Deoptimization: How YJIT Speeds Up Ruby by Slowing Down

In this presentation, Takashi Kokubun discusses how YJIT (Yet Another Just-In-Time compiler) enhances Ruby's performance by employing counterintuitive strategies that sometimes involve slowing down the code execution to optimize overall performance. Kokubun explains the evolution of Ruby's performance optimization, emphasizing the significance of deoptimization as a critical technique.

Key Points Discussed:

• Background and Role: Kokubun, a member of the YJIT team at Shopify, collaborated on enhancing Ruby's infrastructure, including introducing the YJIT and working on the upcoming ZJIT compiler.

• YJIT Overview: YJIT converts Ruby's virtual machine instructions into machine code that can be executed directly, leading to substantial performance improvements. For example, benchmarks indicate that YJIT can make Rails applications twice as fast compared to the traditional interpreter.

• Performance Metrics: In real-world scenarios, such as high-traffic storefronts, YJIT achieves an average speedup of 18% with peaks reaching 33% improvement.

• Deoptimization Explained: The concept of deoptimization is key to YJIT's functionality. This allows the compiler to slow down execution deliberately to handle Ruby's dynamic features more effectively. The idea is to discard optimizations when Ruby's dynamic nature interferes, enabling higher overall throughput.

• Practical Examples: Kokubun shares practical coding examples showcasing how YJIT handles method redefinitions, constant updates, and trace points that can disrupt performance. For instance, he discusses how using line trace points can invalidate optimizations, leading to performance degradation.

• New Deoptimizations in Ruby 34: The talk introduces new deoptimization strategies introduced in Ruby 34, such as escape locals for variables and lazy frame pushes. This involves optimizations that dynamically adjust based on runtime context, significantly enhancing performance while maintaining Ruby's dynamic nature.

• C Trace Attribute: Kokubun highlights the introduction of a C trace attribute in Ruby to ensure smoother transitions between C and Ruby implementations, reducing the likelihood of discrepancies during method execution.

Conclusion:

• The overarching takeaway is that through strategic optimizations and the intelligent use of deoptimization, YJIT manages to enhance Ruby's performance without compromising its core dynamic functionality. The advancements introduced in Ruby 34, especially regarding local variables and method calls, signify a progressive step towards a faster Ruby.

Overall, this presentation underscores the innovative approach Ruby is taking to improve speed and efficiency, with YJIT as a pivotal contributor to these enhancements.