Michael Bien's Weblog

Class Data Sharing as a JVM feature exists for quite some time already, but it became more popular in context of containers. CDS maps a pre-defined class archive into memory and makes it shareable between JVM processes. This can improve startup time but also reduce per-JVM footprint in some cases.

Although basic CDS was already supported in Java 5, this blog entry assumes that Java 11 or later is used, since JVM flags and overall capabilities changed over time. CDS might not be supported on all architectures or GCs: G1, CMS, ShenandoahGC, Serial GC, Parallel GC, and ParallelOld GC do all support CDS on 64bit linux, while ZGC for example doesn't have support for it yet.

JDK Class Data Sharing

[mbien@hulk server]$ java -version
openjdk version "12.0.1" 2019-04-16
OpenJDK Runtime Environment AdoptOpenJDK (build 12.0.1+12)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 12.0.1+12, mixed mode, sharing)

[mbien@hulk server]$ java -version
openjdk version "12.0.1" 2019-04-16
OpenJDK Runtime Environment AdoptOpenJDK (build 12.0.1+12)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 12.0.1+12, mixed mode)

The JDK CDS archive can be manually generated by invoking java -Xshare:dump with root privileges. Recent JVMs are configured to use -Xshare:auto at startup, which will automatically use CDS if available. Enforcing CDS with -Xshare:on will cause the JVM to fail if no archive is found.

Application Class Data Sharing

The basic CDS archive only contains JDK class data. Adding application class data (JEP 310) is fairly easy but comes with some limitations.

• the classpath used at archive creation time must be the same as (or a prefix of) the classpath used at run time
• wildcards or exploded JARs are not allowed in the path (no cheating)

1) create a list of classes (JDK and APP; Java 11+) to include in the shared archive:

$ java -Xshare:off -XX:DumpLoadedClassList=classes.list -jar foo.jar

2) create the archive using that list:

$ java -Xshare:dump -XX:SharedClassListFile=classes.list -XX:SharedArchiveFile=foo.jsa -cp foo.jar

3) launch the applications using the shared archive:

$ java -Xshare:on -XX:SharedArchiveFile=foo.jsa -jar foo.jar

-Xlog:class+load helps to verify that it is working properly.

[0.062s][info][class,load] java.lang.Object source: shared objects file
[0.062s][info][class,load] java.io.Serializable source: shared objects file
[0.062s][info][class,load] java.lang.Comparable source: shared objects file
[0.062s][info][class,load] java.lang.CharSequence source: shared objects file
...

The applications (containers) don't have to be identical to make use of custom CDS archives. The archive is based on the class list of step 1 which can be freely modified or merged with other lists. The main limitation is the classpath prefix rule.

Some Results

As quick test I used OpenJDK 11 + Eclipse Jetty + Apache JSP Wiki and edited a single page (to ensure the classes are loaded), generated a custom archive and started a few more instances with CDS enabled using the archive.

classes.list size = 4296 entries
wiki.jsa size = 57,6 MB 

 VIRT    RES     SHR  %MEM    TIME    COMMAND
2771292 315128  22448  8,3   0:20.54 java -Xmx128m -Xms128m -Xshare:off -cp ... 
2735468 259148  45184  6,9   0:18.38 java -Xmx128m -Xms128m -XX:SharedArchiveFile=wiki.jsa -cp ...
2735296 270044  45192  7,1   0:19.66 java -Xmx128m -Xms128m -XX:SharedArchiveFile=wiki.jsa -cp ...
2735468 259812  45060  6,9   0:18.26 java -Xmx128m -Xms128m -XX:SharedArchiveFile=wiki.jsa -cp ...
2735468 256800  45196  6,8   0:19.10 java -Xmx128m -Xms128m -XX:SharedArchiveFile=wiki.jsa -cp ...

The first JVM is started with CDS turned off as reference. The four other instances have sharing enabled using the custom archive.

Dynamic Class Data Archives

The proposal in JEP 350 which is currently targeted for Java 13 will allow to combine a static base archive with a dynamically generated archive.

The dynamic class data archive is created in a setup phase at application exit (-XX:ArchiveClassesAtExit=dynamic.jsa) and essentially automates step 1 and 2. Just like before -XX:SharedArchiveFile=dynamic.jsa will tell the JVM to map the shared archive. Since the dynamic archive references the static JDK archive, both will be used automatically.

The main advantage (beside the added convenience) is that the dynamic archive will support both builtin class loaders and user-defined class loaders.

now go forth and share class data :)