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Java virtual machine settings

To view this administrative console page, connect to the administrative console and navigate to the Java virtual machine panel.

Information Value
Application server Servers | Server Types | WebSphere Application Servers | server_name | Server Infrastructure section | Java and process management | Process definition | Java virtual machine
Deployment manager System Administration | Deployment manager | Server Infrastructure | Java and process management | Process definition | Java virtual machine
Node agent System Administration | Node agent | node_agent | Server Infrastructure | Java and process management | Process definition | Java virtual machine


Classpath

Standard class path in which the Java virtual machine code looks for classes.

If we need to add a classpath to this field, enter each classpath entry into a separate table row. We do not have to add a colon or semicolon at the end of each entry.

The only classpaths that should be added to this field are the ones that specify the location of the following items:

Processing errors might occur if we add classpaths to this field that specify the location of the following items:

Information Value
Data type String


Boot classpath

Specifies bootstrap classes and resources for JVM code. This option is only available for JVM instructions that support bootstrap classes and resources.

If we need to add a classpath to this field, enter each classpath entry into a table row. We do not need to add the colon or semicolon at the end of each entry.

If we need to add multiple classpaths to this field, we can use either a colon (:) or semi-colon (;), depending on which operating system the node resides, to separate these classpaths.

The only classpaths that should be added to this field are the ones that specify the location of the following items:

Processing errors might occur if we add classpaths to this field that specify the location of the following items:


Verbose class loading

Specifies whether to use verbose debug output for class loading. The default is to not enable verbose class loading.

(Dist) If verbose class loading is enabled, the debug output is sent to one of the native process logs.

Information Value
Data type Boolean
Default false


Verbose garbage collection

Specifies whether to use verbose debug output for garbage collection. The default is to enable verbose garbage collection.

If verbose garbage collection is enabled, the debug output is sent to 10 rotating verbosegc log files, which can contain about 7000 garbage collection cycles, with an approximate size of 10 M. For the z/OS platform, the verbosegc logs are written to the configuration file system. The performance overhead of verbose garbage collection is negligible.

Information Value
Data type Boolean
Default true

To revert the logging behavior for verbose garbage collection to that of releases or fix packs earlier than 9.0.0.3, disable verbose garbage collection and enter the standard JVM option -verbose:gc as a generic JVM argument. This option directs the JVM to send the debug output to a native process log. We can enter extra non-standard JVM options as generic JVM arguments to customize the logging behaviors of verbose garbage collection.

When this field is enabled, a report is written to the output stream each time the garbage collector runs. This report gives you an indication of how the Java garbage collection process is functioning.

We can check the verboseGC report to determine:

(ZOS) On the z/OS platform, we can also issue the MVS console command, modify display, jvmheap, to display JVM heap information. In addition, we can check the server activity and interval SMF records. The JVM heap size is also made available to PMI and can be monitored using the Tivoli Performance Viewer.


Verbose JNI

Specifies whether to use verbose debug output for native method invocation. The default is not to enable verbose Java Native Interface (JNI) activity.

Information Value
Data type Boolean
Default false


Initial heap size

Specifies, in megabytes, the initial heap size available to the JVM code. If this field is left blank, the default value is used.

(ZOS) For z/OS, the default initial heap size for the controller is 256 MB, and the default initial heap size for the servant is 512 MB.

For IBM i and distributed platforms, the default initial heap size is 50 MB.

These default values are sufficient for most applications.bprac

For IBM i, the initial heap size must always be less than the maximum heap size. Never set the initial heap size and maximum heap size properties to the same value.

Increasing this setting can improve startup. The number of garbage collection occurrences are reduced and a 10 percent gain in performance is achieved.

Increasing the size of the Java heap continues to improve throughput until the heap becomes too large to be in physical memory. If the heap size exceeds the available physical memory, and paging occurs, there is a noticeable decrease in performance.


Maximum heap size

Specifies, in megabytes, the maximum heap size available to the JVM code. If this field is left blank, the default value is used.

The default maximum heap size is 25% of the total amount of system memory up to 4 GB, or the JVM default whenever memory size is inaccessible.

Increasing the maximum heap size setting can improve startup. When we increase the maximum heap size, you reduce the number of garbage collection occurrences with a 10 percent gain in performance.

Increasing this setting usually improves throughput until the heap becomes too large to be in physical memory. If the heap size exceeds the available physical memory, and paging occurs, there is a noticeable decrease in performance. Therefore, it is important that the value we specify for this property allows the heap to be contained within physical memory.

(ZOS) For z/OS, the default maximum heap size for the controller is 512 MB, and the default maximum heap size for the servant is 1024 MB. To prevent paging, specify a value for this property that allows a minimum of 256 MB of physical memory for each processor and 512 MB of physical memory for each application server. If processor use is low because of paging, increase the available memory, if possible, instead of increasing the maximum heap size. Increasing the maximum heap size might decrease performance rather than improving performance.

These default values are appropriate for most applications. Enable the Verbose garbage collection property if you think garbage collection is occurring too frequently. If garbage collection is occurring too frequently, increase the maximum size of the JVM heap.bprac

Run HProf

Specifies whether to use HProf profiler support. To use another profiler, specify the custom profiler settings using the HProf Arguments setting. The default is not to enable HProf profiler support.

If we set the Run HProf property to true, then specify command-line profiler arguments as values for the HProf Arguments property.

Information Value
Data type Boolean
Default false

HProf arguments

Specifies command-line profiler arguments to pass to the JVM code that starts the application server process. We can specify arguments when HProf profiler support is enabled.

HProf arguments are only required if the Run HProf property is set to true.


Debug mode

Specifies whether to run the JVM in debug mode. The default is to not enable debug mode support.

If we set the Debug mode property to true, then specify command-line debug arguments as values for the Debug arguments property.

Information Value
Data type Boolean
Default false


Debug arguments

Specifies command-line debug arguments to pass to the JVM code that starts the application server process. We can specify arguments when the Debug mode property is set to true.

If we enable debugging on multiple application servers on the same node, verify that the same value is not specified for the address argument. The address argument defines the port used for debugging. If two servers, for which debugging is enabled, are configured to use the same debug port, the servers might fail to start properly. For example, both servers might still be configured with the debug argument address=7777, which is the default value for the debug address argument.

Information Value
Data type String
Units Java command-line arguments


Generic JVM arguments

Specifies command-line arguments to pass to the Java virtual machine code that starts the application server process.

We can enter the following optional command-line arguments in the Generic JVM arguments field. If you enter more than one argument, enter a space between each argument.

If the argument states that it is only for the IBM Developer Kit only, we cannot use that argument with the JVM from another provider, such as the Microsoft or Hewlett-Packar.

Information Value
Data type String
Units Java command-line arguments


Executable JAR file name

Specifies a full path name for an executable JAR file that the JVM code uses.

Information Value
Data type String
Units Path name


Disable JIT

Specifies whether to disable the just-in-time (JIT) compiler option of the JVM code.

If you disable the JIT compiler, throughput decreases noticeably. Therefore, for performance reasons, keep JIT enabled.

Information Value
Data type Boolean
Default false (JIT enabled)
Recommended JIT enabled


Operating system name

Specifies JVM settings for a given operating system.

When the process starts, the process uses the JVM settings that are specified for the node as the JVM settings for the operating system.

  1. Balanced garbage collection as a new option
  2. Configure the JVM
  3. Tune the application serving environment
  4. Custom property collection
  5. Process logs