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! # Welcome to the JDK!
For build instructions please see the
[online documentation](https://openjdk.org/groups/build/doc/building.html),
or either of these files:
! # Welcome to the Leyden Prototype Repository!
+
+ The purpose of the Leyden repository is to prototype improvements to the
+ startup time, time to peak performance, and footprint of Java programs, as a part of
+ [Project Leyden](https://openjdk.org/projects/leyden). We solicit feedback from
+ the Java community, with the hope that some of these improvements can be eventually
+ incoporated in future JDK releases.
+
+ ## 0. Disclaimers
+
+ - *This repository contains experimental and unstable code. It is not intended to be used
+ in a production environment.*
+ - *This repository is intended for developers of the JDK, and advanced Java developers who
+ are familiar with building the JDK.*
+ - *The experimental features in this repository may be changed or removed without notice.
+ Command line flags and workflows will change.*
+ - *The benchmarks results reported on this page are for illustrative purposes only. Your
+ applications may get better or worse results.*
+
+ ## 1. Overview
+
+ The Leyden "[premain](https://github.com/openjdk/leyden/blob/premain/)" prototype
+ includes many optimizations that shift work from run time to earlier
+ executions of the application, which are
+ called _training runs_. In a training run, we pre-compute various kinds of information.
+ Importantly, we pre-compile
+ bytecode to native code, guided by observations of the application's actual behavior
+ during the training run.
+
+ The Leyden repository closely tracks the JDK main line. We are typically only a few weeks behind
+ the [main-line JDK repo](https://github.com/openjdk/jdk).
+
+ We have implemented the following improvements over the JDK main line:
+
+ - **[Ahead-of-Time Class Loading & Linking (JEP 483)](https://openjdk.org/jeps/483)**:
+ This gives
+ the JVM the ability to put classes in the _linked_ state as soon the application starts up. As a result,
+ we can implement many other time shifting optimizations with considerably simplified assumptions.
+ - This feature is accessed with the new VM flag `-XX:+PreloadSharedClasses`.
+
+ - **[Unified Ahead-of-Time Cache (JEP draft 8320264)](https://openjdk.org/jeps/8320264)**:
+ This enhancement to [CDS] is foundational to the features that follow.
+ - It enables [CDS] to store not only class metadata and heap objects (as before),
+ but also profiling data and compiled code.
+ - This feature is accessed with the new VM flag `-XX:CacheDataStore`.
+ - This option simplifies the creation of the CDS archive, and also the testing
+ of all the prototype features listed here.
+
+ - **[Ahead-of-Time Method Profiling (JEP draft 8325147)](https://openjdk.org/jeps/8325147)**: We store method profiles
+ from training runs in the CDS archive, thereby enabling the JIT to begin compiling earlier during warmup.
+ As a result, Java applications can reach peak performance faster.
+ - This feature is enabled by the new VM flags `-XX:+RecordTraining` and `-XX:+ReplayTraining`.
+
+ - **Ahead-of-time resolution of constant pool entries**: many
+ constant pool entries are resolved during the assembly phase. This allows the application to start up faster. Also,
+ the existence of resolved constant pool entries allows the AOT compiler to generate better code.
+ For diagnostic purposes, you can use `-XX:+UnlockDiagnosticVMOptions -XX:-AOTInvokeDynamicLinking`
+ to disable the AOT linking of constant pool entries for the `invokedynamic` bytecode.
+
+ - **[Ahead-of-Time Code Compilation (JEP draft 8335368)](https://openjdk.org/jeps/8335368)**: Methods that are frequently used during the training run can be
+ compiled and stored along with the CDS archive. As a result, as soon as the application starts up
+ in the production run, its methods can be can be natively executed.
+ - This feature is enabled by the new VM flags `-XX:+StoreCachedCode`, `-XX:+LoadCachedCode`, and `-XX:CachedCodeFile`.
