JEP 132: More-prompt finalization (Re: Another take on Finalization)

[Peter Levart]

Hi Jonathan,

On 06/05/2015 11:11 PM, Jonathan Payne wrote:
> Hi.
>
> I have had an interest in the topic of finalization ever since it caused me to abandon the G1 collector 3 or 4 years ago.
>
> I’ve recently implemented a fix for my interpretation of the problem, which might be very different from the discussion currently ongoing in the thread entitled "JEP 132: More-prompt finalization”.
>
> My problem was that finalization was not being run at all with the G1 collector. Not at all. That would have been fine with me because none of the existing objects in the Finalizer queue actually needed the service anymore: the files, sockets, streams, etc. had all been properly closed by my application, otherwise the server would have long since failed completely. However, those objects started to accumulate in the VM and eventually (8 hours later) brought the server down.

The most probable cause of the described behavior might be that your 
finalizable objects live long enough so that they are promoted to the 
old-generation together with their tracking Finalizer(s) 
(FinalReferences). We know that a finalizable object must undergo at 
least 2 GC cycles to be reclaimed. The 1st cycle merely discovers the 
final-reachable objects and links their FinalReference(s) to the pending 
chain. ReferenceHandler thread then pushes them to the finalization 
queue where their final() methods are invoked by finalizer thread which 
also clears their associated FinalReference(s) and unlinks them from the 
double-linked list. During the 2nd cycle, such finalizable objects are 
usually found unreachable and are reclaimed. It might be that G1 has (or 
had) a performance problem so that it couldn't discover final-reachable 
objects in old-generation soon enough to push them through 2 cycles 
before the system choked.

This is roughly similar to a problem you might have if lots of normal 
objects get promoted to old-generation, but it might be the effects are 
more drastic in G1 when those objects are finalizable. So you might be 
able to tune your app by increasing the young generation and/or the time 
it takes before objects are promoted to old-generation.

I understand that it would be desirable for a finalizable object to be 
made "untracked" as soon as it is manually cleaned-up. This would most 
certainly give a relief to GC as it could reclaim such untracked objects 
immediately like normal objects without pushing them through all the 
finalization steps.

Such feature would need new API. Here's my take on such API incorporated 
in my prototype. This feature is mostly independent of other changes in 
the prototype and could be provided stand-alone. I choose to incorporate 
it into the prototype to compare it's overhead with classical 
finalization in unchanged and changed JDKs:

http://cr.openjdk.java.net/~plevart/misc/JEP132/ReferenceHandling/webrev.03/

The java.lang.ref.Finalizator is a public subclass of package-private 
Finalizer which is a subclass of package-private FinalReference which is 
a subclass of public Reference. The public API therefore consists just 
of two types: Reference and Finalizator and the public methods either 
implemented or inherited by Finalizator. Finalizator is basically just a 
special kind of Reference, which can't be subclassed (is final), can't 
be registeread with a custom reference-queue, and can only be 
constructed using a Finalizator.create(finalizee, thunk) factory method 
taking a 'finalizee' to be tracked and a 'thunk' that is usually just an 
adapter for invoking a private cleanup method on the finalizee. 
Finalizator also implements java.lang.Runnable. It's run() method is 
invoked by finalization infrastructure or manually by user code that 
wishes to promptly trigger clean-up (from AutoCloseable.close() method 
for example).

Here's how a classical finalizable class that also implements 
AutoCloseable might be implemented. Note that the class must implement 
it's own logic to make clean-up idempotent, since finalize() will be 
called even after close() has manually or automatically already been called:

     static final class Finalizable extends AtomicBoolean implements 
AutoCloseable {

         @Override
         protected void finalize() throws Throwable {
             close();
         }

         @Override
         public void close() {
             // close must be idempotent
             if (compareAndSet(false, true)) {
                 // clean-up invoked at most once
             }
         }
     }


And here's how an alternative celanup might be implemented using 
Finalizator. Finalizator already guarantees that it's 'thunk' will be 
called at most once regardless of whether it was triggered by GC and/or 
manually:


     static final class Destroyable implements AutoCloseable {
         final Finalizator<Destroyable> finalizator =
             Finalizator.create(this, Destroyable::destroy);

         void destroy() {
             // clean-up invoked at most once
         }

         @Override
         public void close() {
             // close just runs the finalizator
             finalizator.run();
         }
     }


As soon as Finalizator is run() 1st time, it is cleared and unlinked 
from the doubly-linked list. After that, GC can reclaim it and the 
finalizee right away without pushing them through the discovery and 
reference processing pipeline only to unlink the Finalizer from the 
doubly-linked list.

