1 # Welcome to the Leyden Prototype Repository!
2
3 The purpose of the Leyden repository is to prototype improvements to the
4 startup time, time to peak performance, and footprint of Java programs, as a part of
5 [Project Leyden](https://openjdk.org/projects/leyden). We solicit feedback from
6 the Java community, with the hope that some of these improvements can be eventually
7 incoporated in future JDK releases.
8
9 ## 0. Disclaimers
10
11 - *This repository contains experimental and unstable code. It is not intended to be used
12 in a production environment.*
13 - *This repository is intended for developers of the JDK, and advanced Java developers who
14 are familiar with building the JDK.*
15 - *The experimental features in this repository may be changed or removed without notice.
16 Command line flags and workflows will change.*
17 - *The benchmarks results reported on this page are for illustrative purposes only. Your
18 applications may get better or worse results.*
19
20 ## 1. Overview
21
22 The Leyden "[premain](https://github.com/openjdk/leyden/blob/premain/)" prototype
23 includes many optimizations that shift work from run time to earlier
24 executions of the application, which are
25 called _training runs_. In a training run, we pre-compute various kinds of information.
26 Importantly, we pre-compile
27 bytecode to native code, guided by observations of the application's actual behavior
28 during the training run.
29
30 The Leyden repository closely tracks the JDK main line. We are typically only a few weeks behind
31 the [main-line JDK repo](https://github.com/openjdk/jdk).
32
33 We have implemented the following improvements over the JDK main line:
34
35 - **[Ahead-of-Time Class Loading & Linking (JEP 483)](https://openjdk.org/jeps/483)**:
36 This gives
37 the JVM the ability to put classes in the _linked_ state as soon the application starts up. As a result,
38 we can implement many other time shifting optimizations with considerably simplified assumptions.
39 - This feature is accessed with the new VM flag `-XX:+PreloadSharedClasses`.
40
41 - **[Unified Ahead-of-Time Cache (JEP draft 8320264)](https://openjdk.org/jeps/8320264)**:
42 This enhancement to [CDS] is foundational to the features that follow.
43 - It enables [CDS] to store not only class metadata and heap objects (as before),
44 but also profiling data and compiled code.
45 - This feature is accessed with the new VM flag `-XX:CacheDataStore`.
46 - This option simplifies the creation of the CDS archive, and also the testing
47 of all the prototype features listed here.
48
49 - **[Ahead-of-Time Method Profiling (JEP draft 8325147)](https://openjdk.org/jeps/8325147)**: We store method profiles
50 from training runs in the CDS archive, thereby enabling the JIT to begin compiling earlier during warmup.
51 As a result, Java applications can reach peak performance faster.
52 - This feature is enabled by the new VM flags `-XX:+RecordTraining` and `-XX:+ReplayTraining`.
53
54 - **Ahead-of-time resolution of constant pool entries**: many
55 constant pool entries are resolved during the assembly phase. This allows the application to start up faster. Also,
56 the existence of resolved constant pool entries allows the AOT compiler to generate better code.
57 For diagnostic purposes, you can use `-XX:+UnlockDiagnosticVMOptions -XX:-AOTInvokeDynamicLinking`
58 to disable the AOT linking of constant pool entries for the `invokedynamic` bytecode.
59
60 - **[Ahead-of-Time Code Compilation (JEP draft 8335368)](https://openjdk.org/jeps/8335368)**: Methods that are frequently used during the training run can be
61 compiled and stored along with the CDS archive. As a result, as soon as the application starts up
62 in the production run, its methods can be can be natively executed.
63 - This feature is enabled by the new VM flags `-XX:+StoreCachedCode`, `-XX:+LoadCachedCode`, and `-XX:CachedCodeFile`.
64 - Currently, the native code is stored in a separate file, but our plans is to eventually store the native code
65 inside the CDS archive file.
66
67 - **Ahead-of-time generation of [Dynamic Proxies](https://docs.oracle.com/en/java/javase/22/docs/api/java.base/java/lang/reflect/Proxy.html)**:
68 Dynamic proxies are frequently used by popular application frameworks. We can improve start-up time by generating these proxies ahead of time.
69 - This feature is enabled by the new VM flag `-XX:+ArchiveDynamicProxies`.
70
71 - **Ahead-of-time generation of reflection data**: Reflection data (such as instances of
72 `java.lang.reflect.Method`) are generated by the JVM to support `java.lang.reflect` operations. We can
73 generate these ahead of time to improve start-up.
