NonStop Server for Java 6.0 Programmer's Reference

-Xms : the startup size of memory allocation pool (the GC heap)
-Xmx : the maximum memory allocation pool
The maximum value for the NonStop system depends on the location of the QIO
segment. If the segment is moved to KSEG2, the maximum value can be as high
as 900 MB, otherwise, the maximum value may stay around 350 MB. For
information on QIO segment location, see “Memory Considerations: Moving QIO
to KSEG2” (page 66). Also, for more information on the -Xms and -Xmx options,
see “Managing Generation Size” (page 60).
For large server applications, the default values of the two options listed are usually not adequate.
In general, you should grant as much memory to the JVM as possible.
Another important factor that affects performance is the proportion of the heap that is assigned to
the young generation. The parameters for the young-generation heap are listed below:
• -XX:NewRatio=nn The ratio between the young and old.
• -XX:NewSize=nn The lower size bound.
• -XX:MaxNewSize=nn The upper size bound.
• -XX:SurvivorRatio=nn Tune the survivor spaces (illustrated in the “Layout for Generations”
(page 60)).
For example:
java -Xms512m -Xmx512m -XX:NewSize=256m -XX:MaxNewSize=256m \
-XX:SurvivorRatio=2 class
These options inform the JVM to set the initial size of the heap to 512 MB, the maximum heap to
512 MB, the new generation to 256 MB (128 MB belongs to Eden and 2x64 MB survivor) and
the old generation to 256 MB.
For details about all these parameters, see Java HotSpot VM Options
(http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html)
For details about tuning garbage collection with the Java SE 6 HotSpot virtual machine and for
general performance tuning information, see:
• Java SE 6 HotSpot Virtual Machine Garbage Collection Tuning
(http://www.oracle.com/technetwork/java/javase/tech/index-jsp-136373.html)
• The article Tuning Java I/O Performance
(http://java.sun.com/developer/technicalArticles/Programming/PerfTuning/)
• The article, Turbo-charging Java HotSpot Virtual Machine, v1.4.x to Improve the Performance
and Scalability of Application Servers
(http://java.sun.com/developer/technicalArticles/Programming/turbo/)
• Java Performance Documentation (http://java.sun.com/docs/performance/)
Java Garbage Collector Tuning for Application Performance 63