Ruby 2.6 JIT - Progress and Future

  • Performance
  • Portability
  • Stability
  • Security


At least for Ruby, JIT is introduced to improve performance, specifically for speed in my understanding. So, if it doesn’t make your application fast, it would be useless for you, right? Given the whole year, I believed I would be able to achieve small but meaningful performance improvement on real-world applications in 2.6. However, I failed to make it.

trunk+JIT: 493.8 i/s
2.6.0-preview1+JIT: 246.4 i/s - 2.00x slower
2.6.0-preview1: 86.8 i/s - 5.69x slower
trunk: 86.2 i/s - 5.73x slower
2.5.0: 80.9 i/s - 6.11x slower
2.0.0: 78.4 i/s - 6.30x slower
Optcarrot Lan_Master.nes
2.6.0-preview3+JIT: 86.6 fps
2.6.0-preview2+JIT: 73.9 fps - 1.17x slower
2.6.0-preview1+JIT: 59.2 fps - 1.46x slower
2.6.0-preview3: 54.6 fps - 1.59x slower
2.6.0-preview2: 53.3 fps - 1.62x slower
2.6.0-preview1: 53.0 fps - 1.63x slower
2.5.3: 48.5 fps - 1.78x slower
2.0.0: 34.6 fps - 2.50x slower


As written in the release note of Ruby 2.6-rc2, it aims to support JIT when it's built with following C compilers on non-EOL platforms.

  • GCC
  • Clang
  • Microsoft Visual C++


We've invested testing environment of MJIT as well. To prevent bugs, we have:


Ruby’s JIT compiler translates Ruby method to C code on /tmp, invokes C compiler with it, and dynamically loads the generated object file. Doesn’t it sound like dangerous?

  1. C compiler is replaced to an arbitrary binary
  2. C header files or libraries are replaced
  3. Another user’s program on the same server overwrites C code to be JIT-ed


While I failed to make Ruby's JIT production-ready in 2.6, I don't regret what we've done this year. I needed to develop or lead all of the above things for Ruby 2.6, but I believe I can invest my time mainly on performance improvement in Ruby 2.7.

  • Inline Ruby methods as further as possible to avoid loading code or creating a VM frame many times
  • Optimize object allocations which we've not invested so much in JIT so far



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