IV12155: JAVA JIT - LONG GC PAUSE TIMES ON PPC

APAR status

• Closed as program error.

Error description

• Error Message: Garbage collection pause times can go for as log
  as 12 minutes.
  .
  Stack Trace: N/A
  .
  A TProf during the GC pause time will show that the JIT library
  is consuming a lot of CPU time. There will be a lot of wait time
  if running on a multiprocessor computer as the JIT process that
  is making the GC run for so long is single threaded.
  The JIT methods consuming the CPU time will looks something like
  this:
    Total % For All Processes
  (/opt/IBM/WebSphere/AppServer/java/jre/lib/ppc64/compressedrefs/
  libj9jit24.so) = 22.26
  Subroutine                                            %   Source
  
  ==========                                         ====== ======
  
  .TR_RuntimeAssumption::isAssumingMethod(void*)      20.82
  UnloadAssumption.cpp
  id*,TR_RuntimeAssumption**,TR_RuntimeAssumption**)   0.90
  UnloadAssumption.cpp
  ._ptrgl                                              0.30
  ptrgl_64.s
  .TR_UnloadedClassPicSite::getAssumingPC()            0.14
  UnloadAssumption.cpp
  The problem this APAR addresses is only seen on PowerPC (AIX
  and LinuxPPC). The problem can only occur under very high load
  situations where a large number of methods are JIT compiled. The
  problem is very unlikely to be seen unless the application is
  using thousands of dynamically generated classes.
  

Local fix

• The problem can be avoid by disabling the interpreter profiler
  as this will prevent the JIT from generating excessive runtime
  assumptions.
  -Xjit:disableInterpreterProfiling
  Note: Using the above option will have a slight impact on
  performance. In some cases the use of this option might degrade
  performance by up to 10%, possibly more in extreme cases
  

Problem summary

• The JIT would take a long time trying to delete runtime
  assumptions associated with JIT compiled method bodies that are
  no longer required. This is caused by the JIT generating
  excessive class unload assumptions for profile directed call
  devirtualization guard sites.
  

Problem conclusion

• This defect will be fixed in:
  6.0.0 SR10-FP1
  6.0.1 SR2
  7.0.0 SR1
  .
  The JIT was modified so that it only generate class unload
  assumptions when required to do so to maintain correctness. By
  doing so we eliminate runtime assumptions and reduce GC pause
  times when JIT compiled bodies are being deleted.
  

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

• R600 PSY

     UP

• R260 PSY

     UP