IJ59790: HEALTH CENTER TRIGGERS UNEXPECTED GLOBAL GC WITH COMPACTION ON Z/OS

APAR status

• Closed as program error.

Error description

• Error Message: With Health Center enabled, verbose gc log shows
  global GC with compaction every 30 seconds (reason=rasdump).
  <sys-start reason="rasdump" id="67"
  timestamp="2026-09-03T22:48:33.697" intervalms="30998.808" />
    <compact-info movecount="20808" movebytes="797648"
  reason="rasdump" />
  <sys-start reason="rasdump" id="82"
  timestamp="2026-09-03T22:49:04.698" intervalms="31000.763" />
    <compact-info movecount="1311" movebytes="41984"
  reason="rasdump" />
  .
  Stack Trace: N/A
  .
  

Local fix

• Set the following JVM option to disable Health Center thread
  data collection:
  -Dcom.ibm.diagnostics.healthcenter.data.threads=off
  Note: This prevents the thread data collection that triggers the
  unexpected GC.
  

Problem summary

• Health Center collects thread information every 30 seconds using
  
  
  ThreadMXBean.getThreadInfo() with lockedSynchronizers=true. In
  OpenJ9 v0.60 and later, this operation can require the JVM to
  rebuild its internal Ownable Synchronizer list by walking the
  heap, which triggers Global GC with compaction before the heap
  walk can proceed.
  

Problem conclusion

• Health Center has been updated to stop requesting locked
  synchronizer data, preventing the JVM from triggering a global
  GC with compaction every 30 seconds. A new system property has
  been introduced to allow users to re-enable
  locked synchronizer collection if needed accepting the GC cost.
  
  -Dcom.ibm.java.diagnostics.healthcenter.thread.collection.locked
  
  
  synchronizers=true
  The default value is false.
  .
  This APAR will be fixed in the following Releases:
  .
  IBM Semeru Runtimes
   25              25.0.5.0
   21              21.0.13.0
   17              17.0.21.0
   11              11.0.33.0
  IBM SDK, Java Technology Edition
   8    SR8 FP75  (8.0.8.75)
  .
  Downloads and supplementary documentation can be found at the
  following locations:
  - For non z/OS operating systems:
  - IBM Semeru Runtimes, Version 11 and later
    https://www.ibm.com/semeru-runtimes/downloads/
  - IBM SDK, Java Technology Edition, Version 8
    https://www.ibm.com/support/pages/java-sdk-downloads/
  - For the z/OS operating system:
  - Java SDK Products on z/OS
    https://www.ibm.com/support/pages/java-sdk-products-zos
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

Fix information

• Fixed component name

  J9 COMMON CODE

• Fixed component ID

  620700127

Applicable component levels