WAS v8.5 > Reference > Sets

Java virtual machine settings page

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

To view this dmgr console page, connect to the dmgr console and navigate to the Java virtual machine panel...

Servers | Server Types | WebSphere application servers | server_name | Java and process management | Process definition | Java virtual machine

Classpath

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. We 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 your system.
• JAR files for a product that runs on top WAS v8.5.
• 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 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. We 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.

Information	Value
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 a classpath to this field, enter each classpath entry into a table row. We 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, we 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 WAS v8.5.
• 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 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. We 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.

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.

Information	Value
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 page.

If verbose garbage page is enabled, the debug output is sent to one of the native process logs.

Information	Value
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 page.

  Ideally, you want the JVM to spend less than 5 percent of its processing time doing garbage page. To determine the percentage of time the JVM spends in garbage page, 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 page and if garbage collection is occurring frequently, we might need to increase your Java heap size.

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

  To determine if the allocated heap is growing, look at the percentage of the heap that is remains unallocated after each garbage page cycle, and verify 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 page to garbage page. This situation might indicate the application has a memory leak.

For transitioning users: v7.0 and previous versions use the optthruput garbage page algorithm. In v8.0 and later, the default is set to the generational garbage collector. This garbage page 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 page alogorithm, we can remove -Xgcpolicy:gencon and the default optthruput garbage page 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.

Information	Value
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.

Best practice: 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 the value we specify for this property allows the heap to be contained within physical memory.

Best practice: These default values are appropriate for most applications. Enable the Verbose garbage collection property if you think garbage page 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.

Information	Value
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.

Information	Value
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, verify 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.

Information	Value
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, we cannot use that argument with the JVM from another provider, such as the Microsoft or Hewlett-Packard

• -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, we can recompile to the level of the initial compile in default mode.

  Best practice: 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, we can improve startup time between 15-20 percent.

• -Xverify:none

  Specify-Xverify:none 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 to disable class garbage page. 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 page is typically minimal, and turning off class garbage page 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.

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

  If we 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 to use several garbage page threads at one time. This garbage collection techniques is known as parallel garbage page. 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.

  Best practice: You should use parallel garbage page if your machine has more than one processor.

• -Xnocompactgc

  Specify -Xnocompactgc to disable heap compaction. Heap compaction is the most expensive garbage page operation. If we 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 page policy. This argument is valid only for the IBM Developer Kit.

  Set this argument to optthruput 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 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 page, which allows separate memory pools to contain objects with different ages. The garbage page cycle collects the objects independently from one another depending on age. With additional parameters, we 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 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 page cycle are transferred to another pool. Use theSurvivorRatio parameter to specify the size of the survivor pool.SurvivorRatio. We can use the object statistics the Tivoli Performance Viewer collects, or include the verbose:gc argument in your configuration setting to monitor garbage page 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

  Best practice: 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 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 we 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 we 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=true

  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 dmgr 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.alarmthreadmonitor.hung_alarm_mute=

  This argument specifies the maximum number of times an alarm reports its full stack trace in hung-thread messages in the system logs.

  When a system alarm thread is active longer than the alarm thread monitor threshold, the application server logs a hung-thread message with the name of the alarm thread, the length of time the alarm thread has been active, and the full exception stack trace. The full stack trace is useful for debugging the cause of the delay, but if hung-thread messages are frequently triggered, the repeated long messages can make other information in the system logs hard to find. Set this argument to an integer greater than 0 to specify the maximum number of times any single alarm reports its full stack trace. After this threshold is reached, each subsequent hung-thread message includes only the hung alarm handler's entry.

  The default value of 0 indicates that all hung-thread messages for an alarm include the full stack trace.

  This property specifies a threshold for each alarm handler class, not for the total number of messages or for each alarm handler instance.

• -Dcom.ibm.websphere.native.logging.timestamp=true

  Specify this argument to add a time stamp and thread identifier before all server debug messages that are output to the native_stdout and native_stderr log files. We can use the time stamp and thread identifier to correlate the behaviors of application server bootstrap components with the behaviors of other server mechanisms, which are indicated in the SystemOut and SystemErr log files. This behavior is disabled by default.

  When the server is configured with JVM generic argument -Dws.ext.debug=true, it emits debug messages during its bootstrapping sequence to native_stdout.log and native_stderr.log. If -Dcom.ibm.websphere.native.logging.timestamp is also set to true, the server outputs debug messages with a time stamp and thread identifier, as shown in the following example:

  [6/18/12 16:24:31:453 CDT] 00000000 
  ws.ext.mains.args[0]=-nosplash
  [6/18/12 16:24:31:453 CDT] 00000000 
  ws.ext.mains.args[1]=-application
  [6/18/12 16:24:31:453 CDT] 00000000 
  ws.ext.mains.args[2]=com.ibm.ws.bootstrap.WSLauncher
  [6/18/12 16:24:31:453 CDT] 00000000 
  ws.ext.mains.args[3]=com.ibm.ws.runtime.WsServer

  You should specify -Dws.ext.debug=true only under the direction of IBM support personnel.

• -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 deployment managers, can use the share classes option.

  If we 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 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 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, we can revert to the previous behavior by adding the this argument to the Generic JVM arguments section.

Information	Value
Data type	String
Units	Java command-line arguments

Executable JAR file name

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

Information	Value
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.

Information	Value
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 server as the JVM settings for the operating system.

Related

Configure the JVM
Tune the application serving environment

Reference:

Custom property page
Process logs

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