Analysis on JDK-8022321 java/lang/ref/OOMEInReferenceHandler.java fails intermittently

[David Holmes]

Hi Kalyan,

This is not a hotspot issue so I'm moving this to core-libs, please drop 
hotspot from any replies.

On 20/12/2013 6:26 AM, srikalyan wrote:
> Hi all,  I have been working on the bug JDK-8022321
> <https://bugs.openjdk.java.net/browse/JDK-8022321> , this is a sporadic
> failure and the webrev is available here
> http://cr.openjdk.java.net/~srikchan/Regression/JDK-8022321_OOMEInReferenceHandler-webrev/

I'm really not sure what to make of this. We have a test that triggers 
an out-of-memory condition but the OOME can actually turn up in the 
ReferenceHandler thread causing it to terminate and the test to fail. We 
previously accounted for the non-obvious occurrences of OOME due to the 
Object.wait and the possible need to load the InterruptedException class 
- but still the OOME can appear where we don't want it. So finally you 
have just placed the whole for(;;) loop in a try/catch(OOME) that 
ignores the OOME. I'm certain that makes the test happy, but I'm not 
sure it is really what we want for the ReferenceHandler thread. If the 
OOME occurs while cleaning, or enqueuing then we will fail to clean 
and/or enqueue but there would be no indication that has occurred and I 
think that is a bigger problem than this test failing.

There may be no way to make this test 100% reliable. In fact I'd suggest 
that no memory exhaustion test can be 100% reliable.

David

> *
> **"Root Cause:Still not known"*
> 2 places where there is a possibility for OOME
> 1) Cleaner.clean()
> 2) ReferenceQueue.enqueue()
>
> 1)  The cleanup code in turn has 2 places where there is potential for
> throwing OOME,
>      a) thunk Thread which is run from clean() method. This Runnable is
> passed to Cleaner and appears in the following classes
>          java/nio/DirectByteBuffer.java
>          sun/misc/Perf.java
>          sun/nio/fs/NativeBuffer.java
>          sun/nio/ch/IOVecWrapper.java
>          sun/misc/Cleaner/ExitOnThrow.java
> However none of the above overridden implementations ever create an
> object in the clean() code.
>      b) new PrivilegedAction created in try catch Exception block of
> clean() method but for this object to be created and to be held
> responsible for OOME an Exception(other than OOME) has to be thrown.
>
> 2) No new heap objects are created in the enqueue method nor anywhere in
> the deep call stack (VM.addFinalRefCount() etc) so this cannot be a
> potential cause.
>
> *Experimental change to java.lang.Reference.java* :
> - Put one more guard (try catch with OOME block) in the Reference
> Handler Thread which may give the Reference Handler a chance to cleanup.
> This is fixing the test failure (several 1000 runs with 0 failures)
> - Without the above change the test fails atleast 3-5 times for every
> 1000 run.
>
> *PS*: The code change is to a very critical part of JDK and i am fully
> not aware of the consequences of the change, hence seeking expert help
> here. Appreciate your time and inputs towards this.
>