Network Deployment (Distributed operating systems), v8.0 > Reference > Sets

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 IBM i and distributed platforms, follow one of the following paths.

Application server	Servers > Server Types > WebSphere application servers > server_name Then, in the Server Infrastructure > Java and process management > Process definition > Java virtual machine
Deployment manager	System Administration > Deployment manager. Then, in the Server Infrastructure > Java and process management > Process definition > Java virtual machine
Node agent	System Administration > Node agent > node_agent. Then, in the 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 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:

• An inspection or monitoring tool to the system.
• JAR files for a product that runs on top of this product.
• JVM diagnostic patches or fixes.

Process 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 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 add an extension JAR file to the system, you should use the ws.ext.dirs JVM custom property to specify the absolute path to this JAR file. We can also place the JAR file in...

  WAS_HOME/lib/ext/

  , 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 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.

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 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:

• An inspection or monitoring tool to the system.
• JAR files for a product that runs on top of this product.
• JVM diagnostic patches or fixes.

Process 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 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 add an extension JAR file to the system, you should use the ws.ext.dirs JVM custom property to specify the absolute path to this JAR file. We can also place the JAR file in...

  WAS_HOME/lib/ext/

  , but using the ws.ext.dirs JVM custom property is the recommended approach for specifying the path to an extension JAR file.

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.

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.

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:

• How much time the JVM is spending performing garbage collection.

  Ideally, you want the JVM to spend less than 5 percent of its processing time doing garbage collection.

  To determine the percentage of time the JVM spends in garbage collection, divide the time it took to complete the collection by the length of time since the last AF and multiply the result by 100. For example:

  83.29/3724.32 * 100 = 2.236 percent
  

  If you are spending more than 5 percent of your time in garbage collection and if garbage collection is occurring frequently, you might need to increase your Java heap size.

• If the allocated heap is growing with each garbage collection occurrence.

  To determine if the allocated heap is growing, look at the percentage of the heap that is remains unallocated after each garbage collection cycle, and verify that the percentage is not continuing to decline. If the percentage of free space continues to decline you are experiencing a gradual growth in the heap size from garbage collection to garbage collection. This situation might indicate that the application has a memory leak.

For transitioning users: v7.0 and previous versions use the optthruput garbage collection algorithm. In Version 8.0, 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 you prefer to use the optthruput garbage collection alogorithm, you can remove -Xgcpolicy:gencon and the default 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.

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.

For IBM i and distributed platforms, the default initial heap size is 50 MB.

These default values are sufficient for most applications.

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 you specify for this property allows the heap to be contained within physical memory.

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 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

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 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. We can specify arguments when the Debug mode property is set to true.

If you 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.

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 you enter more than one argument, enter a space between each argument. If the argument states that it is only for the IBM Developer Kit only, you cannot use that argument with the JVM from another provider, such as the Microsoft or Hewlett-Packard

• -DhotRestartSync:

  Specify -DhotRestartSync if 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 dmgr is not active. Therefore, the service does not have to perform a complete repository comparison when the dmgr or node agent servers restart. Enabling this feature improves the efficiency of the first synchronization operation after the dmgr or a node agent restarts, especially for installations that include mixed release cells, use several nodes, and run several applications.

• -Xquickstart

  Specify-Xquickstart if you want the initial compilation to occur 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 percent.

• -Xverify:none

  Specify-Xverify:none if to skip the class verification stage during class loading . Using -Xverify:none disables Java class verification, which can provide a 10-15 percent improvement in startup time. However corrupted or invalid class data is not detected when this argument is specified. If corrupt class data is loaded, the JVM might behave in an unexpected manner, or the JVM might fail.

  • Do not use this argument if you are making bytecode modifications, because the JVM might fail if any instrumentation error occurs.

  • If you experience a JVM failure or the JVM behaves in an unexpected manner while this argument is in affect, remove this argument as your first step in debugging your JVM problem.

• -Xnoclassgc

  Specify-Xnoclassgc if to disable class garbage collection. This argument results in more class reuse and slightly improved performance. However, the resources owned by these classes remain in use even when the classes are not being called. The performance impact of class garbage collection is typically minimal, and turning off class garbage collection in a Java EE based system, with its heavy use of application class loaders, might effectively create a memory leak of class data, and cause the JVM to throw an Out-of-Memory Exception.

  You can use the verbose:gc configuration setting if to monitor garbage collection. We can use the resulting output to determine the performance impact of reclaiming these resources.

  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

  Specify -Xgcthreads if to use several garbage collection threads at one time. This garbage collection techniques is known as parallel garbage collection. This argument is valid only for the IBM Developer Kit.

  When entering this value in the Generic JVM arguments field, also enter the number of processors that are running on your machine.

  Specify -Xgcthreads as follows:

  -Xgcthreads <number of processors> 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.

  You should use parallel garbage collection if your machine has more than one processor.

• -Xnocompactgc

  Specify -Xnocompactgc if to disable heap compaction. Heap compaction is the most expensive garbage collection operation. If you are using the IBM Developer Kit, you should avoid heap compaction. If you disable heap compaction, you eliminate all associated overhead.

• -Xgcpolicy

  Specify-Xgcpolicy to set the garbage collection policy. This argument is valid only for the IBM Developer Kit. -Xgcpolicy

  Specify-Xgcpolicy to set the garbage collection policy. This argument is valid only for the IBM Developer Kit.

  Set this argument to optthruput if to optimize throughput and it does not create a problem if long garbage collection pauses occur. This is the default parameter, recommended setting.

