RFR(M,v3): JDK-8059036 : Implement Diagnostic Commands for heap and finalizerinfo

Derek White derek.white at oracle.com
Fri May 15 22:59:52 UTC 2015

Hi Dmitry, Peter,

I should read my email more frequently - I missed Dmitry's last webrev 

My comments on a preference for volatile over Unsafe are not a strong 
objection to using Unsafe fences. I just don't have enough experience 
with Unsafe fences yet to agree that this use is correct.

While looking over this Reference code I found 3-6 really simple things 
that need cleaning up that have been there for years, so I'm not a big 
cheerleader for more complicated things here :-)

  - Derek

On 5/15/15 6:46 PM, Derek White wrote:
> Hi Peter,
> Some comments below...
> On 5/14/15 3:50 AM, Peter Levart wrote:
>> Hi Derek,
>> On 05/13/2015 10:34 PM, Derek White wrote:
>>> Hi Peter,
>>> I don't have smoking gun evidence that your change introduces a bug, 
>>> but I have some concerns...
>> I did have a concern too, ...
>>> On 5/12/15 6:05 PM, Peter Levart wrote:
>>>> Hi Dmitry,
>>>> You iterate the queue then, not the unfinalized list. That's more 
>>>> logical.
>>>> Holding the queue's lock may pause reference handler and finalizer 
>>>> threads for the entire time of iteration. This can blow up the 
>>>> application. Suppose one wants to diagnose the application because 
>>>> he suspects that finalizer thread hardly keeps up with production 
>>>> of finalizable instances. This can happen if the allocation rate of 
>>>> finalizable objects is very high and/or finalize() methods are not 
>>>> coded to be fast enough. Suppose the queue of Finalizer(s) builds 
>>>> up gradually and is already holding several million objects. Now 
>>>> you initiate the diagnostic command to print the queue. The 
>>>> iteration over and grouping/counting of several millions of 
>>>> Finalizer(s) takes some time. This blocks finalizer thread and 
>>>> reference handler thread. But does not block the threads that 
>>>> allocate finalizable objects. By the time the iteration is over, 
>>>> the JVM blows up and application slows down to a halt doing GCs 
>>>> most of the time, getting OOMEs, etc...
>>>> It is possible to iterate the elements of the queue for diagnostic 
>>>> purposes without holding a lock. The modification required to do 
>>>> that is the following (havent tested this, but I think it should work):
>>>> http://cr.openjdk.java.net/~plevart/misc/Finalizer.printFinalizationQueue/webrev.01/
>>> One issue to watch out for is the garbage collectors inspect the 
>>> Reference.next field from C code directly (to distinguish active vs. 
>>> pending, iterate over oops, etc.). You can search hotspot for 
>>> java_lang_ref_Reference::next*, java_lang_ref_Reference::set_next*.
>>> Your change makes "inactive" References superficially look like 
>>> "enqueued" References. The GC code is rather subtle and I haven't 
>>> yet seen a case where it would get confused by this change, but 
>>> there could easily be something like that lurking in the GC code.
>> ...but then I thought that GC can in no way treat a Reference 
>> differently whether it is still enqueued in a ReferenceQueue or 
>> already dequeued (inactive) - responsibility is already on the Java 
>> side. Currently the definition of Reference.next is this:
>>     /* When active:   NULL
>>      *     pending:   this
>>      *    Enqueued:   next reference in queue (or this if last)
>>      *    Inactive:   this
>>      */
>>     @SuppressWarnings("rawtypes")
>>     Reference next;
>> We see that, unless GC inspects all ReferenceQueue instances and 
>> scans their lists too, the state of a Reference that is enqueued as 
>> last in list is indistinguishable from the state of inactive 
>> Reference. So I deduced that this distinction (enqueued/inactive) 
>> can't be established solely on the value of .next field ( == this or 
>> != this)...
>> But I should inspect the GC code too to build a better understanding 
>> of that part of the story...
