If you monitor your java process with an OS tool like top or taskmanager, you may see the amount of memory you use exceed the amount you have specified for -Xmx. -Xmx limits the java heap size, java will allocate memory for other things, including a stack for each thread. It is not unusual for the total memory consumption of the VM to exceed the value of -Xmx.

(thanks to Rob Lockstone for his comments)

There are essentially two GC threads running. One is a very lightweight thread which does "little" collections primarily on the Eden (a.k.a. Young) generation of the heap. The other is the Full GC thread which traverses the entire heap when there is not enough memory left to allocate space for objects which get promoted from the Eden to the older generation(s).

If there is a memory leak or inadequate heap allocated, eventually the older generation will start to run out of room causing the Full GC thread to run (nearly) continuously. Since this process "stops the world", Resin won't be able to respond to requests and they'll start to back up.

The amount allocated for the Eden generation is the value specified with -Xmn. The amount allocated for the older generation is the value of -Xmx minus the -Xmn. Generally, you don't want the Eden to be too big or it will take too long for the GC to look through it for space that can be reclaimed.

See also:

• Troubleshooting Technique: Garbage Collector
• Sun documentation on garbage collection

Each thread in the VM get's a stack. The stack size will limit the number of threads that you can have, too big of a stack size and you will run out of memory as each thread is allocated more memory than it needs.

The Resin startup scripts (httpd.exe on Windows, wrapper.pl on Unix) will set the stack size to 2048k, unless it is specified explicity. 2048k is an appropriate value for most situations.

JDK 5 includes a number of tools that are useful for monitoring the JVM. Documentation for these tools is available from the Sun website. For JDK's prior to 5, Sun provides the jvmstat tools.

The most useful tool is jconsole. Details on using jconsole are provided in the Administration section of the Resin documentation.

jps and jstack are also useful, providing a quick command line method for obtaining stack traces of all current threads. Details on obtaining and interpreting stack traces is in the Troubleshooting section of the Resin documentation.