+ - Currently, the native code is stored in a separate file, but our plans is to eventually store the native code
+ inside the CDS archive file.
+
+ - **Ahead-of-time generation of [Dynamic Proxies](https://docs.oracle.com/en/java/javase/22/docs/api/java.base/java/lang/reflect/Proxy.html)**:
+ Dynamic proxies are frequently used by popular application frameworks. We can improve start-up time by generating these proxies ahead of time.
+ - This feature is enabled by the new VM flag `-XX:+ArchiveDynamicProxies`.
+
+ - **Ahead-of-time generation of reflection data**: Reflection data (such as instances of
+ `java.lang.reflect.Method`) are generated by the JVM to support `java.lang.reflect` operations. We can
+ generate these ahead of time to improve start-up.
+ - This feature is enabled by the new VM flag `-XX:+ArchiveReflectionData`.
+
+ - **Class Not Found Cache**: Sometimes application frameworks repeatedly try to load classes that do not exist. This optimization allows such failing lookups to be done quickly without repeatedly scanning the class path.
+ - This feature is enabled by the new VM flag `-XX:+ArchiveLoaderLookupCache`.
+
+ The flag `-XX:CacheDataStore` automatically enables the whole bundle
+ of features listed above. This simplifies testing of the whole
+ prototype. If necessary for more detailed testing, each feature can
+ be individually disabled by negating its associated flag.
+
+ The names of all of these VM flags will change in a future EA build as we transition from the old “CDS” terminology to the new “AOT” terminology, as discussed [here](https://openjdk.org/jeps/483#History).
+
+ [CDS]: <https://docs.oracle.com/en/java/javase/22/vm/class-data-sharing.html>
+
+ ## 2. Building the Leyden Repository
+
+ The Leyden Repository can be built in the same way as the main-line JDK repository.
+ Please use the "premain" branch. I.e., [https://github.com/openjdk/leyden/tree/premain](https://github.com/openjdk/leyden/tree/premain).
For build instructions please see the
[online documentation](https://openjdk.org/groups/build/doc/building.html),
or either of these files:
- [doc/building.md](doc/building.md) (markdown version)
See <https://openjdk.org/> for more information about the OpenJDK
Community and the JDK and see <https://bugs.openjdk.org> for JDK issue
tracking.
+
+ ## 3. Trying out Leyden Features
+
+ The easiest way to try out the Leyden features is to build a JVM from the Leyden repository, and use it with your application with the `-XX:CacheDataStore` flag.
+
+ Here's a small benchmark that uses the JDK's built-in
+ [`JavaCompiler`](https://docs.oracle.com/en/java/javase/21/docs/api/java.compiler/javax/tools/JavaCompiler.html)
+ class to compile some Java source files. This benchmark spends a significant amount of start-up time
+ setting up the classes used by `JavaCompiler`, so it will benefit from the Leyden features.
+
+ First, download [JavacBenchApp.java](https://github.com/iklam/jdk/blob/f95f851aed3d2bf06edabab1e7c24e15f4145d0d/test/hotspot/jtreg/runtime/cds/appcds/applications/JavacBenchApp.java)
+ and compile it into a JAR file.
+
+ (Remember to use the `java` program that you built from the Leyden repository.)
+
+ ```
+ $ javac JavacBenchApp.java
+ $ jar cvf JavacBenchApp.jar JavacBenchApp*.class
+ added manifest
+ adding: JavacBenchApp$ClassFile.class(in = 1608) (out= 787)(deflated 51%)
+ adding: JavacBenchApp$FileManager.class(in = 2090) (out= 979)(deflated 53%)
+ adding: JavacBenchApp$SourceFile.class(in = 1351) (out= 671)(deflated 50%)
+ adding: JavacBenchApp.class(in = 7571) (out= 3302)(deflated 56%)
+ ```
+
+ We can run this benchmark without any Leyden features. It takes 893 ms:
+
+ ```
+ $ java -cp JavacBenchApp.jar JavacBenchApp 50
+ Generated source code for 51 classes and compiled them in 893 ms
+ ```
+
+ Now, we can perform a _training run_ and create the Leyden cache files.
