Java virtual machine settings
Overview
Use this page to view and change the Java virtual machine (JVM) configuration for the application server's process.
To view this administrative console page
Servers | Application | Servers | servername | Process Definition | process | Java Virtual Machine
Configuration tab
- Classpath
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
- Bootstrap classes and resources for JVM code.
This option is only available for JVM instructions that support bootstrap classes and resources. We 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
- Initial heap size available to the JVM code, in megabytes.
Increasing the minimum heap size can improve startup. 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.
Data type Integer Default The default is 50. - Maximum Heap Size
- Maximum heap size available to the JVM code, in megabytes.
Increasing the heap size can improve startup. By increasing heap size, one 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.
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
- Command-line debug arguments to pass to the JVM code that starts the application server process. We 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
- 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 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, one cannot use the argument with another JVM, such as the Sun JDK or the HP JDK.
- -Xquickstart
Use -Xquickstart for initial compilation at a lower optimization level than in default mode. Later, depending on sampling results, one 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, one can improve startup time between 15-20%.
The -Xquickstart option is not supported on OS/400.
- -Xverify:none
When using this value, 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. We 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
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, -Xgcthreadsn, 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
Disable 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.
- -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. We can compute a good initial system heap size by knowing the number of classes loaded in the WAS product, which is about 8,000 classes, and their average size. Having knowledge of the applications helps you include them in the calculation. Use this argument only with the IBM Developer Kit.
- -Xgpolicy
Set the garbage collection policy.
If the garbage collection policy 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].We can use this argument only with the IBM Developer Kit.
- -XX
The Sun-based Java Development Kit V1.4.2 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, one 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, one 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)
- -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
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%).
- -server | -client:
Java HotSpot Technology in the Sun-based Java Development Kit V1.4.2 introduces an adaptive JVM containing algorithms for optimizing byte code execution over time. The JVM runs in two modes...
- -server
- -client
If you use the default -client mode, there will be a faster startup time and a smaller memory footprint, but lower extended performance. We 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. We 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
- 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
- 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
See Also
Java virtual machines (JVMs)
Related Tasks
Using the JVM
Tuning the application serving environment