Memory

Memory is one of the first things to look at when you want to optimize performance. If you have any disk swapping, that will have a serious impact on performance. Make sure that your server has an optimal amount of memory and that your JVM is tuned to use it.

There are three JVM command switches that control the amount of memory it uses.

Java heap size:

-Xms

VM heap size:

-Xmx

PermGen space:

-XX:MaxPermSize

These three settings control the amount of memory available to the JVM initially, the maximum amount of memory into which the JVM can grow, and the separate area of the heap called Permanent Generation space.

For example, the default settings can be:

-Xms128m -Xmx1024m -XX:MaxPermSize=128m

This is perfectly reasonable for a moderately sized machine or a developer machine. These settings allow the JVM to initially take 128MB of RAM, grow up to 1024MB of RAM, and have a PermGen space of 128MB. If, however, you have Web Space Server on a server with 4GB of RAM and you are having performance problems, the first thing you might want to look at is increasing the memory available to the JVM. You will be able to tell if memory is a problem by running a profiler (such as Jprobe or YourKit) on the server. If you see Garbage Collection (GC) running frequently, you will definitely want to increase the amount of memory available to the JVM.

Issues with PermGen space can also affect performance. PermGen space contains long-lived classes, anonymous classes and interned Strings. Hibernate, in particular-which Web Space Server uses extensively has been known to make use of PermGen space. If you increase the amount of memory available to the JVM, you may want to increase the amount of PermGen space accordingly.