+
+ <b>Note: Any files `JavacBenchApp.cds*` created by previous tests must
+ be deleted, before new ones are created.</b>:
+
+ ```
+ $ rm -fv JavacBenchApp.cds*
+ $ java -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50
+ $ ls -l JavacBenchApp.cds*
+ -r--r--r-- 1 iklam iklam 30900224 May 20 19:21 JavacBenchApp.cds
+ -r--r--r-- 1 iklam iklam 16895736 May 20 19:21 JavacBenchApp.cds.code
+ ```
+
+ Two files are created:
+
+ - `JavacBenchApp.cds`: This file contains classes, heap objects and profiling data harvested from the training run.
+ - `JavacBenchApp.cds.code`: This file contains AOT-compiled methods, optimized for the execution behaviors observed during the training run.
+ (Data in this file will be merged into `JavacBenchApp.cds` in a future release.)
+
+ Now, we can make a _production run_ of the program with the cache files. It finishes in 423 ms, or more than twice as fast as
+ before.
+
+ ```
+ $ java -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50
+ Generated source code for 51 classes and compiled them in 423 ms
+ ```
+
+ By default, training runs end when the application terminates. You have two other options to end training runs:
+
+ - -XX:AOTEndTrainingOnMethodEntry=<method1,method2,...>[,count=100]
+ - jcmd \<pid> AOT.end_training
+
+ Note that AOTEndTrainingOnMethodEntry uses the same format as CompileOnly and the default count is 1
+
+ ### Optional VM Flags
+
+ When you create the file `JavacBenchApp.cds` with the flag `-XX:CacheDataStore`,
+ all of the other options described
+ in the [Overview](#1-overview) section above are enabled by default. This ensures that you can get all the optimizations
+ without specifying them individually.
+
+ For diagnostic purposes, you can selectively disable some of the options:
+
+ - The `-XX:+LoadCachedCode` and `-XX:+ReplayTraining` flags affect only the production run.
+ - All other options affect only the training run.
+
+ For example, you can disable the loading of the AOT code during the production run. Notice that the benchmark now
+ starts more slowly than it did when AOT code was loaded.
+
+ ```
+ $ java -XX:CacheDataStore=JavacBenchApp.cds -XX:-LoadCachedCode -cp JavacBenchApp.jar JavacBenchApp 50
+ Generated source code for 51 classes and compiled them in 647 ms
+ ```
+
+ You can also disable AOT compilation in the training run:
+
+ ```
+ $ rm -fv JavacBenchApp.cds*
+ $ java -XX:CacheDataStore=JavacBenchApp.cds -XX:-StoreCachedCode -cp JavacBenchApp.jar JavacBenchApp 50
+ $ ls -l JavacBenchApp.cds*
+ -r--r--r-- 1 iklam iklam 30277632 May 20 20:05 JavacBenchApp.cds
+ ```
+
+ Note that the file `JavacBenchApp.cds.code` is no longer created.
+
+ ## 4. Limitations of the Leyden Prototype
+
+ When trying out the Leyden, please pay attention to the following limitations.
+
+ ### The Same Garbage Collector Must be Used between Training and Production Runs
+
+ The CDS archive generated by the Leyden prototype includes machine instructions that are specific to
+ the garbage collector. We recommend that you explicitly specify the same collector during both
+ training and production runs. For example:
+
+ ```
+ # training run
+ $ rm -fv JavacBenchApp.cds*
+ $ java -XX:CacheDataStore=JavacBenchApp.cds -XX:+UseSerialGC -cp JavacBenchApp.jar JavacBenchApp 50
+
+ # production run
+ $ java -XX:CacheDataStore=JavacBenchApp.cds -XX:+UseSerialGC -cp JavacBenchApp.jar JavacBenchApp 50
+ ```
+
+ Otherwise, the CDS archive may not be loaded for the production run, leading to suboptimal performance.