  Set this argument to gencon, if you are using a generational garbage collector. The generational schema attempts to achieve high throughput along with reduced garbage collection pause times.

  To accomplish this goal, the heap is split into new and old segments. Long lived objects are promoted to the old space while short-lived objects are garbage collected quickly in the new space. The gencon policy provides significant benefits for many applications. However, it is not suited for all applications, and is typically more difficult to tune.

  Set this argument to optavgpause, if you want concurrent marking used to track application threads starting from the stack before the heap becomes full. When this parameter is specified, the garbage collector pauses become uniform and long pauses are not apparent. However, using this policy reduces throughput because threads might have to do extra work.

  Set this argument to subpool, if you want to increase performance on multiprocessor systems, that commonly use more then eight processors. This policy is only available on IBM System i, System p, and System z processors. The subpool policy is similar to the optthruput policy except that the heap is divided into subpools that provide improved scalability for object allocation.

• -XX

  The Java Platform, Standard Edition 6 (Java SE 6) 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 objects that have short life cycles, such that the objects in the pool are not kept through more then one garbage collection cycle. Use the NewSize and MaxNewSize parameters to specify the size of the new generation pool.

  Objects that survive the first garbage collection cycle are transferred to another pool. Use theSurvivorRatio parameter to specify the size of the survivor pool.SurvivorRatio. We can use the object statistics that the Tivoli Performance Viewer collects, or include the verbose:gc argument in the configuration setting to monitor garbage collection statistics. If garbage collection becomes a bottleneck, specify the following arguments to customize the generation pool settings to better fit the environment.

  -XX:NewSize=lower_bound
  -XX:MaxNewSize=upper_bound
   -XX:SurvivorRatio=new_ratio_size
  

  The default values are:

  • NewSize=2m
  • MaxNewSize=32m
  • SurvivorRatio=32

  However, if we 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

  Specify-Xminf if to change the minimum free heap size percentage. The heap grows if the free space is below the specified amount. In reset enabled mode, this argument specifies the minimum percentage of free space for the middleware and transient heaps. The valued specified for this argument is a floating point number, 0 through 1. The default is .3 (30 percent).

• -server | -client

  Java HotSpot Technology in Java SE 6 uses an adaptive JVM containing algorithms that, over time, optimize how the byte code performs. The JVM runs in two modes, -server and -client. In most cases, use -server mode, which produces more efficient run-time performance over extended lengths of time.

  If you use the default -client mode, the server startup time is quicker and a smaller memory footprint is created. However, this mode lowers extended performance. Use the -server mode, which improves performance, unless server startup time is of higher importance than performance. We can monitor the process size, and the server startup time to check the performance difference between using the -client and -server modes.

• -Dcom.ibm.CORBA.RequestTimeout=timeout_interval

  Specify the -Dcom.ibm.CORBA.RequestTimeout= timeout_interval argument to set the timeout period for responding to requests sent from the client. This argument uses the -D option. timeout_interval is the timeout period in seconds. If your network experiences extreme latency, specify a large value to prevent timeouts. If you specify a value that is too small, an application server that participates in workload management can time out before it receives a response.

  Specify this argument only if the application is experiencing problems with timeouts. There are no recommended values for this argument.

• -Dcom.ibm.server.allow.sigkill=

  The -Dcom.ibm.server.allow.sigkill=true argument allows the node agent process to use the terminate method of a process when the stop method does not complete within the time interval specified for the Ping interval. This setting is useful when the node agent is monitoring an application server and loses contact with that application server.

  When the monitoring policy for the application server allows the node agent to restart the application server because automatic restart is enabled for the application server, the node agent executes the stop method on the application server process. During stop processing, the node agent monitors the application server and if the application server does not stop within the time interval specified for the Ping interval, and this argument is set to true, which is the default value, the node agent executes the terminate method on the application server process to stop the application server process.

  If you set this argument to false, the node agent continues to monitor the stop process, but does not try to restart the application server.

  To use the administrative console to disable this argument, click System Administration > Node agents > nodeagent_name > Java & Process Management > Process Definition > Java Virtual Machine > Generic JVM Arguments.

• -Dcom.ibm.websphere.wlm.unusable.interval=interval

  This argument only applies for z/OS. Specify the -Dcom.ibm.websphere.wlm.unusable.interval= timeout_interval argument to change the value of the com.ibm.websphere.wlm.unusable.interval property when the workload management state of the client is refreshing too soon or too late. This property specifies, in seconds, the amount of time that the workload management client run time waits after it marks a server as unavailable before it attempts to contact the server again. This argument uses the -D option. . The default value is 300 seconds. If the property is set to a large value, the server is marked as unavailable for a long period of time. This prevents the workload management refresh protocol from refreshing the workload management state of the client until after the time period has ended.

• -Xshareclasses:none

  Specify the-Xshareclasses:none argument to disable the share classes option for a process. The share classes option, which is available with Java SE 6, lets you share classes in a cache. Sharing classes in a cache can improve startup time and reduce memory footprint. Processes, such as application servers, node agents, and dmgrs, 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 WAS_HOME/bin/clearClassCache.bat/sh utility or stop the process and then restart the process.

  • The IBM JVM for J2SE 5 is not supported on Solaris, HP, and IBM i.
  • J2EE application classes running in an application server process are not added to the shared class cache.

• -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 we 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.

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 the process starts, the process uses the JVM settings specified for the node as the JVM settings for the operating system.
Configure the JVM
Tune the application serving environment

Related

Custom property collection
Process logs

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