[wildfly-dev] Reducing Startup Time and Footprint with AppCDS

[Philippe Marschall]

Hello

There has been a lot of talk on this list about how startup time and 
footprint of WildFly can be reduced even further. I have experimented 
with AppCDS and the first results are encouraging.

As you may be aware Application Class-Data Sharing, or AppCDS for short, 
is available in OpenJDK 10 [1].

My work is based on the excellent talk of Volker Simonis on the subject 
at FOSEM [2] and his cl4cds [3] tool. I recommend viewing his 
presentation first, it will provide a lot of background.

To get started you first need download an OpenJDK 10 early access build 
from [4]. It is important to not download an OracleJDK build as AppCDS 
is missing there for some reason.

Then you need to dump the list of loaded classes

export 
PREPEND_JAVA_OPTS="-Xlog:class+load=debug:file=/tmp/wildfly.classtrace"

./bin/standalone.sh

Followed by converting that to a class list suitable for AppCDS, this is 
where the cl4cds tool comes in

$JAVA_HOME/bin/java -jar ~/git/cl4cds/target/cl4cds-1.0.0-SNAPSHOT.jar 
/tmp/wildfly.classtrace /tmp/wildfly.cls

Then you can create the shared archive

export PREPEND_JAVA_OPTS="-Xshare:dump -XX:+UseAppCDS 
-XX:SharedClassListFile=/tmp/wildfly.cls -XX:+UnlockDiagnosticVMOptions 
-XX:SharedArchiveFile=/tmp/wildfly.jsa"

./bin/standalone.sh

and then finally you can start WildFly with the shared archive

export PREPEND_JAVA_OPTS="-Xshare:on -XX:+UseAppCDS 
-XX:+UnlockDiagnosticVMOptions -XX:SharedArchiveFile=/tmp/wildfly.jsa"

./bin/standalone.sh

I checked the startup time reported by an "empty" WildFly 11. I realize 
this is not the most scientific way. The startup time went down from 
about 2000ms to about 1500ms or by about 25%. I did not have a look at 
the memory savings when running multiple WildFly versions is parallel.

One thing I noted is that the Xerces classes should probably be 
recompiled with bytecode major version 49 (Java 1.5) or later, otherwise 
they can not be processed by AppCDS.

Unfortunately AppCDS is quite hard to use, I don't know if the WildFly 
project can somehow help to make this easier. One option would be to 
ship a class list file but I don't know how portable that is. Also as 
WildFly services are lazy it only a fraction of the required classes may 
be in there.

  [1] http://openjdk.java.net/jeps/310
  [2] https://fosdem.org/2018/schedule/event/class_data_sharing/
  [3] https://simonis.github.io/cl4cds/
  [4] http://jdk.java.net/10/

Cheers
Philippe