NonStop Server for Java 5.0 Programmer's Reference

disabled. If you specify this option, the JVM exits with the error: -XX:+AggressiveHeap option is not
supported on this platform.
-XX:GCHeapFreeLimit=space-limit
Specifies the lower limit on the amount of space freed during a garbage collection in percentage of the
maximum heap.
-XX:GCTimeLimit=time-limit
Specifies the upper limit on the amount of time spent in garbage collection in percent of total time.
Also, the following flags of the Garbage Collector Ergonomics technology for a parallel collector are not
supported. These specify performance goals for the application.
-XX:MaxGCPauseMillis=nnn
-XX:GCTimeRatio=nnn
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 5.
The following java command options that specify a concurrent low-pause collector are not valid for
NonStop Server for Java 5:
-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 5 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)