74 - This feature is enabled by the new VM flag `-XX:+ArchiveReflectionData`.
75
76 - **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.
77 - This feature is enabled by the new VM flag `-XX:+ArchiveLoaderLookupCache`.
78
79 The flag `-XX:CacheDataStore` automatically enables the whole bundle
80 of features listed above. This simplifies testing of the whole
81 prototype. If necessary for more detailed testing, each feature can
82 be individually disabled by negating its associated flag.
83
84 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).
85
86 [CDS]: <https://docs.oracle.com/en/java/javase/22/vm/class-data-sharing.html>
87
88 ## 2. Building the Leyden Repository
89
90 The Leyden Repository can be built in the same way as the main-line JDK repository.
91 Please use the "premain" branch. I.e., [https://github.com/openjdk/leyden/tree/premain](https://github.com/openjdk/leyden/tree/premain).
92
93 For build instructions please see the
94 [online documentation](https://openjdk.org/groups/build/doc/building.html),
95 or either of these files:
96
97 - [doc/building.html](doc/building.html) (html version)
98 - [doc/building.md](doc/building.md) (markdown version)
99
100 See <https://openjdk.org/> for more information about the OpenJDK
101 Community and the JDK and see <https://bugs.openjdk.org> for JDK issue
102 tracking.
103
104 ## 3. Trying out Leyden Features
105
106 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.
107
108 Here's a small benchmark that uses the JDK's built-in
109 [`JavaCompiler`](https://docs.oracle.com/en/java/javase/21/docs/api/java.compiler/javax/tools/JavaCompiler.html)
110 class to compile some Java source files. This benchmark spends a significant amount of start-up time
111 setting up the classes used by `JavaCompiler`, so it will benefit from the Leyden features.
112
113 First, download [JavacBenchApp.java](https://github.com/iklam/jdk/blob/f95f851aed3d2bf06edabab1e7c24e15f4145d0d/test/hotspot/jtreg/runtime/cds/appcds/applications/JavacBenchApp.java)
114 and compile it into a JAR file.
115
116 (Remember to use the `java` program that you built from the Leyden repository.)
117
118 ```
119 $ javac JavacBenchApp.java
120 $ jar cvf JavacBenchApp.jar JavacBenchApp*.class
121 added manifest
122 adding: JavacBenchApp$ClassFile.class(in = 1608) (out= 787)(deflated 51%)
123 adding: JavacBenchApp$FileManager.class(in = 2090) (out= 979)(deflated 53%)
124 adding: JavacBenchApp$SourceFile.class(in = 1351) (out= 671)(deflated 50%)
125 adding: JavacBenchApp.class(in = 7571) (out= 3302)(deflated 56%)
126 ```
127
128 We can run this benchmark without any Leyden features. It takes 893 ms:
129
130 ```
131 $ java -cp JavacBenchApp.jar JavacBenchApp 50
132 Generated source code for 51 classes and compiled them in 893 ms
133 ```
134
135 Now, we can perform a _training run_ and create the Leyden cache files.
136
137 <b>Note: Any files `JavacBenchApp.cds*` created by previous tests must
138 be deleted, before new ones are created.</b>:
139
140 ```
141 $ rm -fv JavacBenchApp.cds*
142 $ java -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50
143 $ ls -l JavacBenchApp.cds*
144 -r--r--r-- 1 iklam iklam 30900224 May 20 19:21 JavacBenchApp.cds
145 -r--r--r-- 1 iklam iklam 16895736 May 20 19:21 JavacBenchApp.cds.code
146 ```
147
148 Two files are created:
149
150 - `JavacBenchApp.cds`: This file contains classes, heap objects and profiling data harvested from the training run.
151 - `JavacBenchApp.cds.code`: This file contains AOT-compiled methods, optimized for the execution behaviors observed during the training run.
152 (Data in this file will be merged into `JavacBenchApp.cds` in a future release.)
153
154 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
155 before.
156
157 ```
158 $ java -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50
159 Generated source code for 51 classes and compiled them in 423 ms
160 ```
161
162 ### Optional VM Flags
163
164 When you create the file `JavacBenchApp.cds` with the flag `-XX:CacheDataStore`,
165 all of the other options described
166 in the [Overview](#1-overview) section above are enabled by default. This ensures that you can get all the optimizations
167 without specifying them individually.
168
169 For diagnostic purposes, you can selectively disable some of the options:
170
171 - The `-XX:+LoadCachedCode` and `-XX:+ReplayTraining` flags affect only the production run.
