> ## Documentation Index
> Fetch the complete documentation index at: https://tally.wharflab.com/llms.txt
> Use this file to discover all available pages before exploring further.

# tally/ruby/yjit-not-enabled-on-supported-runtime

> Ruby 3.3+ runtime image doesn't enable YJIT, missing a near-free 15-30% CPU win.

The final Ruby 3.3+ runtime stage doesn't enable YJIT — Ruby's bundled JIT compiler.

| Property | Value                 |
| -------- | --------------------- |
| Severity | Info                  |
| Category | Performance           |
| Default  | Enabled               |
| Auto-fix | Yes (`FixSuggestion`) |

## Description

YJIT is Ruby's bundled JIT compiler. As of Ruby 3.3, it's production-ready and delivers a 15–30% CPU win on
most Rails workloads at near-zero cost. But it's opt-in: you have to set `RUBY_YJIT_ENABLE=1`, pass `--yjit`
to the Ruby/Rails binary, or include `--yjit` in `RUBYOPT`.

The corpus shows only **3 of 196** Rails Dockerfiles enable YJIT. For projects on Ruby 3.3+ this is a
near-free performance gain that's missing from almost every production deployment.

This rule fires when:

* The final stage's effective Ruby version is 3.3 or later (resolved from the base image tag, or from
  `.ruby-version` / `.tool-versions` / `Gemfile.lock`'s `RUBY VERSION` block when the base is ARG-templated).
* The stage looks like a long-running Ruby server (ENTRYPOINT/CMD references `rails`, `puma`, `unicorn`,
  `thrust`, `rackup`, `sidekiq`, `falcon`, `thin`, `passenger`, or `iodine`).
* None of these YJIT signals is present:
  * `ENV RUBY_YJIT_ENABLE=...` (any truthy value).
  * `ENV RUBYOPT="--yjit"` (or contains `--yjit`).
  * ENTRYPOINT/CMD passes `--yjit` to the binary.

### Suppressions

The rule skips:

* Ruby `< 3.3` — YJIT existed but was experimental and had Rails-specific regressions.
* Non-final stages (only the production runtime image matters for YJIT).
* CLI-only Ruby images (no rails/puma/sidekiq/etc. ENTRYPOINT) — JIT warmup dominates short-lived processes.
* Stages explicitly named `dev`, `development`, `test`, `testing`, `ci`, or `debug`.
* Non-Ruby and Windows stages.

## Examples

### Before

```dockerfile theme={null}
FROM ruby:3.3-slim
COPY . .
CMD ["bin/rails", "server"]
```

### After

```dockerfile theme={null}
FROM ruby:3.3-slim
ENV RUBY_YJIT_ENABLE="1"
COPY . .
CMD ["bin/rails", "server"]
```

Equivalent forms also satisfy the rule:

```dockerfile theme={null}
ENV RUBYOPT="--yjit"
```

```dockerfile theme={null}
CMD ["bin/rails", "server", "--yjit"]
```

## Auto-fix

`FixSuggestion`. Inserts `ENV RUBY_YJIT_ENABLE="1"` on the line immediately after the final stage's `FROM`.
The fix is suggestion-level because performance changes — even good ones — are best applied with the user's
explicit consent.

## References

* [Ruby 3.3 release notes](https://www.ruby-lang.org/en/news/2023/12/25/ruby-3-3-0-released/) —
  declares YJIT production-ready.
* [YJIT documentation](https://github.com/ruby/ruby/blob/master/doc/yjit/yjit.md) — opt-in environment
  variable and CLI flag.
* [Mastodon Dockerfile](https://github.com/mastodon/mastodon/blob/main/Dockerfile) — production-grade
  example that enables YJIT.