+ For example, sometimes you may perform the training run on a large development host, and then use
+ a container to run the application in a small production node. In the following scenario, as the collector
+ is not explicitly specified, the VM will automatically pick G1 for the training run, and SerialGC for the
+ production run (due to its limited amount of memory):
+
+ ```
+ # training run (uses G1 by default)
+ $ rm -fv JavacBenchApp.cds*
+ $ java -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50
+
+ # production run (uses SerialGC)
+ $ docker run --rm -v /repos/leyden/build/linux-x64/images/jdk:/jdk -v $(pwd):/test \
+ --memory=1024m \
+ container-registry.oracle.com/java/openjdk \
+ bash -c 'cd /test; /jdk/bin/java -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50'
+ [0.001s][error][cds] CDS archive has preloaded classes. It cannot be used because GC used during dump time (G1)
+ is not the same as runtime (Serial)
+ [0.001s][error][cds] An error has occurred while processing the shared archive file.
+ [0.001s][error][cds] Unable to map shared spaces
+ Error occurred during initialization of VM
+ Unable to use shared archive.
+ ```
+ ### Only G1GC, SerialGC, ParallelGC, EpsilonGC, ShenandoahGC are Supported
+ =======
+
+ Currently, if you use any other garbage collector in combination with `-XX:CacheDataStore`, the VM will
+ exit with an error.
+
+ ```
+ $ java -XX:+UseZGC -XX:CacheDataStore=foo --version
+ Error occurred during initialization of VM
+ Cannot create the CacheDataStore: UseCompressedClassPointers must be enabled, and collector
+ must be G1, Parallel, Serial, Epsilon, or Shenandoah
+ ```
+
+
+ ### -Xshare:on is Enabled by default
+
+ As seen in the example immediately above, in the production run, if the CDS archive cannot be
+ used for any reason, the JVM will report an error and exit. This happens as if `-Xshare:on` was
+ specified in the command-line.
+
+ In the standard JDK, when the CDS archive cannot be used for any reason (for example, the
+ archive was created for a different version of the JDK), the application will
+ continue to run without using CDS.
+ This fall-back strategy ensures that the application will function correctly, though at a lower level of performance.
+
+ With the Leyden prototype, we have changed this fall-back behavior to make it easier to diagnose
+ performance issues. For example, when the start-up time is not as good as one would expect, we
+ want know whether it's caused by a misconfiguration that prevents the CDS archive
+ from being used, or it's caused by a deficiency in the implementation of the Leyden optimizations.
+
+ To revert to the behavior of the standard JDK, you can explicitly add `-Xshare:auto` to the command-line.
+
+ ```
+ $ docker run --rm -v /repos/leyden/build/linux-x64/images/jdk:/jdk -v $(pwd):/test \
+ --memory=1024m \
+ container-registry.oracle.com/java/openjdk \
+ bash -c 'cd /test; /jdk/bin/java -Xshare:auto -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50'
+ [0.001s][error][cds] CDS archive has preloaded classes. It cannot be used because GC used during dump time (G1)
+ is not the same as runtime (Serial)
+ Generated source code for 51 classes and compiled them in 831 ms
+ ```
+
+ See [here](https://docs.oracle.com/en/java/javase/21/vm/class-data-sharing.html) for a discussion of `-Xshare:on` vs `-Xshare:auto`.
+
+
+ ## 5. Benchmarking
+
+ We use a small set of benchmarks to demonstrate the performance of the optimizations in the Leyden repo.
