The Call of C-Tooling: The Secrets Behind Native Image Building (JBCNConf 2022)

(JBCNConf 2022 Version)

You have learned about the "Closed World Assumption". You live by the rule "Thou Shall Sparingly Use Reflection". You know that "From The Powerful defineClass Comes Great Responsibility". And yet you were still left to wonder: what is it still eluding me? What is the secret ingredient that I am still missing? Join us for a short, but deeper dive into the dark magic behind GraalVM's native image builder: heap snapshotting and build-time initialization. And learn more about other obscure projects investigating the craft of static Java compilation.

Transcript

1. The Call of C-Tooling
   The Secrets Behind Native Image Building
   

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2. @evacchi
   About Me
   • Edoardo Vacchi @evacchi
   • Research @ UniMi / MaTe
   • Research @ UniCredit R&D
   • Kogito / Drools / jBPM @ Red Hat
   • evacchi.github.io
   

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3. @evacchi
   Kogito
   

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4. Native Java
   

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5. @evacchi
   Java Applications
   Build Time Run Time
   3 Classloaders
   ~500 Classes
   ~160 Static Init
   100+ Classloaders
   1000+ Classes
   1000+ Static Init
   100++ Classloaders
   1000++ Classes
   1000++ Static Init
   static void Main
   Framework
   Initialization
   Application
   Initialization
   Source: Dan Heidinga - “Starting Fast” (QCon Plus 2021)
   

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6. A Bit of History
   

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7. @evacchi
   Native Java Compilers
   • Compilation into machine code is not innovative per se
   • Prior art: native java compilers early 2000s.
   • GNU Compiler for Java (GCJ)
   • ExcelsiorJET
   • ...
   • More Recently: RoboVM (~2013)
   

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8. @evacchi
   Pros
   • Native code, possibly faster to start-up
   • Smaller memory footprint
   • by avoiding JIT+scratch memory in address space
   • possibly aggressive dead code elimination
   • Self-contained
   • avoid full JDK class library bundle
   

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9. @evacchi
   Cons
   Limitations
   • Not a JDK: different runtime environment, not cross-platform
   • May get out-of-sync with the spec
   • Trade-offs with dynamicity
   • Difference in run-time behavior (dynamic vs static)
   • Possibly need compromises with peak-performance (PGO ?)
   Moreover
   • The benefits of a native compilation are not compelling enough
   • Startup time is negligible
   • "You boot up your application once, you keep it running for a long time"
   • "Disk is cheap"
   • Dynamic Linking vs Static Linking
   • You can still achieve faster startup time through laziness
   

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10. @evacchi
    Laziness
    • Defer initialization to a later stage of execution,
    • Benefits: Shorter Startup Time
    • Downsides: Less predictable performance profile.
    Build Time
    Run Time
    static void Main
    Framework
    Initialization
    Application
    Initialization
    Delayed Inits...
    

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11. Today
    

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12. @evacchi
    Getting Closer to Today
    • Shared Managed Infrastructure
    • Serverless
    • More interest in “Stateless” Apps
    • Suddenly attractive:
    • Fast Startup
    • Smaller Disk Footprint
    • Smaller Memory Footprint
    • Time to revisit?
    

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13. @evacchi
    Run-Time vs Build-Time
    • Generate code at build-time
    • Pre-initialize for boot time
    • e.g. Read config files, turn them into configuration commands
    • e.g. Read annotations, produce code for dependency injection
    • At startup, just execute that code
    • Benefits: faster startup time
    • Downsides
    • you have to write the code that generates code
    • possibly non-trivial, certainly time-consuming
    Build Time
    Run Time
    static void Main
    Framework
    Initialization
    Application
    Initialization
    Codegen
    

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15. SmallTalk VMs
    

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16. @evacchi
    Smalltalk Environment
    • Concept of image
    • At run-time you do not just write code,
    you manipulate the state of such machine
    • contributing to the environment itself
    • possibly altering it or even turning it upside-down
    • When it is shut down, you do not just save the code
    you wrote
    you persist the state of machine to the image
    • When you start it you do not only run a program
    the state is restored, and execution resumes from the
    last saved state
    Run Time
    Load State
    Shutdown
    Save State
    

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17. Checkpointing
    

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18. @evacchi
    CRIU + Java Build Time
    Run Time
    static void Main
    Framework
    Initialization
    Application
    Initialization
    Checkpoint
    • CRIU: Checkpoint and Restore in Userspace
    • https:/
    /www.criu.org
    • Jigawatts:
    • https:/
    /github.com/chflood/jigawatts
    • OpenJ9 Snapshot+Restore
    • https:/
    /danheidinga.github.io/Everyone_wants_fast_startup
    • CRaC: Coordinated Restore at Checkpoint
    • https:/
    /github.com/CRaC/docs#crac
    • https:/
    /openjdk.java.net/projects/crac/
    

