the amazing tales of the search for the invisible man! or, where's my gc root

Jed Wesley-Smith jed at atlassian.com
Fri Apr 17 16:32:25 PDT 2009


Kirk,

Thanks for the info. I will try NetBeans and look at what it tells me  
- this is certainly a lot more useful than what the other tools have  
been able to give me. Unfortunately it is Saturday morning in Sydney  
and I will not be able to attempt with generations on until Monday.

As reported below, we have verified that the IBM JVM does not leak the  
plugin system classes.

cheers,
jed.

On 18/04/2009, at 3:51 AM, kirk wrote:

> Hi Jed,
>
> I've had a quick look at the heap dump. I'm having a little trouble  
> understanding what is in there. What I can see is a large number of  
> java.lang.reflect.Method objects being held. There seems to be two  
> competing patterns of references holding onto these objects. I've  
> attached some screenshots rather than use words.
>
> The scary thing is that the references include ClassLoader.scl,  
> JDK12Hooks.systemClassLoader as well as Apache commons logging  
> LogFactory. With this type of the complex entanglement it would seem  
> unlikely that these objects would ever be collected. The other  
> pattern also includes the spiders web of references. It also  
> includes UberspecImpl and a whole bunch of static collections. IME,  
> static collections are involved in the vast majority of leaks I've  
> diagnosed.
>
> Interestingly enough the a portion of the 2nd largest consumer of  
> memory is also tangled up in the JDK12Hooks. Random sampling leads  
> me to AST parse trees and "no reference". Looks like much of this is  
> tied up with Velocity. In fact the largest consumer of memory at 24%  
> is char[]. I'm failing to find anything that is not tied up with  
> Velocity (AST parsing).
>
> Needs more investigation. Be interesting to run a test with  
> generations turned on. NetBeans generations is a true count unlike  
> that provided by YourKit.
>
> Regards,
> Kirk
>
>
> Jed Wesley-Smith wrote:
>> Classes as well. We end up getting an OOME although the profilers  
>> report only a third of the heap is reachable.
>>
>> Although I indicated we saw this on the IBM jdk analysis of that  
>> dump showed a completely different issue that apparently may not be  
>> a problem (due to reflection optimisation on that jdk) - the dead  
>> objects appear to have been correctly cleared. We are reproducing  
>> this to verify.
>>
>> Additionally we tried running with -client on the sun jvms as we  
>> saw a bug that might have caused it reported against server only  
>> but without success.
>>
>> cheers,
>> jed.
>>
>> On 16/04/2009, at 12:51 AM, Tony Printezis <Antonios.Printezis at sun.com 
>> > wrote:
>>
>>> OK, I'll bite.
>>>
>>> When you say: "a large section of memory (a plugin framework)" do  
>>> you mean only objects in the young / old gen, or also classes in  
>>> the perm gen?
>>>
>>> How do you know that said memory is not being reclaimed? Do you  
>>> eventually get an OOM?
>>>
>>> Given that it happens with two different JVMs (I assume you use  
>>> HotSpot on Linux and Mac, as well as the IBM JDK), it's unlikely  
>>> to be a GC bug, as both JVMs would need to have the same bug. Not  
>>> impossible, but unlikely, IMHO.
>>>
>>> Tony
>>>
>>> Jed Wesley-Smith wrote:
>>>> all,
>>>>
>>>> I am writing to this list in some desperation hoping for some  
>>>> expert advice. We (the JIRA development team at Atlassian) have  
>>>> been hunting memory leaks for some weeks and in the process have  
>>>> tracked down and removed every possible reference to a large  
>>>> section of memory (a plugin framework) that we could find.  
>>>> Starting with all strong references and proceeding to remove soft  
>>>> and weak references - even things like clearing the  
>>>> java.lang.reflect.Proxy cache - and even Finalizer references  
>>>> until both YourKit, Eclipse MAT, JProfiler and jhat all report  
>>>> that the memory in question is dead and should be collectable,  
>>>> but inexplicably _the JVM still holds on to it_. There are no JNI  
>>>> Global references either, yet this memory remains uncollectable!
>>>>
>>>> This happens for the 1.5 and 1.6 JVMs on Linux and Mac, and the  
>>>> IBM 1.6 JDK on Linux.
>>>>
>>>> So my question is, how on earth do I search for what is  
>>>> referencing this uncollectable memory? Are there any other tools  
>>>> that can help find why this memory is not collected? Can I query  
>>>> the VM directly somehow?
>>>>
>>>> I fear this is a JVM GC bug as no known memory analysis tool can  
>>>> find the heap root (i.e. according to "the rules" there is no  
>>>> heap root). Are there any known GC memory leaks caused by  
>>>> ClassLoaders being dropped for instance?
>>>>
>>>> The application is creating and disposing of a lot of  
>>>> ClassLoaders via OSGi (Apache Felix) with Spring OSGi. It creates  
>>>> a lot of java.lang.reflect.Proxy class instances.
>>>>
>>>> We have written this up and added an example heap dump here:
>>>> http://jira.atlassian.com/browse/JRA-16932
>>>>
>>>> Having come to the end of our tethers here, if anyone can help in  
>>>> any way it would be massively appreciated.
>>>>
>>>> cheers,
>>>> Jed Wesley-Smith
>>>> JIRA Team @ Atlassian
>>>
>>> -- 
>>> ---------------------------------------------------------------------
>>> | Tony Printezis, Staff Engineer   | Sun Microsystems  
>>> Inc.          |
>>> |                                  | MS  
>>> UBUR02-311                  |
>>> | e-mail: tony.printezis at sun.com   | 35 Network  
>>> Drive               |
>>> | office: +1 781 442 0998 (x20998) | Burlington, MA 01803-2756,  
>>> USA |
>>> ---------------------------------------------------------------------
>>> e-mail client: Thunderbird (Linux)
>>>
>>>
>>
>




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