+
+ | Benchmark | Source |
+ | ------------- | ------------- |
+ |[helidon-quickstart-se](test/hotspot/jtreg/premain/helidon-quickstart-se) | https://helidon.io/docs/v4/se/guides/quickstart|
+ |[micronaut-first-app](test/hotspot/jtreg/premain/micronaut-first-app) | https://guides.micronaut.io/latest/creating-your-first-micronaut-app-maven-java.html|
+ |[quarkus-getting-started](test/hotspot/jtreg/premain/quarkus-getting-started) | https://quarkus.io/guides/getting-started|
+ |[spring-boot-getting-started](test/hotspot/jtreg/premain/spring-boot-getting-started) | https://spring.io/guides/gs/spring-boot|
+ |[spring-petclinic](test/hotspot/jtreg/premain/spring-petclinic) | https://github.com/spring-projects/spring-petclinic|
+
+ *(FIXME: add a benchmark for javac)*
+
+ ### Benchmarking Against JDK Main-line
+
+ To can compare the performance of Leyden vs the main-line JDK, you need:
+
+ - An official build of JDK 21
+ - An up-to-date build of the JDK main-line
+ - The latest Leyden build
+ - Maven (ideally 3.8 or later, as required by some of the demos). Note: if you are behind
+ a firewall, you may need to [set up proxies for Maven](https://maven.apache.org/guides/mini/guide-proxies.html)
+
+ The same steps are used for benchmarking all of the above demos. For example:
+
+ ```
+ $ cd helidon-quickstart-se
+ $ make PREMAIN_HOME=/repos/leyden/build/linux-x64/images/jdk \
+ MAINLINE_HOME=/repos/jdk/build/linux-x64/images/jdk \
+ BLDJDK_HOME=/usr/local/jdk21 \
+ bench
+ run,mainline default,mainline custom static CDS,premain custom static CDS only,premain CDS + AOT
+ 1,398,244,144,107
+ 2,387,247,142,108
+ 3,428,238,143,107
+ 4,391,252,142,111
+ 5,417,247,141,107
+ 6,390,239,139,127
+ 7,387,247,145,111
+ 8,387,240,147,110
+ 9,388,242,147,108
+ 10,400,242,167,108
+ Geomean,397.08,243.76,145.52,110.26
+ Stdev,13.55,4.19,7.50,5.73
+ Markdown snippets in mainline_vs_premain.md
+ ```
+
+ The above command runs each configuration 10 times, in an interleaving order. This way
+ the noise of the system (background processes, thermo throttling, etc) is more likely to
+ be spread across the different runs.
+
+ As is typical for benchmarking start-up performance, the numbers are not very steady.
+ It is best to plot
+ the results (as saved in the file `mainline_vs_premain.csv`) in a spreadsheet to check for
+ noise and other artifacts.
+
+ The "make bench" target also generates GitHub markdown snippets (in the file `mainline_vs_premain.md`) for creating the
+ graphs below.
+
+ ### Benchmarking Between Two Leyden Builds
+
+ This is useful for Leyden developers to measure the benefits of a particular optimization.
+ The steps are similar to above, but we use the "make compare_premain_builds" target:
+
+ ```
+ $ cd helidon-quickstart-se
+ $ make PM_OLD=/repos/leyden_old/build/linux-x64/images/jdk \
+ PM_NEW=/repos/leyden_new/build/linux-x64/images/jdk \
+ BLDJDK_HOME=/usr/local/jdk21 \
+ compare_premain_builds
+ Old build = /repos/leyden_old/build/linux-x64/images/jdk with options
+ New build = /repos/leyden_new/build/linux-x64/images/jdk with options
+ Run,Old CDS + AOT,New CDS + AOT
+ 1,110,109
+ 2,131,111
+ 3,118,115
+ 4,110,108
+ 5,117,110
+ 6,114,109
+ 7,110,109
+ 8,118,110
+ 9,110,110
+ 10,113,114
+ Geomean,114.94,110.48
+ Stdev,6.19,2.16
+ Markdown snippets in compare_premain_builds.md
+ ```
+
+ Please see [test/hotspot/jtreg/premain/lib/Bench.gmk](test/hotspot/jtreg/premain/lib/Bench.gmk) for more details.
