Java Heap Size Options
You achieve best performance by individually tuning each of your applications. Configuring the JVM heap size options listed in Table 2-2 increases performance for most applications.
The options listed in Table 2-2 may differ depending on your architecture and operating system. See your vendor's documentation for platform-specific JVM tuning options.
Table 2-2 Java Heap Size Options
| Task | Option | Description |
|---|---|---|
| Setting the New generation heap size | -XX:NewSize | Use this option to set the New generation Java heap size. Set this value to a multiple of 1024 that is greater than 1MB. As a general rule, set -XX:NewSize to be one-fourth the size of the maximum heap size. Increase the value of this option for larger numbers of short-lived objects. Be sure to increase the New generation as you increase the number of processors. Memory allocation can be parallel, but garbage collection is not parallel. |
| Setting the maximum New generation heap size | -XX:MaxNewSize | Use this option to set the maximum New generation Java heap size. Set this value to a multiple of 1024 that is greater than 1MB. |
| Setting New heap size ratios | -XX:SurvivorRatio | The New generation area is divided into three sub-areas: Eden, and two survivor spaces that are equal in size. Use the -XX:SurvivorRatio=X option to configure the ratio of the Eden/survivor space size. Try setting this value to 8 and then monitor your garbage collection. |
| Setting minimum heap size | -Xms | Use this option to set the minimum size of the memory allocation pool. Set this value to a multiple of 1024 that is greater than 1MB. As a general rule, set minimum heap size (-Xms) equal to the maximum heap size (-Xmx). |
| Setting maximum heap size | -Xmx | Use this option to set the maximum Java heap size. Set this value to a multiple of 1024 that is greater than 1MB. |
Forcing Garbage Collection
Make sure that full garbage collection is necessary before forcing it on a server. When you force garbage collection, the JVM often examines every living object in the heap.
To use the Administration Console to force garbage collection on a specific server:
Note that the Memory Usage graph displays only for servers that are currently running.
A message displays indicating that the collection operation was successful.
Setting Java HotSpot VM Options
You can use standard java options to improve performance. Be aware that how you use these options depends on how your application is coded. Although command line options are consistent across platforms, some platforms may have different defaults.
You need to test both your client and server JVMs and see what performs better for your particular application.
See Setting Non-Standard Java Command Line Options for more VM options that affect performance.
Standard Options for NT
For NT, WebLogic Server invokes the JVM via the java command. Use the options listed in listed in Table 2-3.
Table 2-3 Standard options for HotSpot VM on NT
| Option | Description |
|---|---|
| -hotspot | Selects the Client HotSpot VM. |
| -server | Selects the server VM. |
| -classic | Selects the classic VM. |
Standard Options for UNIX
For UNIX, the WebLogic Server invokes the JVM via the java command. Use the options listed in listed in Table 2-4.
Table 2-4 Standard options for HotSpot VM on UNIX
| Option | Description |
|---|---|
| -client or -hotspot | Selects the Client HotSpot VM. |
| -server | Selects the server VM. |
Setting Non-Standard Java Command Line Options
You can use non-standard java options to improve performance. Be aware that how you use these options depends on how your application is coded. Although command line options are consistent across platforms, some platforms may have different defaults.
Non-Standard Options for NT
Some examples of non-standard options for improving performance on the Hotspot VM on NT are listed in Table 2-5.
