IV95073: JIT COMPILED TRAP INSTRUCTIONS CAUSE PROBLEMS ON ZOS

APAR status

• Closed as program error.

Error description

• Error Message: The following problems are possible:
  1) CML Lock contention and zIIP eligible Java workloads
  unexpected shifting to GCPs.
  2) SYSDUMP messages showing 0C7 abends as part of normal Java
  execution with no termination.
  3) Data Exceptions causing MVS dump data set creation and 0C7
  abends terminating the Java process.
  4) Performance issues or regressions for code paths which throw
  a lot of Java exceptions
  .
  Stack Trace: N/A
  .
  This problem only applies to the z/OS platform
  

Local fix

• Trap instructions can be disabled using the following Java
  command line option:
  -Xjit:noResumableTrapHandler
  

Problem summary

• Beginning with Java 7.0 with the -Xaggressive option and Java
  7.1/7R1 by default, the JIT compiler started using trap
  instructions to implement the implicit checks required by the
  Java specification done by the Java Virtual Machine for
  exception cases, e.g. NullPointerExceptions and
  ArrayIndexOutOfBounds.
  Like many optimizations, this involves a trade-off which
  benefits the main path but hurts the exception path.
  The implementation creates a 0C7 Data Exception when the Java
  exception needs to be thrown. This Data Exception signal needs
  to be passed to the JVM in order for Java execution to resume.
  The 4 problems that can arise are explained:
  1) When a high volume of exceptions get thrown in parallel, the
  0C7 Data Exceptions get serialized going through z/OS Recovery
  Termination Manager causing CML Lock contention and this also
  can cause the zIIP eligible Java workload to shift to GCPs.
  2) The 0C7 Data Exceptions will get logged as SYSDUMP 0C7 abends
  if that is not suppressed. A Java program can potentially throw
  thousands of exceptions which can cause a lot of noise in system
  logs.
  3) If the 0C7 Data Exception gets intercepted by a handler which
  does not pass it on to the JVM, a 0C7 abend will happen and a
  dump may be produced and the Java process will terminate.
  Correct signal chaining is required by the Java specification to
  avoid this scenario.
  4) Because of the inherent trade-off in using trap instructions,
  if the exception path is heavily used, then this can have a
  significant and noticeable negative effect on performance. To be
  clear, regardless of how exceptions are implemented in the JVM,
  any Java program that heavily relies on throwing exceptions will
  not be following Java programming best practices and will not
  achieve optimal performance, but the use of trap instructions
  exacerbates this.
  

Problem conclusion

• Trap instructions have been disabled by default on the z/OS
  platform. They may be enabled using the following Java command
  line option:
  -Xjit:enableTraps
  .
  This APAR will be fixed in the following Java Releases:
     7    SR10 FP5  (7.0.10.5)
     7 R1 SR4 FP5   (7.1.4.5)
     8    SR4 FP5   (8.0.4.5)
  .
  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