+
+ Note: due to the variability of start-up time, the benefit of minor improvements may
+ be difficult to measure.
+
+ ### Preliminary Benchmark Results
+
+ The following charts show the relative start-up performance of the Leyden/Premain branch vs
+ the JDK main-line.
+
+ For example, a number of "premain CDS + AOT : 291" indicates that if the application takes
+ 1000 ms to start-up with the JDK main-line, it takes only 291 ms to start up when all the
+ current set of Leyden optimizations for CDS and AOT are enabled.
+
+ The benchmark results are collected with `make bench` in the following directories:
+
+ - `helidon-quickstart-se`
+ - `micronaut-first-app`
+ - `quarkus-getting-started`
+ - `spring-petclinic`
+
+ These JDK versions were used in the comparisons:
+
+ - JDK main-line: https://github.com/openjdk/jdk/commit/70944ca54ad0090c734bb5b3082beb33450c4877
+ - Leyden: https://github.com/openjdk/leyden/commit/9fa972214934d30f67db5fd4d1b8007636ac1428
+
+ The benchmarks were executed on an 8-core Intel i7-10700 CPU @ 2.90GHz with 32GB RAM running Ubuntu 22.04.3 LTS.
+
+ ### Helidon Quick Start (SE) Demo (3.44x improvement)
+
+ ```mermaid
+ ---
+ config:
+ xyChart:
+ chartOrientation: horizontal
+ height: 300
+ ---
+ xychart-beta
+ x-axis "variant" ["mainline default", "mainline custom static CDS", "premain custom static CDS only", "premain CDS + AOT"]
+ y-axis "Elapsed time (normalized, smaller is better)" 0 --> 1000
+ bar [1000, 632, 376, 291]
+ ```
+
+ ### Micronaut First App Demo (2.83x improvement)
+
+ ```mermaid
+ ---
+ config:
+ xyChart:
+ chartOrientation: horizontal
+ height: 300
+ ---
+ xychart-beta
+ x-axis "variant" ["mainline default", "mainline custom static CDS", "premain custom static CDS only", "premain CDS + AOT"]
+ y-axis "Elapsed time (normalized, smaller is better)" 0 --> 1000
+ bar [1000, 558, 410, 353]
+ ```
+
+ ### Quarkus Getting Started Demo (3.15x improvement)
+
+ ```mermaid
+ ---
+ config:
+ xyChart:
+ chartOrientation: horizontal
+ height: 300
+ ---
+ xychart-beta
+ x-axis "variant" ["mainline default", "mainline custom static CDS", "premain custom static CDS only", "premain CDS + AOT"]
+ y-axis "Elapsed time (normalized, smaller is better)" 0 --> 1000
+ bar [1000, 568, 395, 317]
+ ```
+
+ ### Spring-boot Getting Started Demo (3.53x improvement)
+
+ ```mermaid
+ ---
+ config:
+ xyChart:
+ chartOrientation: horizontal
+ height: 300
+ ---
+ xychart-beta
+ x-axis "variant" ["mainline default", "mainline custom static CDS", "premain custom static CDS only", "premain CDS + AOT"]
+ y-axis "Elapsed time (normalized, smaller is better)" 0 --> 1000
+ bar [1000, 560, 394, 283]
+ ```
+
+ ### Spring PetClinic Demo (2.72x improvement)
+
+ ```mermaid
+ ---
+ config:
+ xyChart:
+ chartOrientation: horizontal
+ height: 300
+ ---
+ xychart-beta
+ x-axis "variant" ["mainline default", "mainline custom static CDS", "premain custom static CDS only", "premain CDS + AOT"]
+ y-axis "Elapsed time (normalized, smaller is better)" 0 --> 1000
+ bar [1000, 695, 563, 368]
+ ```
+
+ ## 6. More Documentation
+
+ Please see [test/hotspot/jtreg/premain/](test/hotspot/jtreg/premain) for more information.
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