Table 2-5 Non-standard options for HotSpot VM on NT
| Option | Description |
|---|---|
| -Xnoclassgc | This option disables garbage collection for the class, It prevents re-loading of the class when the class is referenced after all references to it have been lost. This option requires a greater heap size. |
| -oss | This option controls the Java thread stack size. Setting it too high (>2MB) severely degrades performance. |
| -ss | This option controls the native thread stack size. Setting it too high (>2MB) severely degrades performance. |
| -Xverbosegc:file=/tmp/gc$$.out | This option redirects -verbosegc messages to a file, allowing you to separate your garbage collection messages from the rest of the messages on stderr. It also provides a performance advantage because writes to files are buffered better than writes to a character stream like stderr. See also Turning On Verbose Garbage Collection and Redirecting Output. |
java [ options ] class [ argument ... ]
java [ options ] -jar file.jar [ argument ... ]
The java tool launches a Java application. It does this by starting a Java runtime environment, loading a specified class, and invoking that class's mainmethod. The method declaration must look like the following:public static void main(String args[])The method must be declared public and static, it must not return any value, and it must accept a String array as a parameter. By default, the first non-option argument is the name of the class to be invoked. A fully-qualified class name should be used. If the -jar option is specified, the first non-option argument is the name of a JAR archive containing class and resource files for the application, with the startup class indicated by the Main-Classmanifest header.The Java runtime searches for the startup class, and other classes used, in three sets of locations: the bootstrap class path, the installed extensions, and the user class path.
Non-option arguments after the class name or JAR file name are passed to the main function.
The launcher has a set of standard options that are supported on the current runtime environment and will be supported in future releases. In addition, the current implementations of the virtual machines support a set of non-standard options options that are subject to change in future releases.
- -client
- Select the Java HotSpot Client VM. This is the default.
- -server
- Select the Java HotSpot Server VM.
- -classpath classpath
- -cp classpath
- Specify a list of directories, JAR archives, and ZIP archives to search for class files. Class path entries are separated by colons (:). Specifying -classpath or -cp overrides any setting of the CLASSPATH environment variable.
If -classpath and -cp are not used and CLASSPATH is not set, the user class path consists of the current directory (
.).For more information on class paths, see Setting the Class Path.
- -Dproperty=value
- Set a system property value.
- -jar
- Execute a program encapsulated in a JAR file. The first argument is the name of a JAR file instead of a startup class name. In order for this option to work, the manifest of the JAR file must contain a line of the form Main-Class: classname. Here, classname identifies the class having thepublic static void main(String[] args) method that serves as your application's starting point. See the Jar tool reference page and the Jar trail of the Java Tutorial for information about working with Jar files and Jar-file manifests.
When you use this option, the JAR file is the source of all user classes, and other user class path settings are ignored.
On Solaris 8, JAR files that can be run with the "java -jar" option can have their execute permissions set so they can be run without using "java -jar".
- -verbose
- -verbose:class
- Display information about each class loaded.
- -verbose:gc
- Report on each garbage collection event.
- -verbose:jni
- Report information about use of native methods and other Java Native Interface activity.
- -version
- Display version information and exit.
- -showversion
- Display version information and continue.
- -?
- -help
- Display usage information and exit.
- -X
- Display information about non-standard options and exit.
-Xconcurrentio: Which generally helps programs with many threads, particularly on Solaris. The main feature turned on with -Xconcurrentio is to use LWP based synchronization instead of thread based synchronization. We have found certain applications to speed up by over 40%. Since 1.4, LWP based synchronization is the default, but -Xconcurrentio can still help since it turns on some other internal options. Finally, there is an alternate thread library which is the default on Solaris 9 and can also be used on Solaris 8 by changing your LD_LIBRARY_PATH to include /usr/lib/lwp before /usr/lib.
- -Xint
- Operate in interpreted-only mode. Compilation to native code is disabled, and all bytecodes are executed by the interpreter. The performance benefits offered by the Java HotSpot VMs' adaptive compiler will not be present in this mode.
- -Xdebug
- Start with the debugger enabled. Refer to jdb description for more details and an example.
- -Xbootclasspath:bootclasspath
- Specify a colon-separated list of directories, JAR archives, and ZIP archives to search for boot class files. These are used in place of the boot class files included in the Java 2 SDK. Note: Applications that use this option for the purpose of overriding a class in rt.jar should not be deployed as doing so would contravene the Java 2 Runtime Environment binary code license.
- -Xbootclasspath/a:path
- Specify a colon-separated path of directires, JAR archives, and ZIP archives to append to the default bootstrap class path.