>> Anyway. It turns out that there is already enough state in Reference 
>> to destinguish between it being enqueued as last in list and already 
>> dequeued (inactive) - the additional state is Reference.queue that 
>> transitions from ReferenceQueue.ENQUEUED -> ReferenceQueue.NULL in 
>> ReferenceQueue.reallyPoll. We need to just make sure the two fields 
>> (r.next and r.queue) are assigned and read in correct order. This can 
>> be achieved either by making Reference.next a volatile field or by a 
>> couple of explicit fences:
>> http://cr.openjdk.java.net/~plevart/misc/Finalizer.printFinalizationQueue/webrev.02/
>> The assignment of r.queue to ReferenceQueue.ENQUEUED already happens 
>> before linking the reference into the queue's head in 
>> ReferenceQueue.enqueue():
>>             r.queue = ENQUEUED;
>>             r.next = (head == null) ? r : head;
>>             head = r;
>> Both stores are volatile.
> This sounds a bit better. I don't have a good handle on the pros and 
> cons of a volatile field over explicit fences via Unsafe in this case. 
> Kim might jump in soon.
> I'd like to suggest an entirely less-clever solution. Since the 
> problem is that forEach() might see inconsistent values for the queue 
> and next fields of a Reference, but we don't want to grab a lock 
> walking the whole queue, we could instead grab the lock as we look at 
> each Reference (and do the "back-up" trick if we were interfered 
> with). Of course this is slower than going lock-free, but it's not any 
> more overhead than the ReferenceHandler thread or FinalizerThreads incur.
> The only benefit of this solution over the others is that it is less 
> clever, and I'm not sure how much cleverness this problem deserves. 
> Given that, and ranking the solutions as "lock" < (clever) "volatile" 
> < "fence", I'd be happier with the "volatile" or "lock" solution if 
> that is sufficient.
>  - Derek
>>>> I also suggest the addition to the ReferenceQueue to be contained 
>>>> (package-private) and as generic as possible. That's why I suggest 
>>>> forEach iteration method with no concrete logic.
>>>> It would be possible to encapsulate the entire logic into a special 
>>>> package-private class (say java.lang.ref.DiagnosticCommands) with 
>>>> static method(s) (printFinalizationQueue)... You could just expose 
>>>> a package-private forEach static method from Finalizer and code the 
>>>> rest in DiagnosticCommands.
>>> That's good for encapsulation. But I'm concerned that if "forEach" 
>>> got exposed beyond careful use in DiagnosticCommands, and the 
>>> Referents were leaked back into the heap, then we could get 
>>> unexpected object resurrection after finalization. This isn't a bug 
>>> on it's own - any finalizer might resurrect its object if not 
>>> careful, but ordinarily /only/ the finalizer could resurrect the 
>>> object. This could invalidate application invariants?
>> Well, it all stays in the confines of package-private API - internal 
>> to JDK. Any future additional use should be reviewed carefully. 
>> Comments on the forEach() method should warn about that.
>>> I agree that using a lock over the ReferenceQueue iteration opens up 
>>> /another/ case of diagnostics affecting application behavior. And 
>>> there are pathological scenarios where this gets severe. This is 
>>> unfortunate but not uncommon. There is enough complication here that 
>>> you should be sure that the fix for diagnostics performance doesn't 
>>> introduce subtle bugs to the system in general. A change in this 
>>> area should get a thorough review from both the runtime and GC sides.
>> Is the webrev.02 proposed above more acceptable in that respect? It 
>> does not introduce any logical changes to existing code.
>>> Better yet, the reference handling code in GC and runtime should 
>>> probably be thrown out and re-written, but that's another issue :-)
>> I may have some proposals in that direction. Stay tuned.
>> Regards, Peter
