The Call of Ctooling: The Secrets Behind Native Image Building

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
2. About Me • Edoardo Vacchi @evacchi • Research @ UniMi / MaTe • Research @ UniCredit R&D • Kogito / Drools / jBPM @ Red Hat
3. Kogito Cloud-Native Business Automation for building intelligent applications, backed by battle-tested capabilities http:/ /kogito.kie.org
4. Native Java
5. 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)
6. A Bit of History
7. Native Java Compilers • Compilation into machine code is not innovative per se • Prior art: native java compilers early 2000s. • GCJ (source code) • ExcelsiorJET (bytecode) • ... • More Recently: RoboVM (~2013)
8. 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
9. 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
10. 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...
11. Today
12. 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?
13. 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
14. SmallTalk VMs
15. 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
16. Checkpointing
17. CRIU + Java Build Time Run Time static void Main Framework Initialization Application Initialization Checkpoint • CRIU: Checkpoint and Restore in Userspace • https:/ /www.criu.org • CRaC: Coordinated Restore at Checkpoint • https:/ /github.com/CRaC/docs#crac • Jigawatts: • https:/ /github.com/chflood/jigawatts • OpenJ9 Snapshot+Restore • https:/ /danheidinga.github.io/Everyone_wants_fast_startup
18. GraalVM
19. 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
20. 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 are not lazy* ! • 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
21. Image Heap Generation
22. Image-Gen Heap That is another embarrassing pun
23. • 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)
24. Static initializers
25. Static initializers public class Example { static { System.out.println("hello"); } public static void main(String... args) { System.out.println("world"); } }
26. Static initializers $ java Example hello world
27. 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
28. 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
29. Static initializers $ ./example world
30. Static initializers public class Example { static { System.out.println("hello"); } public static void main(String... args) { System.out.println("world"); } }
31. Static initializers public class Example { static { System.out.println("hello"); } public static void main(String... args) { System.out.println("world"); } } A string constant
32. 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
33. 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
34. 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
35. Initialization Code First, class initializers are executed. • In Java, every class can have a class initializer ("static initializer") • represented as a method named <clinit> in the class file. • It computes the initial value of static fields. • The developer decides which classes are initialized at image build time
36. 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
37. 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
38. 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
39. Points-To Analysis (Example) • System.out.println("hello") • java.lang.String • System.out • java.io.PrintStream • java.io.FilterOutputStream • java.io.OutputStream • System
40. Ahead-of-Time Compilation • methods marked as reachable by the points-to analysis • placed in the text section of the executable.
41. 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
42. https:/ /twitter.com/reibitto/status/1384795560436113415 Perils of Static Initialization
44. Project Leyden
45. 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
46. 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
47. 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
48. Qbicc: Static Initialization • Ongoing discussion topic • Defining feature • Different approaches being evaluated • Avoid pitfalls
49. mmap + offset Qbicc Build-time serialization + Fast deserialization routines initially static first + opt-out now runtime first + opt-in Qbicc currently all run-time eventually as close to “all build-time” as possible investigating explicit opt-in (code hints? annotations? language changes?) Qbicc: Static Initialization Trade-Offs
50. Further Reading
51. References Andrew Dinn (2021) Leyden: Lessons from Graal Native C. Wimmer et al. (OOPSLA 2019) Initialize Once, Start Fast: Application Initialization at Build Time Dan Heidinga (QCon Plus 2021) Starting Fast Duke Art at OpenJDK Wiki
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