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What is the underlying cause of a PermGen OutOfMemoryError in JBoss?

I am running JBoss AS 4.2.2 in my development environment, and this occurs after redeploying my web application a large number of times.

Christian Vest Hansen's blog gives JVM options that help a lot, but do not solve the problem completely:

-XX:+UseConcMarkSweepGC
-XX:+CMSPermGenSweepingEnabled
-XX:+CMSClassUnloadingEnabled
-XX:MaxPermSize=128m
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3 Answers 3

up vote 5 down vote accepted

As already mentioned, you're probably experiencing leaking class loaders. For some reason your classes are not being unloaded. This can happen for two reasons

  • Objects of said classes still exist on the heap, objects always reference their class, or
  • The class loader is referenced somewhere, for whatever reason, class loaders reference their classes in order to not load them twice

There's no catch-all solution to this problem. One helpful tool to help you find the root cause is the Eclipse Memory Analyzer Tool, which you can apply to a heap dump from your JVM (you can enable heap dumps on OOMs with the -XX:+HeapDumpOnOutOfMemoryError option). Perhaps start looking for java.lang.Class objects from your web app to see why they are being kept alive. Unfortunately, PermGen is not normally part of a JVM heap dump, so you can only try to find correlating artifacts in the rest of the heap (Class-objects are not stored in PermGen if im not mistaken, only the actual byte code is, please correct me if I'm wrong though).

HTH.

Edit:
Dave Cheney suggests in a comment that java.lang.Class-objects are indeed part of PermGen, and not included in a normal hotspot heap dump. Unless you have a JVM which writes this info in the heap dump, you'll need a different approach. You can still look for instances of your objects, but if you're leaking classes/class loaders (they both unfortunately imply each other) it seems you need to look for other signs (meta data objects from JBoss, etc).

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I'm pretty certain that Class objects are stored in the PermGen (that is, objects that are instances of Class). There is a separate area on the Sun JVM called the Code Cache, which is where code that has been compiled to machine code is stored. Both the PermGen and the Code Cache are part of the Non Heap memory that you can see using tools like jconsole –  Dave Cheney May 5 '09 at 12:02
    
@Dave Cheney: You're correct. The leaked Class objects stuck in PermGen are usually the cause of the OP's problem. –  Eddie May 5 '09 at 15:02
    
roe, while I don't think the contents of the PermGen are written to the heap dump (this is using the Sun JVM, I don't have much experience with the others), the hprof dump format certainly knows things like, how many classes were loaded, how many class loaders, etc. You can see this when opening the hprof in Eclipse Memory Analyser). What is probably not written is the large binary class data that makes up the java byte code, what you see in the heap dump is the 'stub' of the Class instance stored on the PermGen. –  Dave Cheney May 5 '09 at 15:05

The underlying cause is references to classes that have been discarded leaking outside their classloader, preventing the JVM from unloading those classes from the perm gen. Those flags you use may cause the JVM to aggressively purge classes that are unloadable, but it will not solve the underlying problem.

There is a good, abeit complex explanation here

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The cause of the PermGen OutOfMemory error is the app redeploys. The underlying cause is leaked Class objects in PermGen from the redeploys.

Of course, the workaround is to restart the JVM after a certain number of redeploys.

This is a very difficult problem to totally solve, although with some sleuthing you can often make big improvements. Here's where you start: When your web app is stopped, make sure that:

  • all Threads that you started are stopped
  • all ThreadPools that you started are shut down
  • all static references that you can release are released

These are some of the things that can cause a Class object to be trapped in PermGen.

Also, note that not all JVMs (or all versions of JVMs) will GC Class objects in PermGen. If you are running a JVM or a version of a JVM that will not GC Class objects in PermGen, then your only choice is to restart the JVM after a certain number of redeploys. This probably does not apply to you, given the JVM options you mention.

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