IBM BS029ML Self Help Guide - Page 157
Tuning advice for the IBM Java Virtual Machine, JVM heap sizes - IBM JVMs only
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For more information about WebSphere Portal Server performance tuning and a detailed explanation of the parameters, refer to the official IBM WebSphere Portal Version 6.0 Tuning Guide, found at: http://www-1.ibm.com/support/docview.wss?uid=swg27008511&aid=1 Alternatively, refer to the WebSphere Application Server InfoCenter for WebSphere Application Server tuning: http://publib.boulder.ibm.com/infocenter/wasinfo/v6r0/index.jsp?topic=/com.ibm.web sphere.nd.doc/info/ae/ae/welc6toptuning.html 5.2.2 Tuning advice for the IBM Java Virtual Machine At the core of the WebSphere Application Server is the Java Virtual Machine (JVM). Depending on your selected platform, you will either be running with an IBM JVM implementation, or if on the Sun Solaris platform, the Sun JVM. For functionality and consistency across the WebSphere range, the JVM is installed as part of the WebSphere Application Server package. No dependency is therefore placed on the version of the JVM installed as part of the operating system (usually referred to as the system JVM or default JVM). JVM heap sizes - IBM JVMs only One common misnomer is that setting a large JVM heap size improves performance. This is simply not the case. We strongly advise that you choose your Java maximum heap settings carefully and then only based on a thorough Java garbage collection (GC) analysis. Remember: If you use a big heap, then garbage collection will be less frequent but much slower, as there is more memory to search through. If you use a small heap, then garbage collection will be more frequent but very fast, as there is less memory to search through. With the IBM JVM, the Java heap is preallocated (in terms of native memory) at the maximum heap size, reducing the overall amount of memory available to the system. The Java garbage collection (GC) cycle, which is a stop-the-world (STW) implementation, will prevent the application server from handling loads for a short period of time. All threads are effectively suspended, with the exception of the garbage collection threads, while GC completes to protect the Java heap from corruption. WebSphere vertical clustering can be used to ensure that the CPU is able to provide execution time for at least one cluster member server that can handle load. The IBM JVM supports multiple garbage collection (GC) helper threads to improve performance during the mark phase of GC. To view or modify the IBM JVM settings from the WebSphere Application Server Administrative Console, select Servers → Application Servers → WebSphere_Portal → Server Infrastructure → Java and Process Management → Process Definition → Java Virtual Machine. The default and recommended values are shown in Table 5-1. Table 5-1 IBM JVM settings Parameter JVM minimum starting heap size JVM maximum heap size Default value n/a n/a Recommended value 768 1792 Chapter 5. WebSphere Portal runtime and services 143
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