Java virtual machine settings

To view, and change the JVM configuration settings of a process for an application server.

Information Value
Application server Servers | Server Types | WebSphere application servers | server_name | Server Infrastructure | Java and process management | Process definition | Java virtual machine
dmgr System Administration | dmgr | 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. You 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 you 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. You do not need to add the colon or semicolon at the end of each entry.

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 you add classpaths to this field that specify the location of the following items:


Verbose class loading

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

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

Whether to use verbose debug output for garbage collection. The default is not to enable verbose garbage collection.

If verbose garbage collection is enabled, the debug output is sent to one of the native process logs.

Information Value
Data type Boolean
Default false

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

We can check the verboseGC report to determine:

For transitioning users: Version 7.0 and previous versions use the optthruput garbage collection algorithm. In Version 8.0 and later, the default is set to the generational garbage collector. This garbage collection algorithm can increase performance. The following JVM option is added to the WAS startup command: -Xgcpolicy:gencon. If we prefer to use the optthruput garbage collection alogorithm, we can remove -Xgcpolicy:gencon and the optthruput garbage collection algorithm is used.trns


Verbose JNI

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.

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 improves throughput until the heap becomes too large to reside 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 that is available to the JVM code. If this field is left blank, the default value is used.

The default maximum heap size is 256 MB. This default value applies for both 32-bit and 64-bit configurations.

Increasing the maximum heap size setting can improve startup. When you 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 reside 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 specified for this property allows the heap to be contained within physical memory.

Best practice: 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.


Run HProf

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

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 we enter more than one argument, enter a space between each argument.

Avoid trouble: 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-Packardgotcha

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

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

If we 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 specified for the node as the JVM settings for the operating system.


Related tasks

  • Configure the JVM
  • Tune the application serving environment

    Custom property collection

    Process logs