Maximum Performance with WebSphere Application Server V5.1 on iSeries

Chapter 7. Tuning the Java virtual machine 219

You may think of a JVM as a special type of operating system. It requires some time to

prepare a runtime environment for execution of a Java program. It starts multiple threads,

loads multiple Java classes, and so on.

Runtime environment

The Java programming language has built-in support for multithreading. This feature implies

that JVM on iSeries has to run in a job that supports multiple threads, for example batch

immediate (BCI).

Figure 7-1 shows a simplified view of JVM. It also shows a special type of a thread called the

garbage collector. This thread is responsible for cleaning up a JVM heap.

Figure 7-1 JVM environment on iSeries

A Java program creates many objects during its execution. These objects are created in a

JVM heap. When an object is dead (meaning that no other object has a reference to this

object), garbage collector removes this object from the heap. This function of the Java

language and JVM is performed automatically.

There are two operating modes for the garbage collector:

 Asynchronous: In this mode, the garbage collector tries to clean up the heap without

stopping other threads. It runs in the background.

 Stop-and-copy: In this mode, all active threads are suspended while the garbage

collector cleans up the heap.

JVM on iSeries uses the asynchronous algorithm for the garbage collector. See 7.2,

“Garbage collection in iSeries JVM” on page 223, for more details about garbage collector.

7.1.2 Runtime modes of execution

Java programs can be executed in different execution modes. An execution mode is the

manner in which a JVM converts Java bytecode into the machine instructions. There are

trade-offs between these modes.

Important: JVM and Java programs create a high number of threads. Make sure that you

have an adequate setting for the activity level in the subsystem where you run JVM.

Also, the health of JVM and garbage collector plays the most important role in ensuring

good performance of your Java and WebSphere applications.

JAVA

CLASS('Hello')

Java Virtual

Machine (JVM)

Application Code

(Hello.class)

Garbage

Collector

Thread X Thread Y

Interactive Job

Batch Immediate Job