Just-in-Time Compilation, Ahead-of-Time Compilation

Péter JeszenszkyFaculty of Informatics, University of Debrecen

jeszenszky.peter@inf.unideb.hu

Last modified: March 7, 2021

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Further Information (1)

● Oracle resources:– The Java Virtual Machine Specification. Java SE 15 Edition. September

2020. https://docs.oracle.com/javase/specs/– Java Virtual Machine Guide. Release 15. September 2020.

https://docs.oracle.com/en/java/javase/15/vm/● OpenJDK resources:

– OpenJDK Wiki – HotSpot Internals https://wiki.openjdk.java.net/display/HotSpot/Main

● HotSpot Runtime Overview https://openjdk.java.net/groups/hotspot/docs/RuntimeOverview.html

● Presentations https://wiki.openjdk.java.net/display/HotSpot/Presentations

– HotSpot Glossary of Terms https://openjdk.java.net/groups/hotspot/docs/HotSpotGlossary.html

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Further Information (2)

● Books:– Scott Oaks. Java Performance: In-Depth Advice for

Tuning and Programming Java 8, 11, and Beyond. 2nd ed. O'Reilly, 2020.

● See Chapter 4, titled Working with the JIT Compiler.● Code:

https://github.com/ScottOaks/JavaPerformanceTuning

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HotSpot VM Implementations (1)

● In Oracle JDK/JRE 8 and earlier, different implementations of the HotSpot JVM were provided (client VM, server VM, and minimal VM).– They can be launched by passing the option -client, -server, or -minimal to the java command, respectively.

● Later JDKs and JREs provide only one implementation, namely, the server VM.

● See:– Java SE 8 Documentation – Java Virtual Machine Technology

https://docs.oracle.com/javase/8/docs/technotes/guides/vm/– Java Virtual Machine Guide, Release 15 – Java Virtual Machine Technology

Overview https://docs.oracle.com/en/java/javase/15/vm/java-virtual-machine-technology-overview.html

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HotSpot VM Implementations (2)

● Information about the JVM:– See also the java.vm.name system property.

$ java -versionopenjdk version "15.0.2" 2021-01-19OpenJDK Runtime Environment (build 15.0.2+7-27)OpenJDK 64-Bit Server VM (build 15.0.2+7-27, mixed mode, sharing)

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Launching the JVM

● The java command launches a Java application starting the JVM.– Usage: JDK 15 Tool Specifications – The java

Command https://docs.oracle.com/en/java/javase/15/docs/specs/man/java.html

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Extra and Advanced JVM Options (1)

● These options aren't guaranteed to be supported by all JVM implementations and are subject to change.– Extra options:

● General purpose options that are specific to the HotSpot JVM.● These options start with -X.

– Advanced options:● Developer options used for tuning specific areas of the

HotSpot JVM operation.● These options start with -XX.

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Extra and Advanced JVM Options (2)

● Examples for using extra options:

$ java --help-extra

$ java -XshowSettings -version$ java -XshowSettings:locale -version$ java -XshowSettings:properties -version$ java -XshowSettings:system -version$ java -XshowSettings:vm -version

$ java -Xmx8g -jar MemoryIntensiveApplication.jar

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Extra and Advanced JVM Options (3)

● Boolean advanced options are used to either enable or disable a feature and are specified in the form -XX:+OptionName or -XX:-OptionName.

● Certain advanced VM options are available only if they are enabled with specifying either the-XX:+UnlockDiagnosticVMOptions or the -XX:+UnlockExperimentalVMOptions option.

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Just-in-time (JIT) Compilation (1)

● Translating Java Virtual Machine code at load-time or during execution into the native instruction set of the host CPU.– See: The Java Virtual Machine Specification, Java

SE 15 Edition. 2020. https://docs.oracle.com/javase/specs/

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Just-in-time (JIT) Compilation (2)

● Empirical observation: programs spend the majority of their time executing a small piece of their code.– See: Donald E. Knuth. An empirical study of

FORTRAN programs. Software: Practice and Experience, 1(2):105–133, 1971. https://doi.org/10.1002/spe.4380010203

● These frequently executed parts of the code are called hot spots.

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Just-in-time (JIT) Compilation (3)

● Tradeoff: resource usage vs. performance of generated code

Source: Tobias Hartmann. The Java HotSpot VM – Under the Hood. August 2017.http://cr.openjdk.java.net/~thartmann/talks/2017-Hotspot_Under_The_Hood.pdf

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Just-in-time (JIT) Compilation (4)

● “Rather than compiling method by method, just in time, the Java HotSpot VM immediately runs the program using an interpreter, and analyzes the code as it runs to detect the critical hot spots in the program. Then it focuses the attention of a global native-code optimizer on the hot spots.”

