NonStop Server for Java Programmer's Reference (NSJ 4.2+)

disabled. If you specify this option, the JVM exits with the error: -XX:+AggressiveHeap option is not
supported on this platform.
Concurrent Low-Pause Collector
A concurrent low-pause collector is for tenured generation. This collector does most of the collection concurrently
with the application execution. This collector divides the collection into different phases and does some phases
concurrently with the application execution and others in "stop the world" mode. This collection technique does
not perform well as the default collector when the phases of the collection cannot run concurrently with the
application thread. This garbage collector, therefore, is disabled in NonStop Server for Java 4.
The following java command options that specify a concurrent low-pause collector are not valid for
NonStop Server for Java 4:
-XX:+UseConcMarkSweepGC
Specifies using the concurrent mark-sweep garbage collector. If you specify this option, the
JVM exits with the error: -XX:+UseConcMarkSweepGC option is not supported on this
platform.
-Xconcgc
Enables a concurrent mark-sweep garbage collector. This option is disabled. If you specify this
option, the JVM exits with the error: -Xconcgc option is not supported on this platform.
Incremental Low-Pause Collector
By careful bookkeeping, the incremental (also called train) low-pause collector collects just a portion of the
tenured generation at each minor collection. The collector tries to spread the large pause of a major collection over
many minor collections. In overall throughput, this collector is slower than the default tenured generation collector.
To use the incremental low-pause collector, specify the following java command option:
-Xincgc
Specifies using the incremental low-pause garbage collector.
Java GC Tuning
The NonStop Server for Java 4 incorporates the HotSpot VM. This topic discusses the options available for tuning
the JVM, suggests flags and values to tune the Java HotSpot VM, and points to HotSpot tuning information on the
Internet.
Since GC takes place when generations fill up, the amount of available total heap memory has direct impact on the
performance. The parameters that affect the heap are listed below.
-Xms : the startup size of memory allocation pool (the GC heap)
-Xmx : the maximum memory allocation pool
The maximum value for the NonStop Kernel 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. Also, for more information on the -Xms and -Xmx options, see Managing Generation Size.
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