NonStop Server for Java 4.2 Programmer's Reference

-XX:GCLogLimits=M,N

where,

is a non-negative integer that specifies the number of rotating GC log records per file. Each GC log record

corresponds to a GC event. A value of 0 specifies an unlimited number of GC log records per file.

is a non-negative integer that specifies the maximum number of rotating GC log files. A value of 0 specifies an

unlimited number of files.

You must use both M and N when you use the -XX:GCLogLimits=M,N option. If this option is not specified, the

default behavior is to write a single GC log file with unlimited size. When rotation is in effect, a sequence number

is appended to the GC filename (0 through N-1).(Examples of file names are: filename.0, filename.1, and

filename.2.

With log rotation, when the specified maximum number of files (N) is reached, logging cycles back to the first file

in the sequence (filename.0), thereby overwriting the old GC data with new data. If the maximum number of files

(N) is never reached, then no log rotation occurs.

For example,

To rotate between two log files, each with a maximum of 100,000 GC records, use:

-XX:GCLogLimits=100000,2

To maintain an unlimited number of smaller files, each with a maximum of 1,000 GC records, use:

-XX:GCLogLimits=1000,0

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 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

generation. The parameters for the young-generation heap are listed below:

XX:NewRatio=nn The ratio between the young and old.

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