IV80558: POOR EXECUTION PERFORMANCE DUE TO OVER-AGGRESSIVE LOOP OPTIMIZAT ION

APAR status

• Closed as program error.

Error description

• Error Message: The performance problem will show up as poor
  execution performance on a computationally bound workload
  running in a relatively tight iterative loop. Profiling will
  show large amounts of execution time spent at high method
  offsets running cold code.
  .
  Stack Trace: N/A
  .
  

Local fix

• This issue can be worked around by applying the following Java
  option to methods which may be suffering from this problem:
  -Xjit:{package/Class.method(signature)}(disableLoopVersioner)
  This option will, in general, cause noticeable performance
  degradation in correctly optimized methods and should be used
  with great care.
  

Problem summary

• The Loop Versioner optimization in the JIT compiler attempts to
  speed up loop execution by speculatively moving tests out of a
  loop body when they can be conservatively tested in a loop
  invariant manner ahead of the loop. In some circumstances it was
  possible for these optimizations to create a mutually exclusive
  sequence of tests leading to the hot loop being recognized as
  dead code and the unoptimized loop being executed at runtime.
  

Problem conclusion

• The loop related optimizations in the JIT compiler have been
  enhanced to avoid generating conflicting tests to allow the
  optimized loop to execute to the greatest extent possible.
  .
  This APAR will be fixed in the following Java Releases:
     7    SR10      (7.0.10.0)
     8    SR2 FP10  (8.0.2.10)
     6    SR16 FP20 (6.0.16.20)
  .
  Contact your IBM Product's Service Team for these Service
  Refreshes and Fix Packs.
  For those running stand-alone, information about the available
  Service Refreshes and Fix Packs can be found at:
             https://www.ibm.com/developerworks/java/jdk/
  

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

• R260 PSY

     UP

• R130 PSY

     UP

• R600 PSY

     UP