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19. GraalVM
    

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20. @evacchi
    GraalVM
    • GraalVM is an umbrella of technologies
    • A just-in-time compiler
    • The Truffle framework to implement dynamic languages
    • they can be seamlessly JITted across language boundaries.
    • SubstrateVM: the native image builder
    • reuses the compilation backend for Ahead-of-Time compilation
    • static init
    • image heap
    

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21. @evacchi
    Native Image Restrictions
    • Native binary compilation
    • Restriction: “closed-world assumption”
    • Limitations on reflection
    • No dynamic code loading: forbidden
    ClassLoader#defineClass(...byte[]...)
    • Allows more aggressive optimization
    (e.g, dead code elimination)
    • Static initializers may be eager* !
    • Evaluated at build time !
    * originally opt-out, now opt-in. In some cases default on (e.g. Quarkus)
    Build Time
    static void Main
    Framework
    Initialization
    Application
    Initialization
    Run Time
    

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22. Image Heap
    Generation
    

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23. Image-Gen
    Heap
    That is another
    embarrassing
    pun
    

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24. @evacchi
    • We run parts of an application at build time and snapshot the objects
    allocated by this initialization code, using an iterative approach that
    is intertwined with points-to analysis.
    • We use points-to analysis results to only AOT-compile the parts of an
    application that are reachable at run time.
    “
    Source: Initialize Once, Start Fast: Application Initialization at Build Time (Wimmer et al. OOPSLA 2019)
    

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25. @evacchi
    Static initializers
    

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26. @evacchi
    Static initializers
    public class Example {
    static {
    System.out.println("hello");
    }
    public static void main(String... args) {
    System.out.println("world");
    }
    }
    

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27. @evacchi
    Static initializers
    $ java Example
    hello
    world
    

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28. @evacchi
    Static initializers
    $ native-image --initialize-at-build-time Example
    [example:23074] classlist: 1,032.11 ms, 1.18 GB
    [example:23074] (cap): 2,301.26 ms, 1.18 GB
    [example:23074] setup: 3,609.57 ms, 1.69 GB
    hello
    [example:23074] (clinit): 82.45 ms, 1.73 GB
    [example:23074] (typeflow): 3,032.00 ms, 1.73 GB
    [example:23074] (objects): 2,923.76 ms, 1.73 GB
    [example:23074] (features): 129.59 ms, 1.73 GB
    [example:23074] analysis: 6,307.81 ms, 1.73 GB
    [example:23074] universe: 277.17 ms, 1.73 GB
    [example:23074] (parse): 525.88 ms, 1.73 GB
    [example:23074] (inline): 877.57 ms, 1.78 GB
    [example:23074] (compile): 3,842.94 ms, 1.87 GB
    [example:23074] compile: 5,504.45 ms, 1.87 GB
    [example:23074] image: 463.22 ms, 1.87 GB
    [example:23074] write: 176.80 ms, 1.87 GB
    [example:23074] [total]: 17,528.27 ms, 1.87 GB
    

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29. @evacchi
    Static initializers
    $ native-image --initialize-at-build-time Example
    [example:23074] classlist: 1,032.11 ms, 1.18 GB
    [example:23074] (cap): 2,301.26 ms, 1.18 GB
    [example:23074] setup: 3,609.57 ms, 1.69 GB
    hello
    [example:23074] (clinit): 82.45 ms, 1.73 GB
    [example:23074] (typeflow): 3,032.00 ms, 1.73 GB
    [example:23074] (objects): 2,923.76 ms, 1.73 GB
    [example:23074] (features): 129.59 ms, 1.73 GB
    [example:23074] analysis: 6,307.81 ms, 1.73 GB
    [example:23074] universe: 277.17 ms, 1.73 GB
    [example:23074] (parse): 525.88 ms, 1.73 GB
    [example:23074] (inline): 877.57 ms, 1.78 GB
    [example:23074] (compile): 3,842.94 ms, 1.87 GB
    [example:23074] compile: 5,504.45 ms, 1.87 GB
    [example:23074] image: 463.22 ms, 1.87 GB
    [example:23074] write: 176.80 ms, 1.87 GB
    [example:23074] [total]: 17,528.27 ms, 1.87 GB
    

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30. @evacchi
    Static initializers
    $ ./example
    world
    

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31. @evacchi
    Static initializers
    public class Example {
    static {
    System.out.println("hello");
    }
    public static void main(String... args) {
    System.out.println("world");
    }
    }
    

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32. @evacchi
    Static initializers
    public class Example {
    static {
    System.out.println("hello");
    }
    public static void main(String... args) {
    System.out.println("world");
    }
    }
    A string constant
    

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33. @evacchi
    Static initializers
    public class Example {
    static {
    System.out.println("hello");
    }
    public static void main(String... args) {
    System.out.println("world");
    }
    }
    A string constant
    A method invocation
    Over a PrintStream
    

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34. @evacchi
    Static initializers
    public class Example {
    static {
    System.out.println("hello");
    }
    public static void main(String... args) {
    System.out.println("world");
    }
    }
    A string constant
    A method invocation
    A field resolution
    Over a subtype of
    OutputStream
    