- -Xbootclasspath/p:path
- Specify a colon-separated path of directires, JAR archives, and ZIP archives to prepend in front of the default bootstrap class path. Note: Applications that use this option for the purpose of overriding a class in rt.jar should not be deployed as doing so would contravene the Java 2 Runtime Environment binary code license.
- -Xfuture
- Perform strict class-file format checks. For purposes of backwards compatibility, the default format checks performed by the Java 2 SDK's virtual machine are no stricter than the checks performed by 1.1.x versions of the JDK software. The -Xfuture flag turns on stricter class-file format checks that enforce closer conformance to the class-file format specification. Developers are encouraged to use this flag when developing new code because the stricter checks will become the default in future releases of the Java application launcher.
- -Xnoclassgc
- Disable class garbage collection.
- -Xincgc
- Enable the incremental garbage collector. The incremental garbage collector, which is off by default, will eliminate occasional garbage-collection pauses during program execution. However, it can lead to a roughly 10% decrease in overall GC performance.
- -Xmsn
- Specify the initial size, in bytes, of the memory allocation pool. This value must be a multiple of 1024 greater than 1MB. Append the letter k or Kto indicate kilobytes, or m or M to indicate megabytes. The default value is 2MB. Examples:
-Xms6291456
-Xms6144k
-Xms6m- -Xmxn
- Specify the maximum size, in bytes, of the memory allocation pool. This value must a multiple of 1024 greater than 2MB. Append the letter k orK to indicate kilobytes, or m or M to indicate megabytes. The default value is 64MB. The upper limit for this value will be approximately 4000m on Solaris 7 and Solaris 8 SPARC platforms and 2000m on Solaris 2.6 and x86 platforms, minus overhead amounts. Examples:
-Xmx83886080
-Xmx81920k
-Xmx80m- -Xssn
- Set thread stack size. Each Java thread has two stacks: one for Java code and one for C code. This option sets the maximum stack size that can be used by C code in a thread to n. Every thread that is spawned during the execution of the program passed to java has n as its C stack size. The default units for n are bytes and n must be > 1000 bytes. To modify the meaning of n, append either the letter k for kilobytes or the letter mfor megabytes. The default stack size is 512 kilobytes (-Xss512k).
- -Xprof
- Profiles the running program, and sends profiling data to standard output. This option is provided as a utility that is useful in program development and is not intended to be be used in production systems.
- -Xrunhprof[:help][:<suboption>=<value>,...]
- Enables cpu, heap, or monitor profiling. This option is typically followed by a list of comma-separated "<suboption>=<value>" pairs. Run the command java -Xrunhprof:help to obtain a list of suboptions and their default values.
- -Xrs
- Reduces use of operating-system signals by the Java virtual machine (JVM). This option is available beginning with J2SE 1.3.1.
In J2SE 1.3.0, the Shutdown Hooks facility was added to allow orderly shutdown of a Java application. The intent was to allow user cleanup code (such as closing database connections) to run at shutdown, even if the JVM terminates abruptly.
Sun's JVM catches signals to implement shutdown hooks for abnormal JVM termination. The JVM uses SIGHUP, SIGINT, and SIGTERM to initiate the running of shutdown hooks.
The JVM uses a similar mechanism to implement the pre-1.2 feature of dumping thread stacks for debugging purposes. Sun's JVM uses SIGQUIT to perform thread dumps.
Applications embedding the JVM frequently need to trap signals like SIGINT or SIGTERM, which can lead to interference with the JVM's own signal handlers. To address this issue, the -Xrs command-line option has been added beginning in J2SE 1.3.1. When -Xrs is used on Sun's JVM, the signal masks for SIGINT, SIGTERM, SIGHUP, and SIGQUIT are not changed by the JVM, and signal handlers for these signals are not installed.
There are two consequences of specifying -Xrs:
- SIGQUIT thread dumps are not available.
- User code is responsible for causing shutdown hooks to run, for example by calling System.exit() when the JVM is to be terminated.
WARNING: Flags -Xdebug and -Xint are mutually exclusive. No more than one of those options should be used on a java command line.
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