Java virtual machine settings

 

Java virtual machine settings

 

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Use this page to view and change the JVM configuration for the application server's process.

To view this administrative console page, click...

Servers | Application Servers | server_name | Server Infrastructure | Process Definition | Java Virtual Machine

Configuration tab

Classpath

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

Enter each classpath entry into a table row. You do not need to add the colon or semicolon at the end of each entry.

Data type String
Units Class path

Boot Classpath

Specifies bootstrap classes and resources for JVM code. This option is only available for JVM instructions that support bootstrap classes and resources. You can separate multiple paths by a colon (:) or semi-colon (;), depending on operating system of the node.

Data type String

Verbose Class Loading

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

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.

Data type Boolean
Default false

Verbose JNI

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

Data type Boolean
Default false

Initial Heap Size

The initial heap size available to the JVM code, in megabytes.

Increasing the minimum heap size can improve startup and runtime. The number of garbage collection occurrences are reduced and a 10% gain in performance is realized.

Increasing the size of the Java heap improves throughput until the heap no longer resides in physical memory, in general. After the heap begins swapping to disk, Java performance suffers drastically. For the iSeries Java Virtual Machine, the minimum heap size must always be less than the maximum heap size. Never set the minimum and maximum heap size to the same value.

Data type Integer
Default The default is 96.

Maximum Heap Size

The maximum heap size available to the JVM code, in megabytes.

Increasing the heap size can improve startup. By increasing heap size, you can reduce the number of garbage collection occurrences with a 10% gain in performance.

Increasing the size of the Java heap improves throughput until the heap no longer resides in physical memory, in general. After the heap begins swapping to disk, Java performance suffers drastically. Set the maximum heap size low enough to contain the heap within physical memory.

On iSeries, the default setting of 0 indicates that no maximum is set. As a result, garbage collector (GC) runs only when the GC threshold has been reached. If a maximum heap size is set, the collector runs whenever the heap reaches that maximum size. However, unlike a normal GC, if the maximum size is reached, all application threads must wait until the collector has finished before they can continue running (synchronous garbage collection). This may result in undesirable pause times. Therefore, it is preferred to use the maximum heap size as a safety net to handle times of unexpected heap growth and ensure that the heap doesn't grow larger than the available memory. The GC threshold should be set so that this maximum size is never actually reached under normal circumstances.

Data type Integer
Default 0 for iSeries, 256 for all other platforms. Keep the value low enough to avoid paging or swapping-out-memory-to-disk.

Debug Mode

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

If you set the Debug Mode property to true, then specify command-line debug arguments as values for the Debug Arguments property.

Data type Boolean
Default false

Debug Arguments

Specifies command-line debug arguments to pass to the JVM code that starts the application server process. You can specify arguments when Debug Mode is enabled.

Debug arguments are only required if the Debug Mode property is set to true. If you enable debugging on multiple application servers on the same node, make sure that the servers are using different address arguments, which define the port for debugging.

For example, if you enable debugging on two servers and leave the default debug port for each server as address=7777, the servers could fail to start properly.

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.

The following are optional command line arguments that you can use by entering them into the General JVM Arguments field. If you enter more than one argument, separate each argument by a space.

If the argument says it is for the IBM Developer Kit only, you cannot use the argument with another JVM, such as the Sun JDK or the HP JDK.

  • -Xquickstart

    Initial compilation at a lower optimization level than in default mode. Later, depending on sampling results, you can recompile to the level of the initial compile in default mode. Use -Xquickstart for applications where early moderate speed is more important than long run throughput. In some debug scenarios, test harnesses and short-running tools, you can improve startup time between 15-20%.

    The -Xquickstart option is not supported on OS/400.

  • -Xverify:none

    The class verification stage is skipped during class loading . By using -Xverify:none with the just in time (JIT) compiler enabled, startup time is improved by 10-15%.

    The -Xverify:none option is not supported on OS/400.

  • -Xnoclassgc

    Disable class garbage collection, which leads to more class reuse and slightly improved performance. The trade-off is that you won't be collecting the resources owned by these classes. You can monitor garbage collection using the verbose:gc configuration setting, which will output class garbage collection statistics. Examining these statistics will help you understand the trade-off between the reclaimed resources and the amount of garbage collection required to reclaim the resources. However, if the same set of classes are garbage collected repeatedly in your workload, you should disable garbage collection. Class garbage collection is enabled by default.

