Tuning Advice For The Ibm Java Virtual Machine - IBM BS029ML - WebSphere Portal Server Self Help Manual

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
Chapter 5. WebSphere Portal runtime and services
Recommended value
768
1792
143