Tuning Java Virtual Machines (JVMs)

 

The Java virtual machine (JVM) is a virtual "execution engine" instance that executes the bytecodes in Java class files on a microprocessor. How you tune your JVM affects the performance of WebLogic Server and your applications.

The following sections discuss JVM tuning options for WebLogic Server:

 

JVM Tuning Considerations

Table 3-1 presents general JVM tuning considerations for WebLogic Server.

Tuning Factor

Information Reference

JVM vendor and version Use only production JVMs on which WebLogic Server has been certified. WebLogic Server 8.1 supports only those JVMs that are J2SE 1.4-compliant.The Supported Configurations pages are frequently updated and contains the latest certification information on various platforms.
Tuning heap size and garbage collection For WebLogic Server heap size tuning details, see JVM Heap Size and Garbage Collection.
Choosing a GC (garbage collection) scheme Depending on your application, there are a number of GC schemes available for managing your system memory, as described in Choosing a Garbage Collection Scheme.
Mixed client/server JVMs Deployments using different JVM versions for the client and server are supported in WebLogic Server. See the support page for Mixed Client/Server JVMs.
UNIX threading models Choices you make about Solaris threading models can have a large impact on the performance of your JVM on Solaris. You can choose from multiple threading models and different methods of synchronization within the model, but this varies from JVM to JVM.See "Performance Documentation For the Java Hotspot Virtual Machine: Threading on Sun Microsystems' Web site.

 

Which JVM for Your System?

Although this section focuses on Sun Microsystems' J2SE 1.4 JVM for the Windows, UNIX, and Linux platforms, the BEA WebLogic JRockit JVM 8.1 was developed expressly for server-side applications and optimized for Intel architectures to ensure reliability, scalability, manageability, and flexibility for Java applications. For more information about the benefits of using JRockit on Windows and Linux platforms, see Introduction to BEA WebLogic JRockit 8.1 SDK.

For more information on JVMs in general, see the Introduction to the JVM specification. For links to related reading for JVM tuning, see Related Reading: Performance Tools and Information.

 

Changing To a Different JVM

When you create a domain, if you choose to customize the configuration, the Configuration Wizard presents a list of SDKs that WebLogic Server installed. From this list, you choose the JVM that you want to run your domain and the wizard configures the BEA start scripts based on your choice. After you create a domain, if you want to use a different JVM, you can modify the scripts as follows. For more information, see "Changing the JVM That Runs Servers.

 

JVM Heap Size and Garbage Collection

Garbage collection is the JVM's process of freeing up unused Java objects in the Java heap.The Java heap is where the objects of a Java program live. It is a repository for live objects, dead objects, and free memory. When an object can no longer be reached from any pointer in the running program, it is considered "garbage" and ready for collection.

The JVM heap size determines how often and how long the VM spends collecting garbage. An acceptable rate for garbage collection is application-specific and should be adjusted after analyzing the actual time and frequency of garbage collections. If you set a large heap size, full garbage collection is slower, but it occurs less frequently. If you set your heap size in accordance with your memory needs, full garbage collection is faster, but occurs more frequently.

The goal of tuning your heap size is to minimize the time that your JVM spends doing garbage collection while maximizing the number of clients that WebLogic Server can handle at a given time. To ensure maximum performance during benchmarking, you might set high heap size values to ensure that garbage collection does not occur during the entire run of the benchmark.

You might see the following Java error if you are running out of heap space: 

java.lang.OutOfMemoryError <<no stack trace available>>



java.lang.OutOfMemoryError <<no stack trace available>>



Exception in thread "main"

To modify heap space values, see Specifying Heap Size Values.

To configure WebLogic Server to detect automatically when you are running out of heap space and to address low memory conditions in the server, see Automatically Logging Low Memory Conditions.

 

Choosing a Garbage Collection Scheme

Depending on which JVM you are using, you can choose from several garbage collection schemes to manage your system memory. For example, some garbage collection schemes are more appropriate for a given type of application. Once you have an understanding of the workload of the application and the different garbage collection algorithms utilized by the JVM, you can optimize the configuration of the garbage collection.

Refer to the following links for in-depth discussions of garbage collection options for your JVM:

 

Using Verbose Garbage Collection to Determine Heap Size

The HotSpot VM's verbose garbage collection option (verbosegc) enables you to measure exactly how much time and resources are put into garbage collection. To determine the most effective heap size, turn on verbose garbage collection and redirect the output to a log file for diagnostic purposes.

The following steps outline this procedure:

  1. Monitor the performance of WebLogic Server under maximum load while running your application.
  2. Use the -verbosegc option to turn on verbose garbage collection output for your JVM and redirect both the standard error and standard output to a log file.

    This places thread dump information in the proper context with WebLogic Server informational and error messages, and provides a more useful log for diagnostic purposes.

