Try Out YJIT for Faster Rubying

Here at Shopify, we’re building a new just-in-time (JIT) implementation on top of CRuby. Maxime talked about it at RubyKaigi and wrote a piece for our Engineering Blog. If you keep careful watch, you may have even seen our page of benchmark results.

YJIT is a Just-In-Time Compiler, so it works by converting your most frequently used code into optimized native machine code. Early results are good—for instance, we’re speeding up a simple hello, world Rails benchmark by 20% or more. Even better, YJIT is pretty reliable: it runs without errors on Shopify’s full suite of unit tests for its main Rails application, and similar tests at GitHub. Matz, Ruby’s chief designer, mentioned us in his EuRuKo keynote. By the time you read this, YJIT should be merged into CRuby in time for Ruby 3.1 release at Christmas.

Maybe you’d like to take YJIT out for a spin. Or maybe you’re just curious about how to compile CRuby (a.k.a. plain Ruby or Matz’s Ruby Interpreter.) Or maybe you just like reading technical blog posts with code listings in them. Reading blog posts feels weirdly productive while you’re waiting for a big test run to finish, doesn’t it? Even if it’s not 100% on-topic for your job.

YJIT is available in the latest CRuby as 3.1.0-dev if you use ruby-build . But let’s say that you don’t or you want to configure YJIT with debugging options, statistics, or other customizations.

Since YJIT is now part of CRuby, it builds and installs the same way that CRuby does. So I’ll tell you how you build CRuby and then you’ll know all about building YJIT too. We’ll also talk about:

  • How mature YJIT is or isn’t 
  • How stable is it 
  • Is it ready for you to use at work 
  • Will this speed turn into real-world benefits, or is it all benchmarks.

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Start Your Engines

If you’re going to build Ruby from source, you’ll need a few things. Autoconf, make, OpenSSL, and GCC or Clang. If you’re on a Mac, XCode and Homebrew will get you these things. I won’t go into full details here, but Google can help out.

brew install autoconf@2.69 openssl@1.1 # Unless you already have them

On Linux, you’ll need a package similar to Debian build-essential plus autoconf and libssl-dev. Again, Google can help you here if you include the name of your Linux distribution. These are all common packages. Note that installing only Ruby from a package isn’t enough to build a new Ruby. You’re going to need Autoconf and a development package for OpenSSL. These are things a pre-built Ruby package doesn’t have.

Now that you have the prerequisites installed, you clone the repo and build:

And that will build you a local YJIT-enabled Ruby. If you’re using chruby, you can now log out, log back in, and switch to it with chruby ruby-yjit. rbenv is similar. With rvm you’ll need to mount it explicitly with a command like rvm mount ~/.rubies/ruby-yjit/bin/ruby -n ruby-yjit and then switch to it with rvm use ext-ruby-yjit.

Note: on Mac, we’ve had a report of Autoconf 2.71 not working with Ruby. So you may need to install version 2.69, as shown above. And for Ruby in general you’ll want OpenSSL 1.1 - Ruby doesn’t work with version 3, which Homebrew installs by default.

How Do I Know if YJIT Is Installed?

Okay… So YJIT runs the same Ruby code, but faster. How do I know I even installed it?

First, and simplest, you can ask Ruby. Just run ruby --enable-yjit -v. You should see a message underneath that YJIT is enabled. If you get a warning that enable-yjit isn’t a thing, you’re probably using a different Ruby than you think. Check that you’ve switched to it with your Ruby version manager.

This message means this Ruby has no YJIT.

You can also pop into irb and see if the YJIT module exists:

You may want to export RUBYOPT=’--enable-yjit’ for this, or export RUBY_YJIT_ENABLE=1 which also enables YJIT. YJIT isn’t on by default, so you’ll need to enable it.

Running YJIT

After you’ve confirmed YJIT is installed, run it on some code. We found it runs fine on our full unit test suites, and a friendly GitHubber verified that it runs without error on theirs. So it’ll probably handle yours without a problem. If you pop into your project of choice and run rake test with YJIT and without, you can compare the times.

If you can’t think of any code to run it on, YJIT has a benchmark suite we like. You could totally use it for that. If you do, you can run things like ruby -Iharness benchmarks/activerecord/benchmark.rb and compare the times. Those are the same benchmarks we use for . You may want to read YJIT’s documentation while you’re there. There are some command-line parameters and build-time configurations that do useful and fun things.

Is YJIT Ready for Production?

Benchmarks are fine, but YJIT doesn’t always deliver the same real-world speedups. We’ve had some luck on benchmarks and minor speedups with production code, but we’re still very much in-progress. So where is YJIT actually at?

First, we’ve had good luck running it on our unit tests, our production-app benchmarking code, and one real web app here at Shopify. We get a little bit of speedup, in the neighbourhood of 6%. That can add up when you multiply by the number of servers Shopify runs… But we aren’t doing it everywhere, just on a small percentage of traffic for a real web service, basically a canary deployment.

Unit tests, benchmarks and a little real traffic is a good start. We’re hoping that early adopters and being included in Ruby 3.1 will give us a lot more real world usage data. If you try YJIT, we’d love to hear from you. File a GitHub issue, good or bad, and let us know!

Hey, YJIT Crashed! Who Do I Talk to?

Awesome! I’ve been fuzz-testing YJIT with AFL for days and trying to crash it. If you could file an issue and tell us as much as possible about how it broke, we’d really appreciate that. Similarly, anything you can tell us about how fast it is or isn’t is much appreciated. This is still early days.

And if YJIT is really slow or gives weird error messages, that’s another great reason to file an issue. If you run your code with YJIT, we’d love to hear what breaks. We’d also love to hear if it speeds you up! You can file an issue and, even if it’s good not bad, I promise we’ll figure out what to do with it.

What if I Want More?

Running faster is okay. But maybe you find runtime_stats up above intriguing. If you compile YJIT with CFLAGS=’-DRUBY_DEBUG=1’ or CFLAGS=’-DYJIT_STATS=1’ you can get a lot more detail about what it’s doing. Make sure to run it with YJIT_STATS set as an environment variable or yjit-stats on the command line. And then YJIT.runtime_stats will have hundreds of entries, not just two.

When you run with statistics enabled, you’ll also get a report printed when Ruby exits showing interesting things like:

  • what percentage of instructions were run by YJIT instead of the regular interpreter 
  • how big the generated code was 
  • how many ISEQs (methods, very roughly) were compiled by YJIT.

The beginning of a YJIT statistics exit report for a trivial one-line print command.

What Can I Do Next?

Right now, it’s really helpful just to have somebody using YJIT at all. If you try it out and let us know what you find, that’s huge! Other than that, just keep watching. Check the benchmarks results now and then. Maybe talk about YJIT a little online to your friends. As a famous copyrighted movie character said, "The new needs friends." We’ll all keep trying to be friendly.

Noah Gibbs wrote the ebook Rebuilding Rails and then a lot about how fast Ruby is at various tasks. Despite being a grumpy old programmer in Inverness, Scotland, Noah believes that some day, somehow, there will be a second game as good as Stuart Smith’s Adventure Construction Set for the Apple IIe. Follow Noah on Twitter and GitHub.

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