Java virtual machine settings

 

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Use this page to view and change the JVM configuration settings of a process for an appserver.

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

For i5/OS and distributed platforms WAS ND:

Application server	Servers | Application Servers | server1 | Server Infrastructure | 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 | nodeagent | Server Infrastructure | Java and Process Management | Process Definition | 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

Specify 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

Specify 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

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

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 idea of what is going on with Java garbage collection. Key things to look for in a verboseGC report are:

• Time spent in garbage collection. Ideally, you want to be spending less than 5% of the time in GC. To determine percentage of time spent in GC, divide the time it took to complete the collection by the time since the last AF and multiply the result by 100. For example,

  83.29/3724.32 * 100 = 2.236%
  
  
  

  If you are spending more than 5% of your time in GC and if GC is occurring frequently, you may need to increase your Java heap size.

• Growth in the allocated heap.

  To determine this, look at the %free. You want to make sure the number is not continuing to decline. If the %free continues to decline you are experiencing a gradual growth in allocated heap from GC to GC which could indicate that your application has a memory leak.

Verbose JNI

Specify 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

Specify the 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	For distributed platforms, the default is 50.

For the majority of applications these defaults should work quite well.

Maximum heap size

Specify 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 usually improves throughput until the heap no longer resides in physical memory. When the heap size exceeds the physical memory, the heap begins swapping to disk which causes Java performance to drastically decrease. Therefore, it is important to set the maximum heap size to a value that allows the heap to be contained within physical memory.

To prevent paging, you should specify a value for this property that allows a minimum of 256MB of physical memory for each processor and 512 MB of physical memory for each appserver. If processor utilization is low because of paging, increase the available memory, if possible, instead of increasing the maximum heap size. Increasing the heap size might make performance worse rather than better.

Data type	Integer
Default	For distributed platforms, the default is 256. Keep the value low enough to avoid paging or swapping-out-memory-to-disk.

For the majority of applications these defaults should work quite well. You can monitor the frequency of garbage collection by enabling Verbose garbage collection field. If garbage collection is occurring too frequently you should increase the maximum size of the JVM heap.

Run HProf

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

Data type	Boolean
Default	false

HProf arguments

Specify command-line profiler arguments to pass to the JVM code that starts the appserver process. You can specify arguments when HProf profiler support is enabled.

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

Data type

String

Debug mode

Specify 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

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

For WAS Network Deployment configurations, Debug arguments are only required if the Debug Mode property is set to true. If you enable debugging on multiple appservers 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

Specify command line arguments to pass to the Java virtual machine code that starts the appserver process. The following are optional command line arguments that you can enter in the Generic JVM arguments field. If you enter more than one argument, enter a space between each argument.

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.

 

• hotRestartSync: You can use hotRestartSync to enable the hot restart sync feature of the synchronization service. This feature indicates to the synchronization service that the installation is running in an environment where configuration updates are not made when the deployment manager is not active. Therefore, the service does not have to perform a complete repository comparison when the deployment manager or node agent servers restart. Enabling this feature improves the efficiency of the first synchronization operation after the deployment manager or a node agent restarts, especially for installations that include mixed release cells, a large numbers of nodes, and run a large numbers of applications.

   

• -Xquickstart

  You can use -Xquickstart for 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%.

   

• -Xverify:none

  You can use -Xverify:none if you want to skip the class verification stage during class loading . Using -Xverify:none with the just in time (JIT) compiler enabled, improves startup time by 10-15%.

   

• -Xnoclassgc

  You can use -Xnoclassgc to disable class garbage collection. This action 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, -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.

   

• -Xnocompactgc

  You can use -Xnocompactgc to 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

  You can use -Xinitsh to 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 WAS 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.

   

• -Xgpolicy

  You can use -Xgpolicy to 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.

   

• -XX

  The Sun-based Java 2 Standard Edition (J2SE) 5 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)
   -XX:SurvivorRatio=NewRatioSize 
  

  The default values are:NewSize=2m MaxNewSize=32m SurvivorRatio=2. However, if you have a JVM with more than 1GB 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.

   

• -Xminf

  You can use -Xminf to 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%).

   

• -server | -client

  Java HotSpot Technology in the Sun-based Java 2 Standard Edition (J2SE) 5 uses 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.

   

• -Xshareclasses:none

  You can use the -Xshareclasses:none argument to disable the share classes option for a process. The share classes option, which is available with Java 2 Standard Edition (J2SE) 5, lets you share classes in a cache. Sharing classes in a cache can improve startup time and reduce memory footprint. Processes, such as appservers, node agents, and deployment managers, can use the share classes option.

  If you use this option, you should clear the cache when the process is not in use. To clear the cache, either call the app_server_root/bin/clearClassCache.bat/sh utility or stop the process and then restart the process.

  The IBM J2SE 5 is currently not used on Solaris, HP, and i5/OS.

Data type	String
Units	Java command line arguments

Executable JAR file name

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

Data type	String
Units	Path name

Disable JIT

Specify 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

Specify JVM settings for a given operating system.

For the ND product, when the process starts, the process uses the JVM settings for the node as the JVM settings for the operating system.

Data type

String

 

Runtime tab

Verbose garbage collection

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

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 idea of what is going on with Java garbage collection. Key things to look for in a verboseGC report are:

• Time spent in garbage collection. Ideally, you want to be spending less than 5% of the time in GC. To determine percentage of time spent in GC, divide the time it took to complete the collection by the time since the last AF and multiply the result by 100. For example,

  83.29/3724.32 * 100 = 2.236%
  
  
  

  If you are spending more than 5% of your time in GC and if GC is occurring frequently, you may need to increase your Java heap size.

• Growth in the allocated heap.

  To determine this, look at the %free. You want to make sure the number is not continuing to decline. If the %free continues to decline you are experiencing a gradual growth in allocated heap from GC to GC which could indicate that your application has a memory leak.

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Related tasks

Configure the JVM
Tuning the application serving environment

 

Related Reference

Custom property collection

 

Reference topic