    For example, on Windows and Solaris, enter the following:

    % java -ms32m -mx200m -verbosegc -classpath $CLASSPATH
    -Dweblogic.Name=%SERVER_NAME% -Dbea.home="C:\bea"
    -Dweblogic.management.username=%WLS_USER%
    -Dweblogic.management.password=%WLS_PW%
    -Dweblogic.management.server=%ADMIN_URL%
    -Dweblogic.ProductionModeEnabled=%STARTMODE%
    -Djava.security.policy="%WL_HOME%\server\lib\weblogic.policy" weblogic.Server
    >> logfile.txt 2>&1

    where the logfile.txt 2>&1 command redirects both the standard error and standard output to a log file.

    On HPUX, use the following option to redirect stderr stdout to a single file:

    -Xverbosegc:file=/tmp/gc$$.out

    where $$ maps to the process ID (PID) of the Java process. Because the output includes timestamps for when garbage collection ran, you can infer how often garbage collection occurs.

  3. Analyze the following data points:
    1. How often is garbage collection taking place? In the weblogic.log file, compare the time stamps around the garbage collection.
    2. How long is garbage collection taking? Full garbage collection should not take longer than 3 to 5 seconds.
    3. What is your average memory footprint? In other words, what does the heap settle back down to after each full garbage collection? If the heap always settles to 85 percent free, you might set the heap size smaller.
  4. If you are using 1.4 Java HotSpot JVM, set the New generation heap sizes.

    See Specifying Heap Size Values and Table 3-2.

    Note: For information about setting the appropriate heap sizes for the BEA WebLogic JRockit JVM, see Tuning WebLogic JRockit JVM.

  5. Make sure that the heap size is not larger than the available free RAM on your system.

    Use as large a heap size as possible without causing your system to "swap" pages to disk. The amount of free RAM on your system depends on your hardware configuration and the memory requirements of running processes on your machine. See your system administrator for help in determining the amount of free RAM on your system.

  6. If you find that your system is spending too much time collecting garbage (your allocated "virtual" memory is more than your RAM can handle), lower your heap size.

    Typically, you should use 80 percent of the available RAM (not taken by the operating system or other processes) for your JVM.

  7. If you find that you have a large amount of available free RAM remaining, run more instances of WebLogic Server on your machine.

    Remember, the goal of tuning your heap size is to minimize the time that your JVM spends doing garbage collection while maximizing the number of clients that WebLogic Server can handle at a given time.

Note: For information about using the BEA WebLogic JRockit JVM -Xgcreport option to print a comprehensive garbage collection report at program completion, see Selecting and Running a Memory Management System.

 

Specifying Heap Size Values

You must specify Java heap size values each time you start an instance of WebLogic Server. This can be done either from the java command line or by modifying the default values in the sample startup scripts that are provided with the WebLogic distribution for starting WebLogic Server, as explained in Using WebLogic Startup Scripts to Set Heap Size.

For example, when you start a WebLogic Server instance from a java command line, you could specify the HotSpot VM heap size values as follows: 

$ java -XX:NewSize=128m -XX:MaxNewSize=128m -XX:SurvivorRatio=8 -Xms512m -Xmx512m

The default size for these values is measured in bytes. Append the letter `k' or `K' to the value to indicate kilobytes, `m' or `M' to indicate megabytes, and `g' or `G' to indicate gigabytes. The example above allocates 128 megabytes of memory to the New generation and maximum New generation heap sizes, and 512 megabytes of memory to the minimum and maximum heap sizes for the WebLogic Server instance running in the JVM. For more information on the heap size options, see Java Heap Size Options.

 

Using WebLogic Startup Scripts to Set Heap Size

Sample startup scripts are provided with the WebLogic Server distribution for starting the server and for setting the environment to build and run the server:

  • startWLS.cmd and setEnv.cmd (Windows).
  • startWLS.sh and setEnv.sh (UNIX and Windows. On Windows, these scripts support the MKS and Cygnus BASH UNIX shell emulators.)

If you used the Configuration Wizard to create your domain, the WebLogic startup scripts are located in the domain-name directory where you specified your domain. By default, this directory is BEA_HOME\user_projects\domain\domain-name, where BEA_HOME is the directory that contains the product installation, and domain-name is the name of the domain directory defined by the selected configuration template. For more information about creating domains using the Configuration Wizard, see "Creating Domains Using the Configuration Wizard.

The WebLogic startup scripts set environment variables, such as the default memory arguments passed to Java (that is, heap size) and the location of the JDK, and then start the JVM with WebLogic Server arguments. Be aware that the WebLogic Server startup scripts specify default heap size parameters; therefore, you need to modify them to fit your environment and applications. For instructions on how to modifying the startup scripts to set the Java heap size options, see "Specifying Java Options for a WebLogic Server Instance.

 

Java Heap Size Options

You achieve best performance by individually tuning each application. However, configuring the Java HotSpot VM heap size options listed in Table 3-2 when starting WebLogic Server increases performance for most applications.

These options may differ depending on your architecture and operating system. See your vendor's documentation for platform-specific JVM tuning options.