172 - All other options affect only the training run.
173
174 For example, you can disable the loading of the AOT code during the production run. Notice that the benchmark now
175 starts more slowly than it did when AOT code was loaded.
176
177 ```
178 $ java -XX:CacheDataStore=JavacBenchApp.cds -XX:-LoadCachedCode -cp JavacBenchApp.jar JavacBenchApp 50
179 Generated source code for 51 classes and compiled them in 647 ms
180 ```
181
182 You can also disable AOT compilation in the training run:
183
184 ```
185 $ rm -fv JavacBenchApp.cds*
186 $ java -XX:CacheDataStore=JavacBenchApp.cds -XX:-StoreCachedCode -cp JavacBenchApp.jar JavacBenchApp 50
187 $ ls -l JavacBenchApp.cds*
188 -r--r--r-- 1 iklam iklam 30277632 May 20 20:05 JavacBenchApp.cds
189 ```
190
191 Note that the file `JavacBenchApp.cds.code` is no longer created.
192
193 ## 4. Limitations of the Leyden Prototype
194
195 When trying out the Leyden, please pay attention to the following limitations.
196
197 ### The Same Garbage Collector Must be Used between Training and Production Runs
198
199 The CDS archive generated by the Leyden prototype includes machine instructions that are specific to
200 the garbage collector. We recommend that you explicitly specify the same collector during both
201 training and production runs. For example:
202
203 ```
204 # training run
205 $ rm -fv JavacBenchApp.cds*
206 $ java -XX:CacheDataStore=JavacBenchApp.cds -XX:+UseSerialGC -cp JavacBenchApp.jar JavacBenchApp 50
207
208 # production run
209 $ java -XX:CacheDataStore=JavacBenchApp.cds -XX:+UseSerialGC -cp JavacBenchApp.jar JavacBenchApp 50
210 ```
211
212 Otherwise, the CDS archive may not be loaded for the production run, leading to suboptimal performance.
213 For example, sometimes you may perform the training run on a large development host, and then use
214 a container to run the application in a small production node. In the following scenario, as the collector
215 is not explicitly specified, the VM will automatically pick G1 for the training run, and SerialGC for the
216 production run (due to its limited amount of memory):
217
218 ```
219 # training run (uses G1 by default)
220 $ rm -fv JavacBenchApp.cds*
221 $ java -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50
222
223 # production run (uses SerialGC)
224 $ docker run --rm -v /repos/leyden/build/linux-x64/images/jdk:/jdk -v $(pwd):/test \
225 --memory=1024m \
226 container-registry.oracle.com/java/openjdk \
227 bash -c 'cd /test; /jdk/bin/java -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50'
228 [0.001s][error][cds] CDS archive has preloaded classes. It cannot be used because GC used during dump time (G1)
229 is not the same as runtime (Serial)
230 [0.001s][error][cds] An error has occurred while processing the shared archive file.
231 [0.001s][error][cds] Unable to map shared spaces
232 Error occurred during initialization of VM
233 Unable to use shared archive.
234 ```
235 ### Only G1GC, SerialGC, ParallelGC, EpsilonGC are Supported
236
237 Currently, if you use any other garbage collector in combination with `-XX:CacheDataStore`, the VM will
238 exit with an error.
239
240 ```
241 $ java -XX:+UseZGC -XX:CacheDataStore=foo --version
242 Error occurred during initialization of VM
243 Cannot create the CacheDataStore: UseCompressedClassPointers must be enabled, and collector
244 must be G1, Parallel, Serial, or Epsilon
245 ```
246
247
248 ### -Xshare:on is Enabled by default
249
250 As seen in the example immediately above, in the production run, if the CDS archive cannot be
251 used for any reason, the JVM will report an error and exit. This happens as if `-Xshare:on` was
252 specified in the command-line.
253
254 In the standard JDK, when the CDS archive cannot be used for any reason (for example, the
255 archive was created for a different version of the JDK), the application will
256 continue to run without using CDS.
257 This fall-back strategy ensures that the application will function correctly, though at a lower level of performance.
258
259 With the Leyden prototype, we have changed this fall-back behavior to make it easier to diagnose
260 performance issues. For example, when the start-up time is not as good as one would expect, we
261 want know whether it's caused by a misconfiguration that prevents the CDS archive
262 from being used, or it's caused by a deficiency in the implementation of the Leyden optimizations.
