IJ38083: UNEXPECTEDLY HIGH CPU IN HEAVILY MULTI-THREADED CODE

APAR status

• Closed as program error.

Error description

• Error Message: After upgrading the JVM to 8 SR5 FP21 or later an
  unexpected increase in cpu usage may be seen with some
  applications.
  .
  Stack Trace: N/A
  .
  

Local fix

• Profiling will typically indicate that a single method is
  consuming most of the cpu. For an example method
  "package.Classname.methodname" the following command line option
  will reduce the time spent in a method exhibiting this problem:
  -Xjit:{package/Classname.methodname(*}(disableInlineCheckcast),d
  ontInline={package/Classname.methodname(*}
  Note that there can only be one Xjit option that is active on
  the command line, so if there is already an existing Xjit: it
  will need to be merged with the above.
  

Problem summary

• This is a rare problem caused by a change to the JIT compiler in
  release 8 SR5 FP21 which affects x86 platforms only. The change
  introduced a class cache slot for inlined instanceof and
  checkcast sequences that is updated with the most recent match.
  In cases where a specific instanceof test is exposed to multiple
  matching values, and the code is actively running in a heavily
  multithreaded application, the continual updates of the class
  cache slot result in poor performance due to the resulting cache
  coherency overheads.
  Instruction level profiling will show an extremely high number
  of cpu ticks on a small number of memory access instructions at
  PC relative addresses, i.e. to data stored at the end of the
  method body. Note that the ticks can be for any access into the
  same cache line as the contended class location, which can make
  the diagnosis a little more confusing.
  

Problem conclusion

• The JIT is updated to avoid contention on a cache slot for the
  class in instanceof and checkcast tests.
  .
  This APAR will be fixed in the following Releases:
  .
  IBM SDK, Java Technology Edition
     8    SR7 FP15  (8.0.7.15)
  .
  Contact your IBM Product's Service Team for these Service
  Refreshes and Fix Packs.
  For those running stand-alone, information about the available
  maintenance can be found at:
             https://www.ibm.com/support/pages/java-sdk
  

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

  JIT

• Fixed component ID

  620700124

Applicable component levels