Most probably, a lot of Java developers have seen OutOfMemory error one time or other. However these errors come in different forms and shapes. The more common is: "Exception in thread "main" java.lang.OutOfMemoryError: Java heap space" and indicates that the Heap utilization has exceeded the value set by -Xmx. This is not the only error message, of this type, however.
One more interesting flavor of the same error message, less common but hence even more troublesome is: "java.lang.OutOfMemoryError: PermGen space". Most of the memory profiler tools are unable to detect this problem, so it is\ even more troublesome and therefor - interesting.
To understand this error message and fix it, we have to remember that, for optimized, more efficient garbage-collecting Java Heap is managed in generations - memory segments holding objects of different ages. Garbage collection algorithms in each generation are different. Objects are allocated in a generation for younger objects - the Young Generation, and because of infant mortality most objects die there. When the young generation fills up it causes a Minor Collection. Assuming high infant mortality, minor collections are garbage-collected frequently. Some surviving objects are moved to a Tenured Generation. When the Tenured Generation needs to be collected there is a Major Collection that is often much slower because it involves all live objects. Each generation contains variables of different length of life and different GC policies are applied to them.
There is a third generation too - Permanent Generation. The permanent generation is special because it holds meta-data describing user classes (classes that are not part of the Java language). Examples of such meta-data are objects describing\ classes and methods and they are stored in the Permanent Generation. Applications with large code-base can quickly fill up this segment of the heap which will cause java.lang.OutOfMemoryError: PermGen no matter how high your -Xmx and how much memory you have on the machine.
Sun JVMs allow you to resize the different generations of the heap, including the permanent generation. On a Sun JVM (1.3.1 and above) you can configure the initial permanent generation size and the maximum permanent generation size.\ \ To set a new initial size on Sun JVM use the -XX:PermSize=64m option when starting the virtual machine. To set the maximum permanent generation size use -XX:MaxPermSize=128m option. If you set the initial size and maximum size to equal values you may be able to avoid some full garbage collections that may occur if/when the permanent generation needs to be resized. The default values differ from among different versions but for Sun JVMs upper limit is typically 64MB.
Some of the default values for Sun JVMs are listed below.
|JDK 1.3.1_06||Initial Size||Maximum Size|
|JDK 1.4.1_01||Initial Size||Maximum Size|
|JDK 1.4.2||Initial Size||Maximum Size|
|JDK 1.5.0||Initial Size||Maximum Size|
Following is a JSP code you can use to monitor memory utilization in different generations (including PermGen):
As of JBoss v3.2.8/4.0.2, the ServerInfo MBean registered under the
jboss.system:type=ServerInfo has been enhanced with a new operation
listMemoryPools(boolean fancy) that presents information about the memory pools managed by the JVM.