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Oracle® Communications Converged Application Server Administration Guide
Release 5.0

Part Number E17647-03
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12 Tuning JVM Garbage Collection for Production Deployments

This chapter describes how to tune Java Virtual Machine (JVM) garbage collection performance for engine tier servers:

Goals for Tuning Garbage Collection Performance

Production installations of Oracle Communications Converged Application Server generally require extremely small response times (under 50 milliseconds) for clients at all times, even under peak server loads. A key factor in maintaining brief response times is the proper selection and tuning of the JVM's Garbage Collection (GC) algorithm for Converged Application Server instances in the engine tier.

Whereas certain tuning strategies are designed to yield the lowest average garbage collection times or to minimize the frequency of full GCs, those strategies can sometimes result in one or more very long periods of garbage collection (often several seconds long) that are offset by shorter GC intervals. With a production SIP Server installation, all long GC intervals must be avoided in order to maintain response time goals.

The sections that follow describe GC tuning strategies for JRockit and Sun's JVM that generally result in best response time performance.


For more information on JRockit, see Introduction to Oracle WebLogic Server in the WebLogic Server documentation.

Modifying JVM Parameters in Server Start Scripts

If you use custom startup scripts to start Converged Application Server engines and replicas, simply edit those scripts to include the recommended JVM options described in the sections that follow.

The Configuration Wizard also installs default startup scripts when you configure a new domain. by default, these scripts are installed in the CAS50_home/user_projects/domains/domain_name/bin directory, where CAS50_home is where you installed the Converged Application Server software anddomain_name is the name of the domain's directory. The /bin directory includes:

If you use the Oracle-installed scripts to start engines and replicas, you can override JVM memory arguments by first setting the USER_MEM_ARGS environment variable in your command shell.


Setting the USER_MEM_ARGS environment variable overrides all default JVM memory arguments specified in the Oracle-installed scripts. Always set USER_MEM_ARGS to the full list of JVM memory arguments you intend to use. For example, when using the Sun JVM, always add -XX:MaxPermSize=128m to the USER_MEM_ARGS value, even if you only intend to change the default heap space (-Xms, -Xmx) parameters.

Tuning Garbage Collection with JRockit

JRockit provides several monitoring tools that you can use to analyze the JVM heap at any given moment, including:

Use these and other tools in a controlled environment to determine the effects of JVM settings before you use the settings in a production deployment.

The following sections describe suggested starting JVM options for use with the JRockit. If you use JRockit with the deterministic garbage collector (recommended), use the options described in "Using Oracle JRockit Real Time (Deterministic Garbage Collection)".

Using Oracle JRockit Real Time (Deterministic Garbage Collection)

Very short response times are most easily achieved by using JRockit Real Time, which implements a deterministic garbage collector.

Oracle recommends using the following JVM arguments for engine tier servers in replicated cluster configurations:

-Xms1024m -Xmx1024m -XgcPrio:deterministic -XpauseTarget=30ms -XXtlasize:min=8k -XXnosystemgc


The above settings are configured by default in the $WL_HOME/common/bin/ file when you use the Configuration Wizard to create a new domain with the JRockit JVM.

You may need to increase the -XpauseTarget value for allocation-intensive applications. The value can be decreased for smaller applications under light loads.

Adjust the heap size according to the amount of live data used by deployed applications. As a starting point, set the heap size from 2 to 3 times the amount required by your applications. A value closer to 3 times the required amount generally yields the best performance.

For replica servers, increase the available memory:

-Xms3072m -Xmx3072m -XgcPrio:deterministic -XpauseTarget=30ms -XXtlasize:min=8k -XXnosystemgc

These settings fix the heap size and enable the dynamic garbage collector with deterministic garbage collection. -XpauseTarget sets the maximum pause time and -XXtlasize=3k sets the thread-local area size. -XXnosystemgc prevents System.gc() application calls from forcing garbage collection.

Using Oracle JRockit without Deterministic Garbage Collection

When using Oracle's JRockit JVM without deterministic garbage collection (not recommended for production deployments), the best response time performance is obtained by using the generational concurrent garbage collector.

The full list of example startup options for an engine tier server are:

-Xms1024m -Xmx1024m -Xgc:gencon -XXnosystemgc -XXtlasize:min=3k -XXkeeparearatio=0 -Xns:48m 


Fine tune the heap size according to the amount of live data used by deployed applications.

The full list of example startup options for a replica server are:

-Xms3072m -Xmx3072m -Xgc:gencon -XXnosystemgc -XXtlasize:min=3k -XXkeeparearatio=0 -Xns:48m

Tuning Garbage Collection with Sun JDK

When using Sun's JDK, the goal in tuning garbage collection performance is to reduce the time required to perform a full garbage collection cycle. You should not attempt to tune the JVM to minimize the frequency of full garbage collections, because this generally results in an eventual forced garbage collection cycle that may take up to several full seconds to complete.

The simplest and most reliable way to achieve short garbage collection times over the lifetime of a production server is to use a fixed heap size with the default collector and the parallel young generation collector, restricting the new generation size to at most one third of the overall heap.

The following example JVM settings are recommended for most engine tier servers:

-server -Xmx1024m -XX:MaxPermSize=128m -XX:+UseParNewGC -XX:+UseConcMarkSweepGC
         -XX:+UseTLAB -XX:+CMSIncrementalMode -XX:+CMSIncrementalPacing
         -XX:CMSIncrementalDutyCycleMin=0 -XX:CMSIncrementalDutyCycle=10
         -XX:MaxTenuringThreshold=0 -XX:SurvivorRatio=256
         -XX:CMSInitiatingOccupancyFraction=60 -XX:+DisableExplicitGC

For replica servers, use the example settings:

-server -Xmx3072m -XX:MaxPermSize=128m -XX:+UseParNewGC -XX:+UseConcMarkSweepGC
         -XX:+UseTLAB -XX:+CMSIncrementalMode -XX:+CMSIncrementalPacing
         -XX:CMSIncrementalDutyCycleMin=0 -XX:CMSIncrementalDutyCycle=10
         -XX:MaxTenuringThreshold=0 -XX:SurvivorRatio=256
         -XX:CMSInitiatingOccupancyFraction=60 -XX:+DisableExplicitGC

The above options have the following effect: