Java virtual machine tuning parameters

 

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Because the application server is a Java process, it requires a Java virtual machine (JVM) to run, and to support the Java applications running on it.

See also:

• JVM settings
• Java memory tuning tips.

If your application experiences slow response times at startup or first touch, you may want to consider using the Java user classloader cache.

You can adjust these settings that determine how the system uses the JVM:

• Class garbage collection (-Xnoclassgc)

  • Description: Disables class garbage collection so that your applications can reuse classes more easily. You can monitor garbage collection using the -verbosegc configuration setting because its output includes class garbage collection statistics.

  • How to view or set:

    1. Go to...

       administrative console | Servers | Application Servers | application_server | Process Definition | Java Virtual Machine

    2. In the Generic JVM arguments field, type -Xnoclassgc.

    3. Click Apply or OK.

    4. Save the configuration.

    5. Stop and restart the application server.

  • Default value: By default, class garbage collection is enabled.

  • Recommended value: Do not disable class garbage collection.

• Initial heap size

  • Description: The initial heap size specifies how often garbage collection runs, and can have a significant effect on performance.

  • How to view or set:

    1. Go to...

       administrative console | Servers | Application Servers | application_server | Process Definition | Java Virtual Machine

    2. In the Initial Heap Size field, specify a value.

    3. Click Apply or OK.

    4. Save the configuration.

    5. Stop and restart the application server.

  • Default value: For iSeries, the default value is 96.

  • Recommended value: 96MB per processor

• Maximum heap size

  • Description: This parameter specifies the maximum heap size available to the JVM code, in megabytes.

  • How to view or set:

    1. Go to...

       administrative console | Servers | Application Servers | application_server | Process Definition | Java Virtual Machine

    2. The value is displayed in the Maximum Heap Size field.

    3. Click Apply or OK.

    4. Save the configuration.

    5. Stop and restart the application server.

  • Default value: For iSeries, the default value is 0. A value of 0 specifies that there is no maximum value.

  • Recommended value: It is recommended that you do not change the maximum heap size. When the maximum heap size triggers a garbage collection cycle, the iSeries JVM's garbage collection stops operating asynchronously. As a result, the application server cannot process user threads until the garbage collection cycle ends, and performance is significantly lower.

• Just-In-Time (JIT) compiler

  • Description: A Just-In-Time (JIT) compiler is a platform-specific compiler that generates machine instructions for each method as needed. See also:
    • Use the Just-In-Time compiler
    • Just-In-Time compiler

  • How to view or set:

    1. Go to...

       administrative console | Servers | Application Servers | application_server | Process Definition | Java Virtual Machine

    2. If you want to disable JIT, select the checkbox for Disable JIT.

    3. Click Apply or OK.

    4. Save the configuration.

    5. Stop and restart the application server.

  • Default value: By default, JIT is enabled.

  • Recommended value: It is recommended that you do not disable the JIT compiler. The os400.jit.mmi.threshold can have a significant effect on performance.