PermGen problem jmap/jhat leads to dead end

Y. Srinivas Ramakrishna Y.S.Ramakrishna at Sun.COM
Mon Dec 14 13:58:33 PST 2009

What's the version of the HotSpot JVM you are running with? (See 4957990.)

-- ramki

On 12/14/09 13:54, Keith McNeill wrote:
> We're having a perm gen problem (maybe also code cache).  Our product 
> generates bytecodes and then loads and optionally unloads them.  Running 
> stability tests where we repeatably load & unload our generated 
> applications we have encountered problems running out of perm gen 
> space.  A few years ago we had similar problems but by using jmap/jhat 
> we were able to debug and fix this.
> This time using jmap/jhat to analyze the problem has lead to a dead end.
> In particular we see:
> 1) There are no instances left of the classes that we load/unload
> 2) The class objects of the classes that we load/unload hang around, 
> they don't seem to get GC'ed
> 3) The ClassLoader of the loaded classes is hanging around.  Doing a 
> "Reference Chains from Rootset" in jhat results in 0 chains.  Unless 
> jhat is lying then the classes/classloader should be able to be GC'ed.  
> Right?
> 4) Using jmap -permstat I see the classloaders in question are marked as 
> 'dead'.  This means that it should be able to be GC'ed
> 5) Tried Current GC, default GC, G1 GC and basically get the same 
> errors.  The G1 GC might error out with code cache problems instead.
> 6) The IBM JVM in the same scenario seems to be happy
> 7) If we load the classes (Class.forName()) but don't instantiate them, 
> PermGen seems happier but we then run out of Code Cache.  I'm not sure 
> if the problems are related.
> The tests are run on linux & linux64.  We are using:  
> -XX:MaxPermSize=128m -Xms800m -Xmx1500m -XX:+CMSClassUnloadingEnabled 
> and appropriate JVM args for the different GC's
> Given #3,4 & #6 above I'm thinking that it maybe a hotspot problem.
> What should I try next to track this problem down?
> Thanks,
> Keith McNeill

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