NonStop Server for Java 6.0 Programmer's Reference

only for a parallel collector and, therefore, is 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 6.0.
The following java command options that specify a concurrent low-pause collector are not valid
for NonStop Server for Java 6.0:
-XX:+UseConcMarkSweepGC
Specifies using the concurrent mark-sweep garbage collector. If you specify this
option, the JVMexits 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 book-keeping, the incremental 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 Garbage Collector Tuning for Application Performance
The NonStop Server for Java 6.0 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. GC tuning is assisted by GC profile data which can
be collected and analyzed as explained in the next section.
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.
62 Implementation Specifics