyze /var/log/production_log -e [email protected]

This option is particularly useful when you invoke pl_analyze from a cron job, for example.

The output file from pl_analyze is divided into three parts:

  • Time spent in each request.

  • Time spent in the database for each request.

  • Time spent rendering the output from each request.

For each controller action, pl_request lists how many times it was invoked, as well as the average time it took to execute. It also gives the min, max and standard deviation, allowing you to see how much the execution time varied over time.

Thus, the production log analyzer shows which actions take the greatest amount of time overall, which take the greatest amount of time in the database (or to render) and how many times each was invoked.

I have found pl_analyzer to be an indispensable tool when trying to determine whether a site is fast enough and where I should focus my attention to improve its speed.

Request Profiler

The production log profiler shows which actions require attention, but it doesn't tell why a particular action might be giving you trouble. For that, you need to dive into the application a bit more, profiling not a set of actions, but one particular action.

This is possible thanks to a built-in script that comes with Rails, script/performance/request. This script follows a set of instructions written in a (presumably short) Ruby program, using a similar set of commands and subroutines that are available for integration tests.

In other words, you use integration-test syntax to describe a short sequence of one or more actions and run this program via the request profiler. Then, the request profiler produces two output files that describe what was going on behind the scenes as those requests were serviced. This information can help you improve the performance of this particular action.

In order for this script to work, first install the ruby-prof gem:

gem install --remote ruby-prof

Once that is installed, you need to create a simple integration test script. This script doesn't need to be wrapped in the same object that the integration tests themselves use. Instead, simply create a file named test.rb, and put it somewhere on the filesystem. I created a directory named test/performance and put it there, with the one-line contents as follows:

get('/')

Notice that I'm using URLs here, rather than names of controllers and actions. Finally, with this in place, invoke the profiler:

script/performance/request -n 10 test/performance/test.rb

Now you should see the program telling you that it's warming up and then reporting as it goes through each of the iterations you specified. In the above example, the -n 10 option indicates the number of times the script should be invoked; by default, it's 100.

Note that the output files are put in the test directory (to which you might not have write access by default). And, indeed, the output files are quite useful, but they can be confusing the first time you look at them.

The first output file, profile-output.txt, is (as the suffix implies) a text file that shows how much time was spent in each method, both as a time measure and as a percentage of the total run time. Consider the following:

%self     total     self     wait    child    calls  name
13.74     58.35    38.13     0.00    20.22   608720  Buffer#read

This means there were 608,720 calls to Buffer#read during the test, which took a total of 38.13 seconds, or 13.75% of the execution time. Because this is a built-in method, you can't optimize it. However, you can try to reduce the number of times it is called, so that it will take even less time.

The question is, how do we know which functions are calling Buffer#read? Perhaps reading from buffers is an inevitable part of a Web application, and we just need to realize that?

If you look at the second file, profile-graph.html, you see a nicely linked description of which methods called which other methods, and how long it took. Each box represents the analysis of one method, and the method being analyzed is printed in bold.

All of the methods above the boldfaced method name are parent methods (that is, methods that called the one in question); whereas, methods below the current one are child methods (that is, methods that are called by the method being analyzed). By looking at who called Buffer#read, you can see which methods (if any) need optimizing or a smaller number of invocations. By going back and forth between methods, their parents and the source code, you can cut down on a great deal of waste, making your sites more efficient than before.

Conclusion

This month, we looked at two basic profiling tools that programmers can use to identify performance problems in Rails-based Web sites. There are, of course, other tools we can use, but the fact that these are so nicely integrated into Rails makes us all the more likely to use them. With constant monitoring and tweaking, we can make our sites run faster without having to resort to buying additional servers.

Resources

The Rails Way, by Obie Fernandez, has become my favorite, because it includes so much useful information, as well as code examples. It doesn't try to teach you Rails, but it does provide a great deal of information that is useful for advanced users as well as newcomers.

Advanced Rails, by Brad Ediger, gives some greater depth to several topics, such as performance optimization, ActiveRecord features, RESTful sites and internationalization, among others.

Rails Analyzer Tools: this is a collection of tools that can help you better understand your Rails-based site. The production log profiler is part of the Rails Analysis Tools set; see rails-analyzer.rubyforge.org for more information.

Reuven M. Lerner, a longtime Web/database developer and consultant, is a PhD candidate in learning sciences at Northwestern University, studying on-line learning communities. He recently returned (with his wife and three children) to their home in Modi'in, Israel, after four years in the Chicago area.

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