IBM Semeru Runtime Certified Edition for z/OS 21: Supplementary documentation

Content

The supplementary documentation is updated only if there are notable changes, such as special advisories, in a particular release. This document was last updated during 21.0.12.0 release.

The documentation to support this version is available in IBM Documentation. For links to downloads, fixes, time zone updates, and a table of OpenJDK versions on which this release is based, see IBM Semeru Runtime Certified Edition for z/OS refreshes.

Supplementary information is available for the following code refreshes:

• 21.0.12.0
• 21.0.9.1
• First release

21.0.12.0 (September 2026)

Health Center might trigger unexpected global GC with compaction

Starting with IBM Semeru Runtime Certified Edition for z/OS release 21.0.12.0 (OpenJ9 v 0.60), enabling the Health Center agent might cause a stop-the-world (STW) global garbage collection (GC) with compaction every 30 seconds. The GC is reported in the verbose GC log with the reason "rasdump".

This scenario occurs because Health Center collects thread information every 30 seconds by using ThreadMXBean.getThreadInfo() with lockedSynchronizers=true. In OpenJ9 v0.6.0 and later, this operation might require the JVM to rebuild its internal Ownable Synchronizer list by walking the heap, which triggers a stop-the-world compacting GC before the heap walk can proceed.

Workaround

Until a fix is available in a future Health Center release, disable Health Center thread data collection by specifying -Dcom.ibm.diagnostics.healthcenter.data.threads=off. This setting prevents the thread data collection that triggers the unexpected GC.

21.0.9.1 (December 2025)

JZOS abnormal end when initiated from JCL

Initiating the Java Batch Launcher and Toolkit for z/OS (JZOS) with System Management Facility (SMF) 121 from a Job Control Language (JCL) script might cause an ABEND with error code S422 U0000 under CN(INTERNAL). Until a resolution is implemented, use the previous Java 21 release (21.0.8.0 or earlier version) for JZOS functions.

First release 21.0.4.0 (November 2024)

Incorrect interpretation of EBCDIC console output from child processes

Under certain scenarios, EBCDIC encoded Standard Output or Standard Error console output of child processes may be garbled when read by its parent process, as the contents are incorrectly interpreted based on UTF-8 being set as the file.encoding property value by default. A possible workaround is to use the command line option -Dfile.encoding=COMPAT to instruct the application to use native encoding instead of the default UTF-8. This known issue will be addressed in a subsequent update. For more information, see File encoding and UTF-8 as default charset.