Classes not unloading correctly?

Tony Printezis Antonios.Printezis at
Fri Jun 12 20:42:07 UTC 2009

Great to see a second Kessler on this list! Welcome, Jacob.


Jacob Kessler wrote:
> I'm working on the GlassFish scripting project, trying to solve an issue 
> with the PermGen around deploying and undeploying applications.
> We use an embedded JRuby interpreter to host each deployed Ruby 
> application, and JRuby makes extensive use of JIT-ed classes to improve 
> performance. A typical JRuby instance takes roughly 20MB of permgen. 
> We'd like for that space to be reclaimed once the application is 
> undeployed, since losing 20MB of permgen each time an application is 
> deployed puts a fairly hard limit on the number of redeploys that the 
> server can take before it has to be restarted.
> We started out using -XX:+UseConcMarkSweepGC and 
> -XX:+CMSClassUnloadingEnabled, and with the aid of a memory analysis 
> tool (YourKit), we confirmed that some classes were being unloaded (not 
> a significant number, though), and we tracked down and fixed two things 
> that were improperly holding references to the classloader that 
> contained JRuby. After fixing those, though, we ended up in a situation 
> where YourKit was reporting the classes as still in memory after a 
> forced full collection, but as having no paths to the GC roots. This 
> situation seemed able to persist indefinitely (so, beyond time for the 
> finalizer queue to drain). Does anyone have any ideas on what might be 
> preventing the classloader from collecting?
> I'm using
> $java -version
> java version "1.6.0_07"
> Java(TM) SE Runtime Environment (build 1.6.0_07-b06)
> Java HotSpot(TM) 64-Bit Server VM (build 10.0-b23, mixed mode)
> Thank you for any help with this.
> _______________________________________________
> hotspot-gc-use mailing list
> hotspot-gc-use at

| Tony Printezis, Staff Engineer   | Sun Microsystems Inc.          |
|                                  | MS UBUR02-311                  |
| e-mail: tony.printezis at   | 35 Network Drive               |
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