This chapter includes the following sections:
The following table presents general JVM tuning considerations for WebLogic Server.
Table 4-1 General JVM Tuning Considerations
| Tuning Factor | Information Reference |
|---|---|
|
Tuning heap size and garbage collection |
For WebLogic Server heap size tuning details, see Garbage Collection. |
|
Mixed client/server JVMs |
Deployments using different JVM versions for the client and server are supported in WebLogic Server. See "Supported Configurations" in What's New in Oracle WebLogic Server for links to the latest supported 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 "Thread Priority on the Solaris Platform" at |
Garbage collection is the VM's process of freeing up unused Java objects in the Java heap. The following sections provide information on tuning your VM's garbage collection:
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. A best practice is to tune the time spent doing garbage collection to within 5% of execution time.
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 and Specifying Heap Size Values.
The 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:
Monitor the performance of WebLogic Server under maximum load while running your application.
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:\Oracle\Middleware" -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.
Analyze the following data points:
How often is garbage collection taking place? In the weblogic.log file, compare the time stamps around the garbage collection.
How long is garbage collection taking? Full garbage collection should not take longer than 3 to 5 seconds.
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.
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.
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.
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.
System performance is greatly influenced by the size of the Java heap available to the JVM. This section describes the command line options you use to define the heap sizes 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.
The following section provides general guidelines for tuning VM heap sizes:
The heap sizes should be set to values such that the maximum amount of memory used by the VM does not exceed the amount of available physical RAM. If this value is exceeded, the OS starts paging and performance degrades significantly. The VM always uses more memory than the heap size. The memory required for internal VM functionality, native libraries outside of the VM, and permanent generation memory (for the Sun VM only: memory required to store classes and methods) is allocated in addition to the heap size settings.
When using a generational garbage collection scheme, the nursery size should not exceed more than half the total Java heap size. Typically, 25% to 40% of the heap size is adequate.
In production environments, set the minimum heap size and the maximum heap size to the same value to prevent wasting VM resources used to constantly grow and shrink the heap. This also applies to the New generation heap sizes.
Although JRockit provides automatic heap resizing heuristics, they are not optimal for all applications. In most situations, best performance is achieved by tuning the VM for each application by adjusting the heaps size options shown in the following table.
Table 4-2 JRockit JVM Heap Size Options
| Task | Option | Description |
|---|---|---|
|
Setting the Nursery |
-Xns |
Optimally, you should try to make the nursery as large as possible while still keeping the garbage collection pause times acceptably low. This is particularly important if your application is creating a lot of temporary objects. The maximum size of a nursery cannot exceed 95% of the maximum heap size. |
|
Setting initial and minimum heap size |
-Xms |
Oracle recommends setting the minimum heap size ( |
|
Setting maximum heap size |
-Xmx |
Setting a low maximum heap value compared to the amount of live data decrease performance by forcing frequent garbage collections. |
|
Setting garbage collection |
-Xgc: parallel |
|
|
Performs adaptive optimizations as early as possible in the Java application run. |
-XXaggressive:memory |
To do this, the bottleneck detector will run with a higher frequency from the start and then gradually lower its frequency. This options also tells JRockit to use the available memory aggressively. |
For example, when you start a WebLogic Server instance from a java command line, you could specify the JRockit VM heap size values as follows:
$ java -Xns10m -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 10 megabytes of memory to the Nursery heap sizes and 512 megabytes of memory to the minimum and maximum heap sizes for the WebLogic Server instance running in the JVM.
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."
For better performance, you may need to increase the heap size for each Managed Server in your environment.
The following sections provide information about how to modify the Java heap size for Managed Servers:
See "Configuring Remote Startup Arguments" in Administering Node Manager for Oracle WebLogic Server.
If you use Node Manager to start the Managed Servers, you can specify a heap size as a Java argument on the Server Start tab for each Managed Server. Your heap size values are then persisted in the startup.properties file for the server.
You can update the startManagedWebLogic script with the required heap size in JAVA_OPTIONS. For example:
JAVA_OPTIONS="-Xms2g -Xmx2g" ${JAVA_OPTIONS}
See "Starting Managed Servers with a Startup Script" in Administering Server Startup and Shutdown for Oracle WebLogic Server.
You can pass JVM parameters when starting a managed server by invoking weblogic.Server class in a Java command. See "weblogic.Server Command-Line Reference" in the Command Reference for Oracle WebLogic Server.
Start scripts and the Administration Console (the startup.properties file) are common ways to configure memory arguments in managed servers. Often, they are set in multiple places and with different values. How do you determine which values are actually used by a running managed server?
A running managed server always uses the last set of memory arguments passed to the server during startup. You can verify this by looking through the log files. If you see the memory arguments listed multiple times, the last set in the output contains the values used by the server. If you used the Administration Console to set the values, see Using the Administration Console to Set Java Heap Size, the startup.properties file is always processed last, after all scripts. This provides a convenient method to tune memory size for an individual managed server when using common scripts. If you prefer not to include memory arguments that are not actually used in the environment, you will need to remove any extraneous memory arguments such as MEM_ARGS and JAVA_OPTONS from the scripts used to start a managed server.