>>> - Derek
>>>> Regards, Peter
>>>> On 05/12/2015 07:10 PM, Dmitry Samersoff wrote:
>>>>> Everybody,
>>>>> Updated version:
>>>>> http://cr.openjdk.java.net/~dsamersoff/JDK-8059036/webrev.03/
>>>>> Now it iterates over queue and output result sorted by number of instances.
>>>>> -Dmitry
>>>>> On 2015-05-07 00:51, Derek White wrote:
>>>>>> Hi Dmitry, Staffan,
>>>>>> Lots of good comments here.
>>>>>> On the topic of what list should be printed out, I think we should focus
>>>>>> on objects waiting to be finalized - e.g. the contents of the
>>>>>> ReferenceQueue. It's more of a pain to walk the ReferenceQueue, but you
>>>>>> could add a summerizeQueue(TreeMap<String, Integer>) method, or a
>>>>>> general iterator and lambda.
>>>>>> A histogram of objects with finalize methods might also be interesting,
>>>>>> but you can get most of the same information from a heap histogram
>>>>>> (ClassHistogramDCmd, and search for count of Finalizer instances).
>>>>>> BTW, by only locking the ReferenceQueue, we should only be blocking the
>>>>>> FinalizerThread from making progress (and I think there's a GC thread
>>>>>> that runs after GC that sorts found References objects that need
>>>>>> processing into their respective ReferenceQueues). But locking the
>>>>>> "unfinalized" list blocks initializing any object with a finalize() method.
>>>>>> The sorting suggestion is a nice touch.
>>>>>>   - Derek White, GC team
>>>>>> On 5/5/15 10:40 AM, Peter Levart wrote:
>>>>>>> Hi Dmitry, Staffan,
>>>>>>> On 05/05/2015 12:38 PM, Staffan Larsen wrote:
>>>>>>>> Dmitry,
>>>>>>>> I think this should be reviewed on hotspot-gc and core-libs-dev as
>>>>>>>> well considering the changes to Finalizer.
>>>>>>>> I’m a little worried about the potentially very large String that is
>>>>>>>> returned from printFinalizationQueue(). A possible different approach
>>>>>>>> would be to write printFinalizationQueue() in C++ inside Hotspot.
>>>>>>>> That would allow for outputting one line at a time. The output would
>>>>>>>> still be saved in memory (since the stream is buffered), but at least
>>>>>>>> the data is only saved once in memory, then. It would make the code a
>>>>>>>> bit harder to write, so its a question of how scared we are of
>>>>>>>> running out of memory.
>>>>>>> If the output is just a histogram of # of instances per class name,
>>>>>>> then it should not be too large, as there are not many different
>>>>>>> classes overriding finalize() method (I counted 72 overriddings of
>>>>>>> finalize() method in the whole jdk/src tree).
>>>>>>> I'm more concerned about the fact that while traversing the list, a
>>>>>>> lock is held, which might impact prompt finalization(). Is it
>>>>>>> acceptable for diagnostic output to impact performance and/or
>>>>>>> interfere with synchronization?
>>>>>>> It might be possible to scan the list without holding a lock for
>>>>>>> diagnostic purposes if Finalizer.remove() didn't set the removed
>>>>>>> Finalizer.next pointer to point back to itself:
>>>>>>>      private void remove() {
>>>>>>>          synchronized (lock) {
>>>>>>>              if (unfinalized == this) {
>>>>>>>                  if (this.next != null) {
>>>>>>>                      unfinalized = this.next;
>>>>>>>                  } else {
>>>>>>>                      unfinalized = this.prev;
>>>>>>>                  }
>>>>>>>              }
>>>>>>>              if (this.next != null) {
>>>>>>>                  this.next.prev = this.prev;
>>>>>>>              }
>>>>>>>              if (this.prev != null) {
>>>>>>>                  this.prev.next = this.next;
>>>>>>>              }
>>>>>>>              // this.next = this; must not be set so that we can
>>>>>>> traverse the list unsynchronized
>>>>>>>              this.prev = this;   /* Indicates that this has been
>>>>>>> finalized */
>>>>>>>          }
>>>>>>>      }
>>>>>>> For detecting whether a Finalizer is already processed, the 'prev'
>>>>>>> pointer could be used instead:
