The Impact of JIT and YJIT in Ruby: Boosting Performance

Ruby is renowned for its elegant syntax and developer productivity, but it has historically lagged in raw performance compared to languages like Java or C++. Enter Just-In-Time (JIT) compilation, a technique that dynamically compiles code at runtime to optimize execution. In Ruby, this has evolved with implementations like MJIT (Method JIT) and the more recent YJIT (Yet Another JIT), developed by Shopify and integrated into CRuby starting from Ruby 3.1.

What is JIT and YJIT?

• JIT Compilation: Traditionally, Ruby interprets code line-by-line, which is flexible but slow for repeated executions. JIT analyzes and compiles hot code paths (frequently run sections) into machine code, reducing interpretation overhead.

• YJIT: A lightweight, lazy JIT compiler that focuses on simplicity and speed. Unlike older MJIT, YJIT uses a basic block versioning approach, making it faster to start up and more efficient for web apps. As of Ruby 3.3 (and beyond in 2025), YJIT is maturing with features like better inlining and register allocation.

Positive Impacts

The biggest win? Performance gains without sacrificing Ruby's dynamism. YJIT can deliver 20-40% speedups in real-world scenarios, especially for CPU-bound tasks. This means:

• Faster web applications (e.g., Rails servers handle more requests per second).

• Lower resource usage, reducing cloud costs.

• Better scalability for long-running processes like background jobs or APIs.

These improvements make Ruby more competitive for production environments, attracting teams that prioritize both speed and ease of use.

A Simple Example

Let's look at a benchmark using a Fibonacci sequence, a classic CPU-intensive task. Without JIT:

def fib(n)
  return n if n <= 1
  fib(n-1) + fib(n-2)
end
start = Time.now
fib(40)
puts "Time: #{Time.now - start} seconds"

On Ruby 3.4 (interpreted): ~25 seconds.

With YJIT enabled (ruby --yjit script.rb): ~8 seconds—a 3x speedup!

In a Rails app, YJIT has shown 30% faster response times in benchmarks like Optcarrot or Railsbench, translating to snappier user experiences and higher throughput.

In summary, JIT and YJIT are game-changers for Ruby, bridging the gap between productivity and performance. If you're running Ruby apps, enable YJIT today—it's a low-effort upgrade with high rewards!