  • -Xgcthreads

    You can use several garbage collection threads at one time, also known as parallel garbage collection. When entering this value in the Generic JVM Arguments field, also enter the number of processors that your machine has, for example, -Xgcthreads n, where n is the number of processors. On a node with n processors, the default number of threads is n. You should use parallel garbage collection if your machine has more than one processor. This argument is valid only for the IBM Developer Kit.

    The -Xgcthreads option is not supported on OS/400.

  • -Xnocompactgc

    Disables heap compaction which is the most expensive garbage collection operation. Avoid compaction in the IBM Developer Kit. If you disable heap compaction, you eliminate all associated overhead.

    The -Xnocompactgc option is not supported on OS/400.

  • -Xinitsh:

    Set the initial heap size where class objects are stored. The method definitions and static fields are also stored with the class objects. Although the system heap size has no upper bound, set the initial size so that you do not incur the cost of expanding the system heap size, which involves calls to the operating system memory manager. You can compute a good initial system heap size by knowing the number of classes loaded in the WebSphere Application Server product, which is about 8,000 classes, and their average size. Having knowledge of the applications helps you include them in the calculation. You can use this argument only with the IBM Developer Kit.

    The -Xinitsh option is not supported on OS/400.

  • -Xgpolicy:

    Set the garbage collection policy. If the garbage collection policy (gcpolicy) is set to optavgpause, concurrent marking is used to track application threads starting from the stack before the heap becomes full. The garbage collector pauses become uniform and long pauses are not apparent. The trade-off is reduced throughput because threads might have to do extra work. The default, recommended value is optthruput.

    Enter the value as -Xgcpolicy:[optthruput|optavgpause]. You can use this argument only with the IBM Developer Kit.

    The -Xgpolicy option is not supported on OS/400.

  • -XX

    The Sun-based Java Development Kit (JDK) Version 1.4.2 has generation garbage collection, which allows separate memory pools to contain objects with different ages. The garbage collection cycle collects the objects independently from one another depending on age. With additional parameters, you can set the size of the memory pools individually. To achieve better performance, set the size of the pool containing short lived objects so that objects in the pool do not live through more then one garbage collection cycle. The size of new generation pool is determined by the NewSize and MaxNewSize parameters. Objects that survive the first garbage collection cycle are transferred to another pool. The size of the survivor pool is determined by parameter SurvivorRatio.

    If garbage collection becomes a bottleneck, you can try customizing the generation pool settings. To monitor garbage collection statistics, use the object statistics in Tivoli Performance Viewer or the verbose:gc configuration setting.

    Enter the following values: -XX:NewSize (lower bound) , -XX:MaxNewSize (upper bound), and -XX:SurvivorRatio=NewRatioSize. The default values are:NewSize=2m MaxNewSize=32m SurvivorRatio=2 However, if you have a JVM with more than 1 GB heap size, you should use the values: -XX:newSize=640m -XX:MaxNewSize=640m -XX:SurvivorRatio=16, or set 50 to 60% of total heap size to a new generation pool.

    The -XX option is not supported on OS/400.

  • -Xminf

    Specify the minimum free heap size percentage. The heap grows if the free space is below the specified amount.

    In reset enabled mode, this option specifies the minimum percentage of free space for the middleware and transient heaps. This is a floating point number, 0 through 1. The default is .3 (30%).

    The -Xminf option is not supported on OS/400.

  • -server | -client

    Java HotSpot Technology in the Sun-based Java Development Kit (JDK) Version 1.4.2 introduces an adaptive JVM containing algorithms for optimizing byte code execution over time. The JVM runs in two modes, -server and -client. If you use the default -client mode, there will be a faster startup time and a smaller memory footprint, but lower extended performance. You can enhance performance by using -server mode if a sufficient amount of time is allowed for the HotSpot JVM to warm up by performing continuous execution of byte code. In most cases, use -server mode, which produces more efficient run-time execution over extended periods. You can monitor the process size and the server startup time to check the difference between -client and -server.

    The -server | -client option is not supported on OS/400.

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.

Data type String
Units Path name

Disable JIT

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.

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

Operating System Name

Specifies JVM settings for a given operating system. When started, the process uses the JVM settings for the operating system of the node .

Data type String



Related concepts
Java virtual machines (JVMs)

Related tasks
Using the JVM
Tuning the application serving environment

Related reference
Custom property collection