Sun Microsystems 820434310 manual Heap Tuning Parameters, Xmsvalue Xmxvalue

Managing Memory and Garbage Collection

memory structures. The difference between the maximum address space and the total of those values is the amount of memory that can be allocated to the heap.

You can improve performance by increasing your heap size or using a different garbage collector. In general, for long-running server applications, use the J2SE throughput collector on machines with multiple processors (-XX:+AggressiveHeap) and as large a heap as you can fit in the free memory of your machine.

Heap Tuning Parameters

You can control the heap size with the following JVM parameters:

■-Xmsvalue

■-Xmxvalue

■-XX:MinHeapFreeRatio=minimum

■-XX:MaxHeapFreeRatio=maximum

■-XX:NewRatio=ratio

■-XX:NewSize=size

■-XX:MaxNewSize=size

■-XX:+AggressiveHeap

The -Xms and -Xmx parameters define the minimum and maximum heap sizes, respectively. Since GC occurs when the generations fill up, throughput is inversely proportional to the amount of the memory available. By default, the JVM grows or shrinks the heap at each GC to try to keep the proportion of free space to the living objects at each collection within a specific range. This range is set as a percentage by the parameters -XX:MinHeapFreeRatio=minimumand -XX:MaxHeapFreeRatio=maximum; and the total size bounded by -Xms and -Xmx.

Set the values of -Xms and -Xmx equal to each other for a fixed heap size. When the heap grows or shrinks, the JVM must recalculate the old and new generation sizes to maintain a predefined NewRatio.

The NewSize and MaxNewSize parameters control the new generation’s minimum and maximum size. Regulate the new generation size by setting these parameters equal. The bigger the younger generation, the less often minor collections occur. The size of the young generation relative to the old generation is controlled by NewRatio. For example, setting -XX:NewRatio=3 means that the ratio between the old and young generation is 1:3, the combined size of eden and the survivor spaces will be fourth of the heap.

By default, the Enterprise Server is invoked with the Java HotSpot Server JVM. The default NewRatio for the Server JVM is 2: the old generation occupies 2/3 of the heap while the new generation occupies 1/3. The larger new generation can accommodate many more short-lived objects, decreasing the need for slow major collections. The old generation is still sufficiently large enough to hold many long-lived objects.

To size the Java heap:

88 Sun GlassFish Enterprise Server 2.1 Performance Tuning Guide • January 2009