>>>>>>>      private boolean hasBeenFinalized() {
>>>>>>>          return (prev == this);
>>>>>>>      }
>>>>>>> Also, to make sure a data race dereferencing 'unfinalized' in
>>>>>>> unsynchronized printFinalizationQueue() would get you a fully
>>>>>>> initialized Finalizer instance (in particular the next pointer), you
>>>>>>> would have to make the 'unfinalized' field volatile or insert an
>>>>>>> Unsafe.storeFence() before assigning to 'unfinalized':
>>>>>>>      private void add() {
>>>>>>>          synchronized (lock) {
>>>>>>>              if (unfinalized != null) {
>>>>>>>                  this.next = unfinalized;
>>>>>>>                  unfinalized.prev = this;
>>>>>>>              }
>>>>>>>              // make sure a data race dereferencing 'unfinalized'
>>>>>>>              // in printFinalizationQueue() does see the Finalizer
>>>>>>>              // instance fully initialized
>>>>>>>              // (in particular the 'next' pointer)
>>>>>>>              U.storeFence();
>>>>>>>              unfinalized = this;
>>>>>>>          }
>>>>>>>      }
>>>>>>> By doing these modifications, I think you can remove
>>>>>>> synchronized(lock) {} from printFinalizationQueue().
>>>>>>>> I see you are traversing the ‘unfinalized’ list in Finalizer, whereas
>>>>>>>> the old SA code for ‘-finalizerinfo' traverses the ‘queue’ list. I am
>>>>>>>> not sure of the difference, perhaps someone from the GC team can help
>>>>>>>> out?
>>>>>>> The 'queue' is a ReferenceQueue of Finalizer (FinalReference)
>>>>>>> instances pending processing by finalizer thread because their
>>>>>>> referents are eligible for finalization (they are not reachable any
>>>>>>> more). The 'unfinalized' is a doubly-linked list of all Finalizer
>>>>>>> instances for which their referents have not been finalized yet
>>>>>>> (including those that are still reachable and alive). The later serves
>>>>>>> two purposes:
>>>>>>> - it keeps Finalizer instances reachable until they are processed
>>>>>>> - it is a source of unfinalized instances for
>>>>>>> running-finalizers-on-exit if requested by
>>>>>>> System.runFinalizersOnExit(true);
>>>>>>> So it really depends on what one would like to see. Showing the queue
>>>>>>> may be interesting if one wants to see how the finalizer thread is
>>>>>>> coping with processing the finalize() invocations. Showing unfinalized
>>>>>>> list may be interesting if one wants to know how many live +
>>>>>>> finalization pending instances are there on the heap that override
>>>>>>> finalize() method.
>>>>>>> Regards, Peter
>>>>>>>> For the output, it would be a nice touch to sort it on the number of
>>>>>>>> references for each type. Perhaps outputting it more like a table
>>>>>>>> (see the old code again) would also make it easier to digest.
>>>>>>>> In diagnosticCommand.hpp, the new commands should override
>>>>>>>> permission() and limit access:
>>>>>>>>     static const JavaPermission permission() {
>>>>>>>>       JavaPermission p = {"java.lang.management.ManagementPermission",
>>>>>>>>                           "monitor", NULL};
>>>>>>>>       return p;
>>>>>>>>     }
>>>>>>>> The two tests don’t validate the output in any way. Would it be
>>>>>>>> possible to add validation? Perhaps hard to make sure an object is on
>>>>>>>> the finalizer queue… Hmm.
>>>>>>>> Thanks,
>>>>>>>> /Staffan
>>>>>>>>> On 5 maj 2015, at 12:01, Dmitry Samersoff
>>>>>>>>> <dmitry.samersoff at oracle.com>  wrote:
>>>>>>>>> Everyone,
>>>>>>>>> Please review the fix:
>>>>>>>>> http://cr.openjdk.java.net/~dsamersoff/JDK-8059036/webrev.01/
>>>>>>>>> heap dcmd outputs the same information as SIGBREAK
>>>>>>>>> finalizerinfo dcmd outputs a list of all classes in finalization queue
>>>>>>>>> with count
>>>>>>>>> -Dmitry
>>>>>>>>> -- 
>>>>>>>>> Dmitry Samersoff
>>>>>>>>> Oracle Java development team, Saint Petersburg, Russia
>>>>>>>>> * I would love to change the world, but they won't give me the sources.

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