RFC 7038914: VM could throw uncaught OOME in ReferenceHandler thread (original) (raw)
David Holmes david.holmes at oracle.com
Thu May 9 10:02:46 UTC 2013
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Hi Peter,
Thanks for clarifying the test details. A few follow ups:
The reference class does get initialized early in the VM startup well before Main will be loaded. But the use of the weakref doesn't hurt.
500ms may not be enough on some platforms, particularly some embedded systems. Ideally we could code up a handshake using another weakref that would guarantee that the handler thread really survived past the OOM. But at some point this just becomes a no-reg-hard situation :)
I'm not certain that setting waste=null is sufficient to guarantee the next allocation will succeed under all GC's. GC folk can confim/deny that.
Thanks, David
On 9/05/2013 6:35 PM, Peter Levart wrote:
On 05/09/2013 07:53 AM, David Holmes wrote:
Hi Thomas,
On 9/05/2013 1:28 AM, Thomas Schatzl wrote: Hi,
please review the latest webrev for this patch that is available at http://cr.openjdk.java.net/~tschatzl/7038914/webrev.2/ As mentioned, it incorporates the fix and reproducer from Peter Levart. Fix is fine. I'm not sure about the test (sorry I didn't pay it attention earlier). Have you instrumented the code to verify that the test actually triggers an OOME? It may be possible that if the OOME did kill the reference thread that we wouldn't necessarily detect it using the sleeps etc. Also I don't understand the need for the actual weakRef usage and System.gc() etc. If the heap is full then the interrupt will wake the reference handler thread and the allocation will trigger the OOME. It doesn't matter if there are any references to process. The main logic might then reduce to: referenceHandlerThread.interrupt(); for (int i = 0; i < 10; i++) {_ _if (!referenceHandlerThread.isAlive())_ _throw new Exception("Reference-handler thread died");_ _Thread.sleep(1000);_ _}_ _which just gives it 10s to die else assumes all ok. ?_ _But I can live with existing test (it just might vacuously pass)_ _Cheers,_ _David_ _Hi David, Thomas,_ _Yes, this was just a left-over from my bug reproducer that demonstrated_ _the situation where WeakReference was cleared, but nothing was enqueue-d_ _into the ReferenceQueue. Reference Handler thread otherwise just sleeps_ _in lock.wait() most of the time if there's nothing to do and_ _interrupting it's thread will trigger allocation of InterruptedException_ _and consequently OOME nevertheless._ _One think to watch: Reference Handler thread is started in the_ _j.l.r.Reference static initializer, so the Reference class must be_ _initialized. I don't know if there is any guarantee that this is done_ _before entering main(). So there should be something explicit in main()_ _method that ensures it._ _Also, throwing test-fail exception should be done after releasing the_ _heap or we'll just trigger another OOME without any message to describe_ _the situation._ _Waiting in a loop for ReferenceHandler thread is not needed since the_ _test will normally succeed and so the whole loop will execute to the_ _end. Only when the test fails the loop will exit faster. In my_ _experience, the thread dies between 10-20ms after interrupting, so_ _waiting for about 500ms is enough I think. Also no System.gc() and_ _additional waiting is needed to report the outcome - just releasing the_ _reference to 'waste' variable is enuugh in my experience to restore the_ _heap into working condition._ _Here's the updated test that does all that:_ _public class OOMEInReferenceHandler {_ _static Object[] fillHeap() {_ _Object[] first = null, last = null;_ _int size = 1 << 20;_ _while (size > 0) { try { Object[] array = new Object[size]; if (first == null) { first = array; } else { last[0] = array; } last = array; } catch (OutOfMemoryError oome) { size = size >>> 1; } } return first; } public static void main(String[] args) throws Exception { // ensure ReferenceHandler thread is started new WeakReference(new Object()); ThreadGroup tg = Thread.currentThread().getThreadGroup(); for (ThreadGroup tgn = tg; tgn != null; tg = tgn, tgn = tg.getParent()); Thread[] threads = new Thread[tg.activeCount()]; Thread referenceHandlerThread = null; int n = tg.enumerate(threads); for (int i = 0; i < n; i++) { if("Reference Handler".equals(threads[i].getName())) { referenceHandlerThread = threads[i]; } } if (referenceHandlerThread == null) { throw new IllegalStateException("Couldn't find Reference Handler thread."); } Object waste = fillHeap(); referenceHandlerThread.interrupt(); Thread.sleep(500L); // release waste so we can allocate and throw normal exceptions waste = null; // check that the handler is still alive if (!referenceHandlerThread.isAlive()) { throw new Exception("Reference Handler thread died."); } } } Regards, Peter Bugs.sun.com: http://bugs.sun.com/viewbug.do?bugid=7038914 JIRA: https://jbs.oracle.com/bugs/browse/JDK-7038914 Testing: JPRT, with the new reproducer passing In need of reviewers and a sponsor for this patch; as mentioned earlier I will set Peter (plevart) as author for this patch, i.e. send a patchset to the sponsor for this change with that author set. Thanks, Thomas
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