Gaurav Varma

Ruby 3.4 introduces several improvements to YJIT, Ruby’s just-in-time compiler designed to improve execution speed while maintaining Ruby’s developer-friendly experience.

YJIT was first introduced to bring better runtime performance to Ruby without requiring major changes to application code. With Ruby 3.4, the YJIT engine becomes faster, more memory-efficient, and more stable.

These improvements continue Ruby’s long-term effort to make the language competitive with other high-performance runtimes while preserving its simplicity. Ruby 4.0 continues this work with the new ZJIT compiler and further Ractor enhancements.

What is YJIT?

YJIT is a Just-In-Time (JIT) compiler designed specifically for Ruby.

Instead of interpreting Ruby code line-by-line, YJIT compiles frequently executed code paths into optimized machine code.

This reduces execution overhead and improves performance for many real-world workloads.

Unlike earlier Ruby JIT experiments, YJIT focuses on:

low memory overhead
fast startup
compatibility with existing Ruby code

Improvements in Ruby 3.4

Ruby 3.4 introduces several enhancements to YJIT.

Key improvements include:

reduced memory usage
faster code generation
improved instruction caching
better stability across workloads

These changes allow Ruby applications to benefit from performance gains without significantly increasing memory usage.

Enabling YJIT

YJIT can be enabled when starting Ruby:

1ruby --yjit app.rb

Rails applications can also enable YJIT in production environments.

Example:

1RUBYOPT="--yjit" bundle exec rails server

Once enabled, Ruby automatically compiles frequently executed code paths.

Example performance benefits

Applications that perform many repeated operations often benefit the most.

For example:

1100_000.times do
2  [1,2,3].map { |n| n * 2 }
3end

Loops, method calls, and object allocations can see noticeable improvements with YJIT enabled.

Real-world impact

YJIT improvements benefit many Ruby workloads, including:

Rails applications
background job processing
API servers
data processing scripts

Because YJIT works automatically, most developers can benefit simply by enabling it.

Summary

Ruby 3.4 continues improving runtime performance with enhancements to YJIT. By reducing memory usage and improving execution speed, Ruby applications can run faster while keeping the same simple development experience.

Ruby 3.4 optimizes YJIT for better performance (memory and speed improvements)