Getting Started with WebSphere Application Server

190 Getting Started with WebSphere Application Server Community Edition
A value too high results in high memory consumption and a low number of major slow
garbage collection cycles. A value too low results in a high number of minor fast garbage
collection cycles.
The goal of properly tuning the heap size is to minimize the overhead of GC to improve the
server response and the throughput. The IBM JDK which comes with Community Edition
allows you to change several parameters that are related to the heap size:
• Initial heap size
The initial heap size specifies the initial amount of memory that is allocated in the heap
when the JVM starts.
To set the initial heap size include the following option in the JAVA_OPTS
environment variable where mem is the heap size in megabytes -Xmsm
• Maximum heap size
The maximum heap size specifies the maximum amount of memory that the JVM can
use.
To set maximum heap size, include the following option in the JAVA_OPTS
environment variable where mem is maximum heap size in megabytes -Xmxm
Table 10.1 shows the IBM JVM default heap size values on Linux and Windows.
JVM setting Linux Windows
Initial Heap Size (-Xms) 4 MB 4 MB
Maximum Heap Size (-
Xmx)
Half the real memory with a
minimum of 16 MB and a
maximum of 512 MB
Half the real memory with
a minimum of 16 MB and
a maximum of 2 GB
Table 10.1 - IBM JVM default heap size values
• Heap with large pages
Note:
It is also possible to configure the IBM JVM to allocate the heap using large pages.
Use the following option –Xlp
For information about the JAVA_OPTS environment variable see
http://publib.boulder.ibm.com/wasce/V2.1.1/en/javaopts.html
You can add the JAVA_OPTS variable to the following files on Windows:
/bin/geronimo.bat
/bin/setenv.bat
On Linux the filenames are the same but have a .sh extension.