Java virtual machine settings

Use this page to view and change the Java virtual machine (JVM) configuration settings of a process for an application server.

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

For i5/OS and distributed platforms base WebSphere Application Server and the WebSphere Application Server - Express, click Servers > Application Servers > server1. Then, under Server Infrastructure, click Java and Process Management > Process definition > Java Virtual Machine

Configuration tab

Classpath

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

If you 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.

Enter each classpath entry into a table row. You do not need to add the 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:
  • An inspection or monitoring tool to your system.
  • JAR files for a product that runs on top of this product.
  • JVM diagnostic patches or fixes.
Processing errors might occur if you add classpaths to this field that specify the location of the following items:
  • JAR files for resource providers, such as DB2. The paths to these JAR files should be added to the relevant provider class paths.
  • A user JAR file that is used by one or more of the applications that you are running on the product. The path to this type of JAR file should be specified within each application that requires that JAR file, or in server-associated shared libraries.
  • An extension JAR file. If you need to add an extension JAR file to your system, you should use the ws.ext.dirs JVM custom property to specify the absolute path to this JAR file. You can also place the JAR file in the WAS_HOME/lib/ext/ directory, but using the ws.ext.dirs JVM custom property is the recommended approach for specifying the path to an extension JAR file.
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 you need to add multiple classpaths to this field, you can use either a colon (:) or semi-colon (;), depending on which operating system the JVM 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:
  • An inspection or monitoring tool to your system.
  • JAR files for a product that runs on top of this product.
  • JVM diagnostic patches or fixes.
Processing errors might occur if you add classpaths to this field that specify the location of the following items:
  • JAR files for resource providers. such as DB2. The paths to these JAR files should be added to the relevant provider class paths.
  • A user JAR file that is used by one or more of the applications that you are running on the product. The path to this type of JAR file should be specified within each application that requires that JAR file, or in server-associated shared libraries.
  • An extension JAR file. If you need to add an extension JAR file to your system, you should use the ws.ext.dirs JVM custom property to specify the absolute path to this JAR file. You can also place the JAR file in the WAS_HOME/lib/ext/ directory, but using the ws.ext.dirs JVM custom property is the recommended approach for specifying the path to an extension JAR file.
Data type String

Verbose class loading

Specifies 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

Specifies 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

Specifies 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

Specifies 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

Specifies 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.

Data type Integer
Default 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

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.

This setting only applies to the base WebSphere Application Server and the WebSphere Application Server - Express products.

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

Data type Boolean
Default false

HProf arguments

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

This setting only applies to the base WebSphere Application Server and the WebSphere Application Server - Express products.

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

Data type String

Debug mode

Specifies 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 you must 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.

For base WebSphere Application Server and the WebSphere Application Server - Express configurations, Debug arguments are only required if the Debug Mode property is set to true. If you enable debugging on multiple application servers, 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 enter in the Generic JVM arguments field. If you enter more than one argument, enter a space between each argument.
Important: 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

    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 disables Java class verification, which can provide a 10-15% improvement in startup time. However corrupted or invalid class data is not detected when this option is specified. If corrupt class data is loaded, the Java Virtual Machine (JVM) might behave in an unexpected manner, or the JVM might fail.

    Avoid trouble:
    • Do not use this option if you are making bytecode modifications, because the JVM might fail if any instrumentation error occurs.
    • If, while this option is in affect, you experience a JVM failure or the JVM behaves in an unexpected manner, remove this option as your first step in debugging your JVM problem.
    gotcha
  • -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. Class garbage collection is enabled by default.

    [Version 6.0.2] If the same set of classes are garbage collected repeatedly in your workload, you should disable garbage collection.

    If you specify the -Xnoclassgc argument, whenever you redeploy an application, you should always restart the application server to clear the classes and static data from the pervious version of the application.

  • -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 are running on your machine.

    [jun2010] Specify -Xgcthreads as follows:

    -Xgcthreads<number of processors>

    Avoid trouble: Do not add a space between --Xgcthreads and the n value for the number of processors.

    -Xgcthreads5 is an example of specifying -Xgcthreads with 5 processors.

    gotcha
    [jun2010]
    jun2010

    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 this value 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 WebSphere 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 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
    -XX:SurvivorRatio=new_ratio_size 
    The default values are:
    • NewSize=2m
    • MaxNewSize=32m
    • SurvivorRatio=32
    Best practice: However, if you have a JVM with more than 1 GB heap size, you should use the following values:
    • -XX:NewSize=640m
    • -XX:MaxNewSize=640m
    • -XX:SurvivorRatio=16
    bprac

    Alternatively, you could set 50% to 60% of the 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 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.

  • [jun2010] -XXallowvmshutdown:false

    Use the -XXallowvmshutdown:false argument to revert to a previous behavior for the JVM that is not correct. Java 5.0 SR10 and Java 6 SR5 correct issues in which the Java Virtual Machine (JVM) does not shut down correctly. If you have an application that depends on the old behavior, you can revert to the previous behavior by adding the this argument to the Generic JVM arguments section.

    [jun2010]
    jun2010
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

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.

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

Operating system name

Specifies JVM settings for a given operating system.

For the base WebSphere Application Server and the WebSphere Application Server - Express products, when the process starts, the process uses the JVM settings specified for the server as the JVM settings for the operating system.

Data type String



Related tasks
Configuring the JVM
Tuning the application serving environment
Related reference
Custom property collection
Reference topic    

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Last updated: Aug 29, 2010 6:22:59 PM CDT
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