#116 Takashi Kokubun on ZJIT, Ruby Performance, and the Path from Japan to Shopify

Takashi Kokubun is a Staff Developer on the Ruby JIT team at Shopify, where he works on ZJIT, the next-generation Just-In-Time compiler for Ruby that shipped with Ruby 4.0. Before going deep on compilers full time, he worked on distributed systems and infrastructure, and along the way earned a Master of Science in Computer Science from Georgia Tech while working full time in the United States.His path into compiler engineering started with Haml, a Ruby template engine. Optimizing rendering performance taught him how template engines think like compilers: parse input, transform intermediate representations, generate optimized output. That hands-on experience gave him a foundation for understanding Ruby internals and eventually led to full-time work on YJIT and then ZJIT at Shopify.In this episode, Takashi explains JIT compilation without jargon: why interpreters are slow, how native machine code helps, and what the real tradeoffs are around warmup, memory, caching, and deployment. He also talks about how Shopify's Ruby infrastructure team works, what changed architecturally between YJIT and ZJIT, and what it means for a team to contribute to open source at this scale.The conversation also covers his move from Japan to the US, what drew him to Silicon Valley, and what he learned from earning a graduate degree while working full time. If you work in Ruby, care about language performance, or are just curious how a developer goes from hobbyist to contributor to world-class compiler engineer, this one is for you.About Takashi Kokubun 🔧⚡🐦Staff Developer, Ruby JIT Team at Shopify | ZJIT and YJIT Contributor- 🎤 ZJIT talk at SF Ruby: https://www.youtube.com/watch?v=XSdBCKepWHM- 📝 ZJIT launch post: https://railsatscale.com/2025-12-24-launch-zjit/About Federico Ramallo ✨👨‍💻🌎🚀 Software Engineering Manager | 🛠 Founder of DensityLabs.io & PreVetted.ai | 🤝 Connecting 🇺🇸 U.S. teams with top nearshore 🌎 LATAM engineers- 💼 https://www.linkedin.com/in/framallo/- 🌐 https://densitylabs.io- ✅ https://prevetted.ai🎙 PreVetted Podcast 🎧📡- 🎯 https://prevetted.ai/podcast- 🐦 https://x.com/PrevettedPod- 🔗 https://www.linkedin.com/company/prevetted-podcast00:00 Introduction to Takashi Kokubun and His Work03:35 Journey into Software Engineering11:21 The Evolution of Compiler Interest20:23 Optimizing Ruby with JIT Compilers25:22 Optimizing Compilers: Parallels and Lessons Learned27:49 Caching Strategies: When to Optimize vs. Cache29:48 Development vs. Production: Managing Trade-offs34:29 ZGIT vs. YJIT: Architectural Improvements40:28 Cultural Insights: Moving from Japan to the US