Enabling YJIT

Introduction

I just did this today. So, this post will not include the results of enabling YJIT.

As finance folks say

This is not investment programming advice.

Why

In one acronym, FOMO.

A while back I read this PR, Enable YJIT by default if running Ruby 3.3+. It made it look like a low hanging fruit. I thought, why not?

The why yes list included:

• This Shopify post
• This Rails at Scale (also Shopify) post
• This 37 signals post

How

On the server(s)

My Ruby on Rails (Ruby 3.3.0 -via rbenv-) app was running on an Ubuntu server. So, what I did was:

First, I checked if my Ruby had YJIT. I did not remember if I had installed it with it 🤷‍♂️.

ruby --enable-yjit -v

I didn’t. As a matter of fact, I did not have Rust installed either. So, I did that first.

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

And then I added Rust to my path.

nano ~/.bashrc

export PATH="$HOME/.cargo/bin:$PATH"

source ~/.bashrc

rustc --version

Finally, I installed Ruby with YJIT.

RUBY_CONFIGURE_OPTS="--enable-yjit" rbenv install 3.3.0

On the app

I just added the initializer to enable YJIT (taken from the PR mentioned above ☝️).

# config/initializers/enable_yjit.rb

if defined? RubyVM::YJIT.enable
  Rails.application.config.after_initialize do
    RubyVM::YJIT.enable
  end
end

Conclusion

Everything went so smooth that it feels like the fruit fell into my hands.

I did not mentioned it because it’s kind of a given, but having a good test coverage provides a lot of confidence when trying out this things. And, of course, I tried everything on my local machine first and on my staging server later.

The only regret so far is not having done this 30 minutes later. I finished the whole thing just minutes before Ruby 3.3.1 was released 🤦‍♂️.