Task

Option

Recommended Value

Setting the New generation heap size -XX:NewSize 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 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. 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 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) to minimize garbage collections.
Setting maximum heap size -Xmx Set the maximum size of the memory allocation pool. Set this value to a multiple of 1024 that is greater than 1MB.

Note: For information about setting the appropriate heap sizes for WebLogic's JRockit JVM, see Tuning WebLogic JRockit JVM.

 

Automatically Logging Low Memory Conditions

WebLogic Server enables you to automatically log low memory conditions observed by the server. WebLogic Server detects low memory by sampling the available free memory a set number of times during a time interval. At the end of each interval, an average of the free memory is recorded and compared to the average obtained at the next interval. If the average drops by a user-configured amount after any sample interval, the server logs a low memory warning message in the log file and sets the server health state to "warning."

If the average free memory ever drops below 5 percent of the initial free memory recorded immediately after you start the server, WebLogic Server logs a message to the log file.

You configure each aspect of the low memory detection process using the Administration Console:

  1. Start the Administration Server if it is not already running.
  2. Access the Administration Console for the domain.
  3. Expand the Servers node in the navigation tree to display the servers configured in your domain.
  4. Click the name of the server instance that you want to configure. Note that you configure low memory detection on a per-server basis.
  5. Select the Configuration - > Tuning tab in the right pane.
  6. On the Advanced Options bar, click Show to display additional attributes.
  7. Modify the following Memory Option attributes as necessary to tune low memory detection for the selected server instance:

    • Low Memory GCThreshold: Enter a percentage value (0-99 percent) to represent the threshold after which WebLogic Server logs a low memory warning and changes the health state to "warning." By default, Memory GCThreshold is set to 5 percent. This means that by default the server logs a low memory warning after the average free memory reaches 5 percent of the initial free memory measured at the server's boot time.
    • Low Memory Granularity Level: Enter a percentage value (1-99 percent) to use for logging low memory conditions and changing the server health state to "warning." By default this value is set to 5 percent. This means that if the average free memory drops by 5 percent or more over two measured intervals, the server logs a low memory warning in the log file and changes the server health state to "warning."
    • Low Memory Sample Size: Enter the number of times the server samples free memory during a fixed time period. By default, the server samples free memory 10 times each interval to acquire the average free memory. Using a higher sample size can increase the accuracy of the reading.
    • Low Memory Time Interval: Enter the time, in seconds, that define the interval over which the server determines average free memory values. By default WebLogic Server obtains an average free memory value every 3600 seconds.
  8. Click Apply to apply your changes.
  9. Reboot the server to use the new low memory detection attributes.

 

Manually Requesting Garbage Collection

Make sure that full garbage collection is necessary before manually selecting it on a server. When you perform garbage collection, the JVM often examines every living object in the heap.

To use the Administration Console to request garbage collection on a specific server instance:

  1. On the Administration Console, expand the server instance node for the server whose memory usage you want to view. A dialog box in the right pane shows the tabs associated with this instance.
  2. Select the Monitoring - > Performance tab.
  3. Check the Memory Usage graph for high usage. Note that the Memory Usage graph displays information only for a server that is currently running.
  4. Click the Force garbage collection button to request garbage collection. The Force garbage collection button calls the JVM's System.gc() method to perform garbage collection. Note, however, that the JVM implementation itself decides whether or not the request actually triggers garbage collection.

 

Setting Java HotSpot VM Options

You can use standard java command-line options to improve the performance of your JVM. 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.

Test both your client and server JVMs to see which options perform better for your particular application. The Sun Microsystems Java HotSpot VM Options document provides information on the command-line options and environment variables that can affect the performance characteristics of the Java HotSpot Virtual Machine.

For a brief discussion of additional "non-standard" VM options that can also affect performance, see Non-Standard HotSpot VM Options for Windows, Solaris, and Linux.

Note: For information about using the WebLogic JRockit JVM command-line options to improve performance, see Tuning WebLogic JRockit JVM.

 

Standard HotSpot VM Options for Windows, Solaris, and Linux

You can use standard java options to improve performance on Windows, Solaris, and Linux platforms. These options are supported on the current Java the run-time environment and will also be supported in future releases of HotSpot. When specifying one of the standard options listed Table 3-3 through the java command, WebLogic Server invokes a particular version of the JVM.

Option

Platform

Description

-client WindowsSolaris Selects the Java HotSpot Client VM. This is the default for Windows and Solaris.
-server WindowsSolarisLinux Selects the Java HotSpot Server VM.
-hotspot Linux Selects the Java HotSpot Client VM. This is the default for Linux.

For additional examples of the standard HotSpot VM options, see:

Sun Microsystems' Java Virtual Machine document provides a detailed discussion of the Client and Server implementations of the Java virtual machine for J2SE 1.4.

 

Non-Standard HotSpot VM Options for Windows, Solaris, and Linux

You can also use non-standard java options to improve performance. 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. Note that non-standard command-line options are subject to change in future releases.

For examples of the non-standard options for improving performance on the Hotspot VM on Windows, Solaris, and Linux, see:

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