Sun Microsystems 820434310 manual Rebasing DLLs on Windows, Survivor Ratio Sizing

Managing Memory and Garbage Collection

■Decide the total amount of memory you can afford for the JVM. Accordingly, graph your own performance metric against young generation sizes to find the best setting.

■Make plenty of memory available to the young generation. The default is calculated from NewRatio and the -Xmx setting.

■Larger eden or younger generation spaces increase the spacing between full GCs. But young space collections could take a proportionally longer time. In general, keep the eden size between one fourth and one third the maximum heap size. The old generation must be larger than the new generation.

For up-to-date defaults, see Java HotSpot VM Options.

EXAMPLE 4–1Heap Configuration on Solaris

This is an exmple heap configuration used by Enterprise Server on Solaris for large applications:

-Xms3584m

-Xmx3584m

-verbose:gc

-Dsun.rmi.dgc.client.gcInterval=3600000

Survivor Ratio Sizing

The SurvivorRatio parameter controls the size of the two survivor spaces. For example, -XX:SurvivorRatio=6 sets the ratio between each survivor space and eden to be 1:6, each survivor space will be one eighth of the young generation. The default for Solaris is 32. If survivor spaces are too small, copying collection overflows directly into the old generation. If survivor spaces are too large, they will be empty. At each GC, the JVM determines the number of times an object can be copied before it is tenured, called the tenure threshold. This threshold is chosen to keep the survivor space half full.

Use the option -XX:+PrintTenuringDistribution to show the threshold and ages of the objects in the new generation. It is useful for observing the lifetime distribution of an application.

Rebasing DLLs on Windows

When the JVM initializes, it tries to allocate its heap using the -Xms setting. The base addresses of Application Server DLLs can restrict the amount of contiguous address space available, causing JVM initialization to fail. The amount of contiguous address space available for Java memory varies depending on the base addresses assigned to the DLLs. You can increase the amount of contiguous address space available by rebasing the Application Server DLLs.

Chapter 4 • Tuning the Java Runtime System 89