● “By avoiding compilation of infrequently executed code (most of the program), the Java HotSpot compiler can devote more attention to the performance-critical parts of the program, without necessarily increasing the overall compilation time. This hot spot monitoring is continued dynamically as the program runs, so that it literally adapts its performance on the fly to the user's needs.”

● See:– HotSpot Runtime Overview

https://openjdk.java.net/groups/hotspot/docs/RuntimeOverview.html– The Java HotSpot Performance Engine Architecture

https://www.oracle.com/java/technologies/whitepaper.html

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Just-in-time (JIT) Compilation (5)

● The HotSpot JVM contains two JIT compilers:– Client/C1:

● A fast JIT compiler that performs only basic optimizations.● Originally, it was intended to be used for desktop applications.● It is written in C++ and can be found under the src/hotspot/share/c1/ directory of the OpenJDK source tree.

– Server/C2/“opto”:● A slower JIT compiler with high resource demands that performs

aggressive optimizations.● Originally, it was intended to be used for long-running server applications.● It is written in C++ and can be found under the src/hotspot/share/opto/ directory of the OpenJDK source tree.

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Just-in-time (JIT) Compilation (6)

● Tiered compilation combines the C1 and C2 JIT compilers.– It was introduced in Java SE 7.

● See: Java HotSpot Virtual Machine Performance Enhancements https://docs.oracle.com/javase/7/docs/technotes/guides/vm/performance-enhancements-7.html

– It is the default mode for the server VM since Java SE 8.● See: Java HotSpot Virtual Machine Performance

Enhancements https://docs.oracle.com/javase/8/docs/technotes/guides/vm/performance-enhancements-7.html

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Just-in-time (JIT) Compilation (7)

● GraalVM compiler:– A high-performance JIT compiler written in Java that

integrates with the HotSpot JVM.– A component of GraalVM. – It was introduced in JDK 10.

● See: JDK 10 https://openjdk.java.net/projects/jdk/10/– JEP 317: Experimental Java-Based JIT Compiler

https://openjdk.java.net/jeps/317

– It is an experimental feature and is currently supported only on Linux x64 systems.

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Just-in-time (JIT) Compilation (8)

● GraalVM compiler (continued):– It is part of the JDK build and it is delivered as an

internal module, jdk.internal.vm.compiler.

– It communicates with the JVM using the JVM Compiler Interface (JVMCI).

● The JVMCI is also part of the JDK build and it is contained within the internal module: jdk.internal.vm.ci.

– See: Java Virtual Machine Guide, Release 15 – Graal: a Java-Based JIT Compiler https://docs.oracle.com/en/java/javase/15/vm/java-hotspot-virtual-machine-performance-enhancements.html

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Just-in-time (JIT) Compilation (9)

● GraalVM compiler (continued):– Further information:

● https://github.com/oracle/graal/blob/master/compiler/README.md

● Oracle GraalVM Enterprise Edition 21 Guide – GraalVM Compiler https://docs.oracle.com/en/graalvm/enterprise/21/docs/reference-manual/compiler/

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Just-in-time (JIT) Compilation (10)

● Disabling JIT compilation:– The -Xint option of the java command runs the

application in interpreted-only mode.– Compilation to native code is disabled, and all

bytecode is executed by the interpreter.– The performance benefits offered by the JIT

compiler are not available in this mode.

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Just-in-time (JIT) Compilation (11)

● Specifying the JIT compiler to be used:

JIT Compiler VM Option

C1 -client

C2 -serverTiered -XX:+TieredCompilationGraal -XX:+UnlockExperimentalVMOptions

-XX:+EnableJVMCI-XX:+UseJVMCICompiler

- -Xint

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Just-in-time (JIT) Compilation (12)

● Showing the methods that are compiled to native code:– How to interpret the output:

● Stephen Fox. Quick reference for -XX:+PrintCompilation JVM flag. 29 December 2018. https://foxstephen.net/quick-ref-print-compilation

$ java -XX:+PrintCompilation pkg.Main

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Just-in-time (JIT) Compilation (13)

● Showing assembly code generated by the JIT compiler:– Requires the hsdis JVM plugin:

http://hg.openjdk.java.net/jdk/jdk/file/tip/src/utils/hsdis/

● The hsdis-amd64.so/hsdis-amd64.dll file must be copied to the lib/server/ directory of the JDK.