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35. @evacchi
    Static initializers
    public class Example {
    static {
    System.out.println("hello");
    }
    public static void main(String... args) {
    System.out.println("world");
    }
    }
    A string constant
    A method invocation
    A field resolution
    A static class initializer
    Over a subtype of
    OutputStream
    

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36. @evacchi
    Initialization Code
    First, class initializers are executed.
    • In Java, every class can have a class initializer ("static initializer")
    • represented as a method named in the class file.
    • It computes the initial value of static fields.
    • The developer decides which classes are initialized at image build time
    

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37. @evacchi
    Heap Snapshotting
    • Builds an object graph i.e., the transitive closure of reachable objects
    • starts with root pointers e.g. static fields.
    • This object graph is written into the native image as the image heap
    

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38. @evacchi
    Heap Snapshotting
    • Builds an object graph i.e., the transitive closure of reachable objects
    • starts with root pointers e.g. static fields.
    • This object graph is written into the native image as the image heap
    

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39. @evacchi
    Points-To Analysis
    • determine which classes, methods, and fields are reachable at run time.
    • starts with all entry points, e.g., the main method of the application,
    • iteratively processes all transitively reachable methods until a fixed point is
    reached
    

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40. @evacchi
    Points-To Analysis (Example)
    • System.out.println("hello")
    • java.lang.String
    • System.out
    • java.io.PrintStream
    • java.io.FilterOutputStream
    • java.io.OutputStream
    • System
    

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41. @evacchi
    Ahead-of-Time Compilation
    • methods marked as reachable by the points-to analysis
    • placed in the text section of the executable.
    

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42. @evacchi
    Image Heap at Run-Time
    • Execution at run-time starts with an already
    pre-populated Java heap
    • Relocatable: references relative to the start of the
    image heap
    • Objects of the image heap and objects allocated at
    run-time
    • i.e., also objects allocated at run time use relative
    references
    • (use of a fixed register r14 on x64 architectures).
    Build Time
    static void Main
    Framework
    Initialization
    Application
    Initialization
    Run Time
    

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43. @evacchi
    https:/
    /twitter.com/reibitto/status/1384795560436113415
    Perils of Static Initialization
    

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44. @evacchi
    

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45. Project Leyden
    

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46. @evacchi
    Project Leyden
    • Goals
    • Address Java’s slow startup time
    • Reduce time to peak performance
    • Reduce memory footprint
    • Introduce static images at spec level (TCK)
    • stand-alone
    • closed-world
    

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47. @evacchi
    Project Leyden
    • Goals
    • Address Java’s slow startup time
    • Reduce time to peak performance
    • Reduce memory footprint
    • Introduce static images at spec level (TCK)
    • stand-alone
    • closed-world “a spectrum of constraints”
    

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48. @evacchi
    Qbicc
    • Experimental sandbox project for Leyden
    • Intended for compiler developers and experts
    • Goal: prototype approaches to native Java
    • New self-contained codebase
    • Allows to experiment with different trade-offs
    • GraalVM’s choices are known,
    • possible to explore different trade-offs of the solution space
    • Currently: Java-based compiler to LLVM IR
    • Future: different backend? (e.g. C2)
    https://github.com/qbicc/qbicc
    https://github.com/qbicc/qbicc/discussions
    https://qbicc.zulipchat.com
    

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49. @evacchi
    Qbicc: Architecture
    • Points-to analysis (static entry points)
    • Flow graph copied between phases, dropping unreachable nodes
    • Approaches to static init being investigated
    ADD ANALYZE LOWER GENERATE
    ● TRANSFORM
    ● CORRECT
    ● OPTIMIZE
    ● INTEGRITY
    ● TRANSFORM
    ● CORRECT
    ● OPTIMIZE
    ● INTEGRITY
    ● TRANSFORM
    ● CORRECT
    ● OPTIMIZE
    ● INTEGRITY
    ● TRANSFORM
    ● CORRECT
    ● OPTIMIZE
    ● INTEGRITY
    

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50. @evacchi
    mmap + offset
    Qbicc Build-time serialization + Fast deserialization routines
    initially static first + opt-out
    now runtime first + opt-in
    Qbicc
    as close to “all build-time” as possible
    investigating explicit opt-in (build-time, run-time, reinit)
    (code hints? annotations? language changes?)
    Qbicc: Static Initialization Trade-Offs
    

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51. Further Resources
    

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52. @evacchi
    References
    David Lloyd (J4K 2021)
    qbicc: Exploring the possibilities of Java native images
    Andrew Dinn (2021)
    Leyden: Lessons from Graal Native
    Static Java, GraalVM Native and OpenJDK
    C. Wimmer et al. (OOPSLA 2019)
    Initialize Once, Start Fast: Application Initialization at Build Time
    Dan Heidinga (QCon Plus 2021)
    Starting Fast and Recent Blog Posts Cover Art by François Baranger
    Duke Art at OpenJDK Wiki
    @evacchi
    

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