263
264 To revert to the behavior of the standard JDK, you can explicitly add `-Xshare:auto` to the command-line.
265
266 ```
267 $ docker run --rm -v /repos/leyden/build/linux-x64/images/jdk:/jdk -v $(pwd):/test \
268 --memory=1024m \
269 container-registry.oracle.com/java/openjdk \
270 bash -c 'cd /test; /jdk/bin/java -Xshare:auto -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50'
271 [0.001s][error][cds] CDS archive has preloaded classes. It cannot be used because GC used during dump time (G1)
272 is not the same as runtime (Serial)
273 Generated source code for 51 classes and compiled them in 831 ms
274 ```
275
276 See [here](https://docs.oracle.com/en/java/javase/21/vm/class-data-sharing.html) for a discussion of `-Xshare:on` vs `-Xshare:auto`.
277
278
279 ## 5. Benchmarking
280
281 We use a small set of benchmarks to demonstrate the performance of the optimizations in the Leyden repo.
282
283 | Benchmark | Source |
284 | ------------- | ------------- |
285 |[helidon-quickstart-se](test/hotspot/jtreg/premain/helidon-quickstart-se) | https://helidon.io/docs/v4/se/guides/quickstart|
286 |[micronaut-first-app](test/hotspot/jtreg/premain/micronaut-first-app) | https://guides.micronaut.io/latest/creating-your-first-micronaut-app-maven-java.html|
287 |[quarkus-getting-started](test/hotspot/jtreg/premain/quarkus-getting-started) | https://quarkus.io/guides/getting-started|
288 |[spring-boot-getting-started](test/hotspot/jtreg/premain/spring-boot-getting-started) | https://spring.io/guides/gs/spring-boot|
289 |[spring-petclinic](test/hotspot/jtreg/premain/spring-petclinic) | https://github.com/spring-projects/spring-petclinic|
290
291 *(FIXME: add a benchmark for javac)*
292
293 ### Benchmarking Against JDK Main-line
294
295 To can compare the performance of Leyden vs the main-line JDK, you need:
296
297 - An official build of JDK 21
298 - An up-to-date build of the JDK main-line
299 - The latest Leyden build
300 - Maven (ideally 3.8 or later, as required by some of the demos). Note: if you are behind
301 a firewall, you may need to [set up proxies for Maven](https://maven.apache.org/guides/mini/guide-proxies.html)
302
303 The same steps are used for benchmarking all of the above demos. For example:
304
305 ```
306 $ cd helidon-quickstart-se
307 $ make PREMAIN_HOME=/repos/leyden/build/linux-x64/images/jdk \
308 MAINLINE_HOME=/repos/jdk/build/linux-x64/images/jdk \
309 BLDJDK_HOME=/usr/local/jdk21 \
310 bench
311 run,mainline default,mainline custom static CDS,premain custom static CDS only,premain CDS + AOT
312 1,398,244,144,107
313 2,387,247,142,108
314 3,428,238,143,107
315 4,391,252,142,111
316 5,417,247,141,107
317 6,390,239,139,127
318 7,387,247,145,111
319 8,387,240,147,110
320 9,388,242,147,108
321 10,400,242,167,108
322 Geomean,397.08,243.76,145.52,110.26
323 Stdev,13.55,4.19,7.50,5.73
324 Markdown snippets in mainline_vs_premain.md
325 ```
326
327 The above command runs each configuration 10 times, in an interleaving order. This way
328 the noise of the system (background processes, thermo throttling, etc) is more likely to
329 be spread across the different runs.
330
331 As is typical for benchmarking start-up performance, the numbers are not very steady.
332 It is best to plot
333 the results (as saved in the file `mainline_vs_premain.csv`) in a spreadsheet to check for
334 noise and other artifacts.
335
336 The "make bench" target also generates GitHub markdown snippets (in the file `mainline_vs_premain.md`) for creating the
337 graphs below.
