OpenJ9 tomcat won't start with high -Xmx heap option

Question

I have a Spring app running in a Tomcat 9.0.6 on Linux 64. Because it needs a lot of memory, I would like to try the OpenJ9 JVM which is supposedly more efficient in that regard (current heap limit with Hotspot: -Xmx128G). I installed the 64-bit adoptopenjdk-8-jdk-openj9:

/usr/lib/jvm/adoptopenjdk-8-jdk-openj9/bin/java -version
openjdk version "1.8.0_212"
OpenJDK Runtime Environment (build 1.8.0_212-b04)
Eclipse OpenJ9 VM (build openj9-0.14.2, JRE 1.8.0 Linux amd64-64-Bit Compressed References 20190521_315 (JIT enabled, AOT enable
OpenJ9   - 4b1df46fe
OMR      - b56045d2
JCL      - a8c217d402 based on jdk8u212-b04)

Starting the tomcat causes the following error:

This JVM package only includes the '-Xcompressedrefs' configuration. Please run the VM without specifying the '-Xnocompressedrefs' option or by specifying the '-Xcompressedrefs' option.

After I set this option I get the following error:

JVMJ9GC028E Option too large: '-Xmx'
JVMJ9VM015W Initialization error for library j9gc29(2): Failed to initialize
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

Documentation isn't that clear, but I found this: https://www.ibm.com/support/knowledgecenter/SSYKE2_8.0.0/com.ibm.java.vm.80.doc/docs/mm_gc_compressed_refs.html

Compressed references are used by default on a 64-bit IBM SDK when the value of -Xmx, which sets the maximum Java heap size, is in the correct range. Start of changes for service refresh 2 fix pack 10On AIX®, Linux and Windows systems, the default range is 0 - 57 GB. For larger heap sizes, you can try to use compressed references by explicitly setting -Xcompressedrefs.End of changes for service refresh 2 fix pack 10 However, larger heap sizes might result in an out of memory condition at run time because the VM requires some memory at low addresses. You might be able to resolve an out of memory condition in low addresses by using the -Xmcrs option.

So basically, at least this build of the JDK only supports compressedrefs, and in order to use that, I must set it manually since my Xmx is above the range where it is enabled by default, but that fails because my OS already allocated to much of <4GB memory ranges, but some is needed to use compressedrefs. Since I can never guarantee that that won't be the case, is there any way I can use OpenJ9 without compressedrefs? And will that even yield the benefits in terms of memory consumption? Or is there any way I can use compressedrefs with very high Xmx settings?

I also tried setting this option, but it didn't help: https://www.ibm.com/support/knowledgecenter/SSYKE2_8.0.0/openj9/xmcrs/index.html?view=embed

How do I find the correct size for it? 1G and 64m failed. Even if I find the correct setting, how would this value guarantee that the OS hasn't already allocated all the lower memory addresses?

Answer 1

I found this build which allows noncompressedrefs and thus solves my issues: https://adoptopenjdk.net/releases.html?variant=openjdk8&jvmVariant=openj9#linuxxl

Answer 2

The limit to use the compressed refs JVM is 57G and you can't run it if the -Xnocompressedrefs option is specified.

The 57G division is documented here: https://www.eclipse.org/openj9/docs/xcompressedrefs/

The -Xnocompressedrefs problem is mentioned in the release notes: https://github.com/eclipse/openj9/blob/master/doc/release-notes/0.15/0.15.md

With a reference to: https://github.com/eclipse/openj9/issues/479

Creating a single JVM that supports both is covered by: https://github.com/eclipse/openj9/issues/643

https://github.com/eclipse/openj9/pull/7505

(With thanks to the help from the Eclipse OpenJ9 slack community, especially to Peter Shipton)