Specifications

26 IBM System z Personal Development Tool: Volume 3 Additional Topics

About Java versions

The exact version of Java that your application uses greatly affects how you can go about

tweaking it. If you have Java installed, you can check the version on your system by running

the java executable from an OMVS prompt:

$ (cd to the directory with the Java bin files

)

$ (if Java is in your search path there is no need to cd to it)

$ java -version

Java(TM) SE Runtime Environment (build pxz6460sr6-20090729_05(SR6))

IBM J9 VM (build 2.6, JRE 1.6.0 IBM J9 2.6 Linux s390x-64 20090731_039920

(JIT enabled, AOT enabled)

J9VM - 20090731_039920

JIT - dev_20090804_1730

GC - R26_head_20090731_1122_B39896

J9CL - 20090715_1250)

JCL - 20090727_01

The tips presented in this section apply specifically to the current IBM J9 Java Virtual

Machine. The older Classic or Sovereign JVMs are not relevant.

Unless you are using a JVM bundled with a middleware application, you should try to install

and use the latest version of the IBM JVM, because it is likely to give you the best

performance.

Also, note that many Java-based middleware applications ship with their own copies of the

JVM. As a first step you will have to determine exactly which JVM is being invoked by your

application. Furthermore, you may have to locate either the startup script or the JVM options

file that the middleware may be using. Unfortunately, this varies from application to application

and cannot be generally documented.

General principles

The JVM used with z/OS is tuned for large systems and maximum throughput, rather than

being tuned for a quick startup. It does not perform too well on zPDT as far as startup

performance is concerned; startup of a GUI-based application may seem quite sluggish. The

primary way to improve this is by telling the JVM to optimize for startup performance rather

than throughput. In a nutshell, the most important change is to provide the '-Xquickstart'

option to the JVM. This can be done in two ways.

JVM command line

If you can locate the command line being used by your program to invoke the JVM, you can

simply add this option near the start of the JVM command statement. This command line may

be present in the start-up script of the application. For example,

$ java -Xquickstart <other JVM options> MyProgram <program options>

This is the best method to provide the option. However, it may not be trivial to locate the exact

startup script that launches your application and the location of the JVM command inside it.

Environment variable

If you cannot locate the JVM command line responsible for running your application, you can

achieve a similar effect by setting an environment variable. Note that this environment

variable must be set inside the OS (zOS, Linux for System z) running inside zPDT.

In an earlier AD-CD system we found a set of Java executables in /usr/lpp/java/J6.0/bin. This was after IPLing

our AD system with the BC parameter. This detail tends to change with every AD-CD release.