I have done some testing and the results of creating and destroying 100M 
objects with a sustained rate of ~90 objects/ms with or without 
performing (AutoCloseable) clean-up immediately after construction gives 
the following results:


Finalization, ORIGINAL

real    2m5.958s
user    0m33.855s
sys     0m1.982s


AutoCloseable combined with Finalization, ORIGINAL

real    2m0.952s
user    0m32.103s
sys     0m1.730s


Finalization, PATCHED

real    2m0.519s
user    0m16.664s
sys     0m1.240s


AutoCloseable combined with Finalization, PATCHED

real    1m55.641s
user    0m16.872s
sys     0m1.218s


Finalizator-based cleanup, PATCHED

real    2m1.379s
user    0m17.422s
sys     0m1.321s


AutoCloseable combined with Finalizator-based cleanup, PATCHED

real    1m55.169s
user    0m4.167s
sys     0m1.139s



We see what I have already shown before that my prototype practically 
halves the CPU overhead of finalization infrastructure. Just making an 
object AutoCloseable and promptly doing the clean-up can not reduce this 
overhead if the object is also finalizable. But if manual clean-up also 
"unregisters" the Finalizator from the doubly-linked list and clears it, 
it spares the finalization infrastructure from processing it as a 
finalizable object which further reduces the CPU overhead for a factor 
of 4, totaling just 1/8th of overhead of classic finalization with 
current JDK. Besides greatly reduced CPU overhead, such objects are also 
potentially more promptly reclaimed by GC, freeing memory for other more 
useful things...


> Which brings me to a few points:
>
> Finalization as conceived in the early JDKs was a bad idea. To make matters worse, the way we then made use of it in those early days was A REALLY REALLY bad idea.
> None of this mattered in those days because the GC ran often and quickly and finalization occurred during every GC cycle.
> There may be situations where finalization as a feature actually matters, but in the intervening years the JDK has added new technologies that provide a way to accomplish finalization on your own, in your own code. A few helper classes and it might even be easy when it’s necessary, which is hopefully almost never.
> Many of the uses of finalize() in the JDK today are bad and should be deleted.
>
> My fix, BTW, was to use a back door (that I added to SharedSecrets) in all the JDK classes that had a finalize() method, so that when a resource is properly closed, by calling the close() method for example, the back door would remove the Finalizer for the specified object from the linked list of Finalizer objects, thus removing it from the finalization equation altogether. I implemented this, and then the various tests of creating a huge number of objects with a finalize() method ran quickly and flawlessly with no horrific GCs or even a growing memory pool. The main problem with my solution was that there was this nasty SharedSecrets back door, so it has been rejected and probably rightly so.
>
> However, it proved a point.
>
> But now I am wondering why the actual right thing to do is not simply this:
>
> 	Remove the finalize() method from all the worst offenders in the JDK.
>
> I cannot remember all the places I patched when I implemented my fix, but the majority of them were pieces of code that absolutely had a close() method. If you don’t close objects when you’re done with them, your program PROBABLY SHOULD BE BROKEN. But even if you do not accept that, for all practical purposes, the program IS broken today because finalization is absolutely NOT run in a timely enough fashion.

I have shown that we can have a cake and eat it too. Combining 
Finalizator-based clean-up with manual (or AutoCloseable) clean-up is a 
win-win situation. Programs that forget to call close() still work and 
those that do prompt close()ing will not be affected by finalization 
overhead. The migration of internal JDK code from finalize() methods to 
Finalizator-based cleanup should be simple and straight-forward.

So what are we waiting for? ;-)

> BTW - I never understood why CMS and other GC’s had absolutely no problem running finalization in a very timely fashion while the G1 collector just never seemed to get around to it. My interpretation of that fact has always led me to believe that it’s not a throughput issue with the finalization thread (not in real world examples, anyway) but rather a GC implementation that didn’t feel the need to be thorough enough to make sure something is ready to be finalized. I mean, when the G1 collector was forced to run a full collection (a death sentence on a 15Gb heap but it did occur) all the finalizable objects were found AND finalized immediately, all 15 or 20 million of them.
>
> So in summary:
>
> (1) The problem with finalization is that people use it. And more importantly, that the JDK is filled with inappropriate uses of it.
>
> (2) The main solution is probably just to delete the inappropriate uses in the JDK. But if that’s not OK, then some sort of patch like what I did which allows the JDK classes to unregister the Finalizer’s when they are no longer needed, i.e., when the object knows that it has cleaned itself up.
>
> I am curious to hear your thoughts.

Thanks for the description of the problem you have with finalization. 
JDK has an internal alternative to finalization called sun.misc.Cleaner, 
which has basically the same API and implementation as my presented 
Finalizator with the following differences:

- Cleaner is a PhantomReference which means that the referent is not 
obtainable any more when it is triggered, so clean-up code can only work 
on state that is not part of the referent (captured by Cleaner's thunk 
at the time of construction). This is suitable sometimes but not always.
- Cleaner(s) are executed by the ReferenceHandler thread direclty which 
makes them unsuitable for public consumption as their thunk's must 
guarantee to be executed quickly or else the whole reference processing 
infrastructure blocks. Finalizator(s) are executed by the same thread(s) 
as Finalizer(s).

While it would be possible to retro-fit internal JDK classes to use 
Cleaner(s) instead of finalize() methods, this would require more 
refactoring which is always tricky. Finalizators, on the other hand, 
could be used as a drop-in replacement for finalize() method.

> JP
>

Regards, Peter