Chapter 16. JVM

16.1. About JVM

16.1.1. About JVM Settings

Configuration of Java Virtual Machine (JVM) settings varies between the managed domain and standalone server instances. In a managed domain, the JVM settings are declared in host.xml and domain.xml configuration files, and determined by the domain controller components responsible for starting and stopping server processes. In a standalone server instance, the server startup processes can pass command line settings at startup. These can be declared from the command line or via the System Properties screen in the Management Console.
Managed Domain

An important feature of the managed domain is the ability to define JVM settings at multiple levels. You can configure custom JVM settings at the host level, by server group, or by server instance. The more specialized child elements will override the parent configuration, allowing for the declaration of specific server configurations without requiring exclusions at the group or host level. This also allows the parent configuration to be inherited by the other levels until settings are either declared in the configuration files or passed at runtime.

Example 16.1. JVM settings in the domain configuration file

The following example shows a JVM declaration for a server group in the domain.xml configuration file. 
<server-groups>
       <server-group name="main-server-group" profile="default">
           <jvm name="default">
               <heap size="64m" max-size="512m"/>
           </jvm>
           <socket-binding-group ref="standard-sockets"/>
       </server-group>
       <server-group name="other-server-group" profile="default">
           <jvm name="default">
               <heap size="64m" max-size="512m"/>
           </jvm>
           <socket-binding-group ref="standard-sockets"/>
       </server-group>
</server-groups>
In this instance a server group called main-server-group is declaring a heap size of 64 megabytes, and a maximum heap size of 512 megabytes. Any server that belongs to this group will inherit these settings. You can change these settings for the group as a whole, by the host, or the individual server.

Example 16.2. Domain settings in the host configuration file

The following example shows a JVM declaration for a server group in the host.xml configuration file. 
<servers>
  <server name="server-one" group="main-server-group" auto-start="true">
    <jvm name="default"/>
  </server>
  <server name="server-two" group="main-server-group" auto-start="true">
    <jvm name="default">
      <heap size="64m" max-size="256m"/>
    </jvm>
    <socket-bindings port-offset="150"/>
  </server>
  <server name="server-three" group="other-server-group" auto-start="false">
    <socket-bindings port-offset="250"/>
  </server>
</servers>
In this instance, a server named server-two belongs to the server group named main-server-group, inheriting the JVM settings from the default JVM group. In the previous example, the main heap size for main-server-group was set at 512 megabytes. By declaring the lower maximum heap size of 256 megabytes, server-two can override the domain.xml settings to fine-tune performance as desired.
Standalone server settings at runtime

The JVM settings for standalone server instances can be declared at runtime by setting the JAVA_OPTS environment variable before starting the server. An example of setting the JAVA_OPTS environment variable at the Linux command-line is: 

[user@host bin]$ export JAVA_OPTS="-Xmx1024M"
The same setting can be used in a Microsoft Windows environment, as follows: 
C:\> set JAVA_OPTS="Xmx1024M"
Alternatively, JVM settings can be added to the standalone.conf file found in the EAP_HOME/bin folder, which contains examples of options to pass to the JVM.

Warning

Setting the JAVA_OPTS environment variable will re-define the default values for the JAVA_OPTS environment variable. This can break or terminate the start of JBoss EAP.
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16.1.2. Display the JVM Status in the Management Console

Prerequisites

Java Virtual Machine (JVM) status can be displayed in the Management Console for either the standalone server or a managed domain. The console shows the heap usage, non heap usage, and thread usage of the server. While the statistics are not displayed in real-time, you can refresh the console display to provide an up-to-date overview of JVM resources.
The JVM status shows the following values.

Table 16.1. JVM Status Attributes

Type Description
Max The maximum amount of memory that can be used for memory management. The maximum availabe memory is shown by the light grey bar.
Used The amount of used memory. The amount of used memory is shown by the dark grey bar.
Committed The amount of memory that is committed for the Java virtual machine to use. The committed memory is shown by the dark grey bar.
Init The amount of memory that the Java virtual machine initially requests from the operating system for memory management. The init amount is shown by the dark grey bar.

Procedure 16.1. Display the JVM Status in the Management Console

    • Display the JVM status for a standalone server instance

      Select the Runtime tab from the top of the screen. Expand the Status menu, then expand the Platform menu. Select JVM.

    • Display the JVM status for a managed domain

      Select the Runtime tab from the top of the screen. Expand the Server Status menu, then expand the Platform menu. Select JVM.
  2. The managed domain can provide visibility of all server instances in the server group, but will only allow you to view one server at a time by selecting from the server menu. To view the status of other servers in your server group, click Change Server left of the screen to select from the host and servers displayed in your group. Select the required server or host and the JVM details will change. Click Close to finish.
Result

The status of the JVM settings for the server instance are displayed.

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16.1.3. Configuring JVM

The <jvm></jvm> tags support the usage of <jvm-options></jvm-options>, which can be used to add parameters to the JVM configuration using the <option value="VALUE"/> tag.

Example 16.3. Use of <jvm-options>

<jvm name="default">
  <heap size="1303m" max-size="1303m"/>
  <permgen max-size="256m"/>
  <jvm-options>
    <option value="-XX:+UseCompressedOops"/>
  </jvm-options>
</jvm>
Configuring JVM Using CLI

To configure JVM using CLI, use the following syntax: 

# cd /server-group=main-server-group/jvm=default

# :add-jvm-option(jvm-option="-XX:+UseCompressedOops")
{
    "outcome" => "success",
    "result" => undefined,
    "server-groups" => {"main-server-group" => {"host" => {"master" => {
        "server-one" => {"response" => {
            "outcome" => "success",
            "response-headers" => {
                "operation-requires-restart" => true,
                "process-state" => "restart-required"
            }
        }},
        "server-two" => {"response" => {
            "outcome" => "success",
            "response-headers" => {
                "operation-requires-restart" => true,
                "process-state" => "restart-required"
            }
        }}
    }}}}
}

# :read-resource

# Expected Result:

[domain@localhost:9999 jvm=default] :read-resource                                      
  {
    "outcome" => "success",
    "result" => {
    "agent-lib" => undefined,
    "agent-path" => undefined,
    "env-classpath-ignored" => undefined,
    "environment-variables" => undefined,
    "heap-size" => "1303m",
    "java-agent" => undefined,
    "java-home" => undefined,
    "jvm-options" => ["-XX:+UseCompressedOops"],
    "max-heap-size" => "1303m",
    "max-permgen-size" => "256m",
    "permgen-size" => undefined,
    "stack-size" => undefined,
    "type" => undefined
  }
}

Removing jvm-options Entry

To remove jvm-options entry, use the following syntax: 

# cd /server-group=main-server-group/jvm=default

# :remove-jvm-option(jvm-option="-XX:+UseCompressedOops")

# Expected Result:

[domain@localhost:9999 jvm=default] :remove-jvm-option(jvm-option="-XX:+UseCompressedOops")
{
    "outcome" => "success",
    "result" => undefined,
    "server-groups" => {"main-server-group" => {"host" => {"master" => {
        "server-one" => {"response" => {
            "outcome" => "success",
            "response-headers" => {
                "operation-requires-restart" => true,
                "process-state" => "restart-required"
            }
        }},
        "server-two" => {"response" => {
            "outcome" => "success",
            "response-headers" => {
                "operation-requires-restart" => true,
                "process-state" => "restart-required"
            }
        }}
    }}}}
}

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