Make Ruby 2.6 Faster with JIT

An introduction to MJIT in Ruby 2.6 and up. What does it do? How do you use it? How well does it work? What can go wrong?

Transcript

1. Make Ruby
   2.6 Faster
   with JIT
   Noah Gibbs, AppFolio
   @codefolio / engineering.appfolio.com
   Pictures are by Arthur Rackham, and are in the Public Domain.
   

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2. Thank You, AppFolio!
   AppFolio pays me to write and
   speak. Thank you, AppFolio! If
   you’re looking for a Ruby job in
   Southern California, talk to us!
   

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3. I Often Talk Too Fast.
   If you call out “hey, slow
   down!”, I’ll try to go slower.
   And it will help your fellow audience members!
   

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4. JIT:
   What Is It?
   “Effortlessly Optimizing Ruby”
   

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5. Compiling
   Ruby interprets your
   code as it runs. C++
   (and others) compile to
   machine code before.
   “Compiling a C++ Program”
   

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6. JIT
   “Just-In-Time” compiling
   translates part of your
   app to fast machine
   code.
   “Running Faster Without Effort”
   

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7. Does JIT Work?
   TruffleRuby JIT gives 10x+
   speedup over interpreted.
   

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8. Does JIT Work? (2)
   Ruby 2.6 experimental
   JIT is still prerelease.
   But it gets 2.5x+ for
   the same benchmark.
   “Ruby JIT Shows Early Promise”
   

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9. Tradeoffs of JIT
   Nothing is perfect. JIT
   has two main
   disadvantages: memory
   usage and warmup.
   “Considering JIT’s Tradeoffs”
   

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10. Memory Usage (1)
    JIT tracks method calls:
    ● How many calls?
    ● How much runtime?
    ● With what argument types?
    

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11. Memory Usage (2)
    JIT keeps 2+ copies of
    a method and switches
    back to un-optimized if
    required.
    “De-optimizing a Method”
    

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12. Warmup
    JITted programs speed up as
    they compile methods. There
    is “warmup time” as new
    programs start out slower.
    

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13. Warmup vs Startup
    If compiling is high
    priority, early program
    speed is very slow. JVM
    is infamous for this.
    “Ruby Dancing with the JVM”
    

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14. Long Programs
    Servers and batch jobs
    benefit from JIT. Small
    quick programs get
    little or nothing.
    “Rake task dodging a JIT penalty”
    

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15. Why Didn’t
    Ruby Get
    JIT Sooner?
    “Ruby 2.5 Mocked for No JIT”
    

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16. JIT and Ruby Core
    JIT’s disadvantages are
    Ruby advantages -
    memory, startup. JIT
    was a tough sell.
    “Core Dev Inspects JIT Attempt”
    

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17. JIT and Ruby Core
    After early JIT
    prototypes looked
    good, there was a
    bake-off. MJIT won.
    “Newborn Hope for Ruby Speed”
    

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18. MJIT:
    Ruby’s JIT
    “MJIT In a Hurry”
    

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19. How Does MJIT Work?
    MJIT uses a C compiler from a
    background job with a
    Ruby-to-C translator.
    

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20. MJIT - the Basics (1)
    

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21. MJIT - the Basics (2)
    

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22. How MJIT Differs (1)
    Each language’s JIT
    picks the CPU priority
    for compiling. MJIT is
    conservative.
    “MJIT’s Compiler at App Start”
    

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23. How MJIT Differs (2)
    MJIT compiles a bit at a
    time. So it has normal
    fast Ruby startup but
    long warmup.
    “JIT’s Usual Troubles”
    

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24. How MJIT Differs (3)
    JVM-style JIT compiles
    in-process. That runs
    faster, but can’t easily
    shed excess memory.
    “Attempting to Free Memory”
    

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25. How MJIT Differs (4)
    MJIT’s background job
    lets it discard memory
    after compile. But it pays
    a compiler speed penalty.
    “Invoking the Compiler”
    

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26. Using JIT in Ruby 2.6
    In Ruby 2.6, you can turn JIT
    on with “--jit”. Try “ruby --help”
    to get args for JIT debugging
    and tuning.
    

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27. Ruby 2.6?
    With rvm, you can install
    “ruby-2.6.0-preview2”.
    Ruby-install is similar.
    

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28. JIT With Rails?
    Takashi’s working on
    it. But for now, it
    slows Rails down.
    “Rails, Broken on JIT”
    

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29. JIT Problems
    What trouble can
    JIT cause?
    “Debugging JIT Deoptimization”
    

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30. Want JIT Off?
    In 2.6, JIT is off by
    default. Later, you can
    disable with
    “--disable-jit”.
    “Ruby JIT, Told to Go Away”
    

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31. Ahead-of-Time
    AoT is compiling fully at
    app start. Can Ruby JIT
    do that?
    Not really, no.
    “Minor Problems in Ruby AoT”
    

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32. Questions?
    “If You Have a Question, Somebody Else Wants to Know Too”
    These slides: https://bit.ly/southeast2018-gibbs
    

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