338
339 ### Benchmarking Between Two Leyden Builds
340
341 This is useful for Leyden developers to measure the benefits of a particular optimization.
342 The steps are similar to above, but we use the "make compare_premain_builds" target:
343
344 ```
345 $ cd helidon-quickstart-se
346 $ make PM_OLD=/repos/leyden_old/build/linux-x64/images/jdk \
347 PM_NEW=/repos/leyden_new/build/linux-x64/images/jdk \
348 BLDJDK_HOME=/usr/local/jdk21 \
349 compare_premain_builds
350 Old build = /repos/leyden_old/build/linux-x64/images/jdk with options
351 New build = /repos/leyden_new/build/linux-x64/images/jdk with options
352 Run,Old CDS + AOT,New CDS + AOT
353 1,110,109
354 2,131,111
355 3,118,115
356 4,110,108
357 5,117,110
358 6,114,109
359 7,110,109
360 8,118,110
361 9,110,110
362 10,113,114
363 Geomean,114.94,110.48
364 Stdev,6.19,2.16
365 Markdown snippets in compare_premain_builds.md
366 ```
367
368 Please see [test/hotspot/jtreg/premain/lib/Bench.gmk](test/hotspot/jtreg/premain/lib/Bench.gmk) for more details.
369
370 Note: due to the variability of start-up time, the benefit of minor improvements may
371 be difficult to measure.
372
373 ### Preliminary Benchmark Results
374
375 The following charts show the relative start-up performance of the Leyden/Premain branch vs
376 the JDK main-line.
377
378 For example, a number of "premain CDS + AOT : 291" indicates that if the application takes
379 1000 ms to start-up with the JDK main-line, it takes only 291 ms to start up when all the
380 current set of Leyden optimizations for CDS and AOT are enabled.
381
382 The benchmark results are collected with `make bench` in the following directories:
383
384 - `helidon-quickstart-se`
385 - `micronaut-first-app`
386 - `quarkus-getting-started`
387 - `spring-petclinic`
388
389 These JDK versions were used in the comparisons:
390
391 - JDK main-line: https://github.com/openjdk/jdk/commit/70944ca54ad0090c734bb5b3082beb33450c4877
392 - Leyden: https://github.com/openjdk/leyden/commit/9fa972214934d30f67db5fd4d1b8007636ac1428
393
394 The benchmarks were executed on an 8-core Intel i7-10700 CPU @ 2.90GHz with 32GB RAM running Ubuntu 22.04.3 LTS.
395
396 ### Helidon Quick Start (SE) Demo (3.44x improvement)
397
398 ```mermaid
399 ---
400 config:
401 xyChart:
402 chartOrientation: horizontal
403 height: 300
404 ---
405 xychart-beta
406 x-axis "variant" ["mainline default", "mainline custom static CDS", "premain custom static CDS only", "premain CDS + AOT"]
407 y-axis "Elapsed time (normalized, smaller is better)" 0 --> 1000
408 bar [1000, 632, 376, 291]
409 ```
410
411 ### Micronaut First App Demo (2.83x improvement)
412
413 ```mermaid
414 ---
415 config:
416 xyChart:
417 chartOrientation: horizontal
418 height: 300
419 ---
420 xychart-beta
421 x-axis "variant" ["mainline default", "mainline custom static CDS", "premain custom static CDS only", "premain CDS + AOT"]
422 y-axis "Elapsed time (normalized, smaller is better)" 0 --> 1000
423 bar [1000, 558, 410, 353]
424 ```
425
426 ### Quarkus Getting Started Demo (3.15x improvement)
427
428 ```mermaid
429 ---
430 config:
431 xyChart:
432 chartOrientation: horizontal
433 height: 300
434 ---
435 xychart-beta
436 x-axis "variant" ["mainline default", "mainline custom static CDS", "premain custom static CDS only", "premain CDS + AOT"]
437 y-axis "Elapsed time (normalized, smaller is better)" 0 --> 1000
438 bar [1000, 568, 395, 317]
439 ```
440
441 ### Spring-boot Getting Started Demo (3.53x improvement)
442
443 ```mermaid
444 ---
445 config:
446 xyChart:
447 chartOrientation: horizontal
448 height: 300
449 ---
450 xychart-beta
451 x-axis "variant" ["mainline default", "mainline custom static CDS", "premain custom static CDS only", "premain CDS + AOT"]
452 y-axis "Elapsed time (normalized, smaller is better)" 0 --> 1000
453 bar [1000, 560, 394, 283]
454 ```
455
456 ### Spring PetClinic Demo (2.72x improvement)
457
458 ```mermaid
459 ---
460 config:
461 xyChart:
462 chartOrientation: horizontal
463 height: 300
464 ---
465 xychart-beta
466 x-axis "variant" ["mainline default", "mainline custom static CDS", "premain custom static CDS only", "premain CDS + AOT"]
467 y-axis "Elapsed time (normalized, smaller is better)" 0 --> 1000
468 bar [1000, 695, 563, 368]
469 ```
470
471 ## 6. More Documentation
472
473 Please see [test/hotspot/jtreg/premain/](test/hotspot/jtreg/premain) for more information.