6.8 Just-in-Time compiler
The IBM JVM includes the Just-in-Time compiler (JIT), which is enabled by default. The JIT compiler dynamically generates machine code for frequently used byte code sequences in Java applications. The JIT is tuned for long-running applications, such as WebSphere applications.
Note: The JIT should always be used in production environments.
Modifying JIT optimization levels is normally not recommended. Tuning the JIT should only be necessary for short-running applications.
Use -Xquickstart for short-running applications in which execution time is not concentrated into a small number of methods. -Xnoquickstart to be used for applications that demand that application methods be compiled at a high level of optimization very early in the execution period.
Refer to IBM Java Diagnostics Guide for more details about tuning the JIT:
http://publib.boulder.ibm.com/infocenter/javasdk/v5r0/index.jsp