$ java -XX:+UnlockDiagnosticVMOptions \ -XX:+PrintAssembly pkg.Main

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Just-in-time (JIT) Compilation (14)

● Logging of compilation activity:– Output is written to hotspot_pid<pid>.log.

$ java -XX:+UnlockDiagnosticVMOptions \ -XX:+LogCompilation pkg.Main

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Just-in-time (JIT) Compilation (15)

● Tools:– JITWatch (license: Simplified BSD License)

https://github.com/AdoptOpenJDK/jitwatch/● A tool for analyzing and visualizing the compilation

activity of the HotSpot JIT compiler.● Instructions:

https://github.com/AdoptOpenJDK/jitwatch/wiki/Instructions

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Just-in-time (JIT) Compilation (16)

● JIT compilation in other JVMs:– Eclipse OpenJ9:

● The JIT compiler https://www.eclipse.org/openj9/docs/jit/● -Xjit/-Xnojit https://www.eclipse.org/openj9/docs/xjit/

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Ahead-of-Time Compilation (1)

● Ahead-of-Time (AOT) compilation means compiling Java classes to native code prior to launching the virtual machine.

● AOT compilation was introduced in JDK 9.● It is an experimental feature and is currently

supported only on Linux x64 systems.● See:

– JEP 295: Ahead-of-Time Compilation https://openjdk.java.net/jeps/295

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Ahead-of-Time Compilation (2)

● AOT compilation is done by the jaotc command line tool.– See: JDK 15 Tool Specifications – The jaotc

Command https://docs.oracle.com/en/java/javase/15/docs/specs/man/jaotc.html

– The tool produces native code in the form of a shared library for the Java methods in specified Java class files.

● The JVM can load these AOT libraries and use native code from them when corresponding Java methods are called.

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Ahead-of-Time Compilation (3)

● Example:

// Hello.java:public class Hello {

public static String getGreeting(String name) { return String.format("Hello, %s!", name); }

public static void main(String[] args) { System.out.println(getGreeting("World")); }

}

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Ahead-of-Time Compilation (4)

● Example (continued):

$ javac Hello.java$ jaotc --output libHello.so Hello.class$ java -XX:AOTLibrary=./libHello.so HelloHello, World!

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Ahead-of-Time Compilation (5)

● Example (continued):$ java -XX:+PrintAOT -XX:AOTLibrary=./libHello.so Hello 5 1 loaded ./libHello.so aot library[Found [B in ./libHello.so] 32 1 aot[ 1] Hello.<init>()V 32 2 aot[ 1] Hello.main([Ljava/lang/String;)V 32 3 aot[ 1] Hello.getGreeting(Ljava/lang/String;)Ljava/lang/StringHello, World!

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Ahead-of-Time Compilation (6)

● Example (continued):– If class bytecode changes but the corresponding

AOT library is not updated, the native code for that particular class is not used during execution.

# The parameter "Hello" has been changed to "Buddy" in Hello.java$ javac Hello.java# jaotc is not executed$ java -XX:+PrintAOT -XX:AOTLibrary=./libHello.so Hello 4 1 loaded ./libHello.so aot library[Found [B in ./libHello.so]Hello, Buddy!

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Ahead-of-Time Compilation (7)

● Example (continued):– Modules can also be AOT-compiled:

$ jaotc --output libjava.base.so --module java.base$ java -XX:AOTLibrary=./libHello.so,./libjava.base.so HelloHello, World!

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Ahead-of-Time Compilation (8)

● Further recommended reading:– Igor Veresov, Vladimir Kozlov. Ahead of Time

Compilation. 2018. http://cr.openjdk.java.net/~thartmann/offsite_2018/AOT_offsite_2018.pdf

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Monitoring the JVM

● jconsole https://docs.oracle.com/en/java/javase/15/docs/specs/man/jconsole.html– It is part of the JDK.

● JDK Mission Control (written in: Java; license: BSD-style/UPL) https://openjdk.java.net/projects/jmc/ https://wiki.openjdk.java.net/display/jmc/Main https://github.com/openjdk/jmc

● VisualVM (written in: Java; license: GPLv2) https://visualvm.github.io/ https://github.com/oracle/visualvm– Installation (SDKMAN!): sdk install visualvm

– Usage: Getting Started with VisualVM https://visualvm.github.io/gettingstarted.html– IDE support:

● IntelliJ IDEA plugin: VisualVM Launcher https://plugins.jetbrains.com/plugin/7115-visualvm-launcher