IZ76742: DELAYED CLASSLOADER UNLOADING WITH -XGCPOLICY:GENCON

APAR status

• Closed as program error.

Error description

• Error Message: N/A
  .
  Stack Trace: N/A
  .
  Not enough global GC's are kicked off with the gencon GC policy
  causing a large number of classloaders to accumulate.  This
  build up of classloaders causes a native memory increase which
  could possibly lead to an out of memory.
  

Local fix

• Use a different GC policy (e.g. use -Xgcpolicy:optavgpause)
  

Problem summary

• The problem is caused when there are a large number of
  classloaders that are being created within a small timeframe.
  The gencon gcpolicy is a based on a generational garbage
  collection strategy which is designed for maximizing application
  runtime and minimizing frequent global gcs.
  As classloader unloading only occurs during a global gc, if
  there are thousands of classloaders being created and discarded
  between global gc events this can lead to high native memory
  overhead, as a portion of the classloader is stored in native
  memory, and potentially lead to an out of memory.
  This can also lead to pause times in a global gc if there are a
  large number of classloaders to be unloaded.  This can be seen
  in the verbosegc logs with the GCMV analyzer or viewing the logs
  and spotting events like this :
      <classloadersunloaded count="249805" timetakenms="2047.082"
  />
  Time and number of classloaders unloaded will be application
  dependent.
  

Problem conclusion

• This defect will be fixed in:
  5.0.0 SR9
  6.0.0 SR3
  .
  There is a java command line option available for tuning the
  frequency that a global gc will take place based on the
  classloader creation rate.
  -Xgc:classUnloadingKickoffThreshold=<number>
  The setting of this value will be completely dependent on the
  application and the desired frequency of global gc. More
  information on gcpolicys can be found here :
  http://www.ibm.com/developerworks/java/library/j-ibmjava2/
  Analyzing if this option is for you can be performed on
  verbosegc logs by Garbage Collection and Memory Visualizer
  (GCMV) available through the IBM Support Assistant.
  http://www-01.ibm.com/software/support/isa/
  .
  To obtain the fix:
  Install build 20100603 or later
  

Temporary fix

Comments

APAR Information

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

  IZ76735

• 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

• R600 PSN

     UP