RFR: JDK-8149925 We don't need jdk.internal.ref.Cleaner any more

Peter Levart peter.levart at gmail.com
Thu Mar 31 21:12:29 UTC 2016

Hi Roger,

On 03/31/2016 09:12 PM, Roger Riggs wrote:
> Hi Peter,
> It would be simpler to understand the changes if we solve the problems 
> one at a time,
> at least for review purposes.

Right. We can focus on one aspect at a time, but I'm still trying to 
keep the whole thing in a working condition at all times...

> To your question in the 2nd part about the Cleaner. (webrev.11.part2)
> I don't think the communication between the memory reserving thread 
> and the unreserving thread
> should be mixed into the Cleaner design or implementation.  The logic 
> for the communication
> between reserveMemory and unreserveMemory methods should be in those 
> two methods
> and isolated to Bits.java.  I understand the intent for the reserving 
> thread to poll for available memory
> and it might as well do something useful while it is waiting and get a 
> hint about unreserved memory.
> But it mixes together the implementations. (too much)

The problem with reserving thread getting information from unreserving 
thread solely by communication implemented in methods reserveMemory and 
unreserveMemory is that this information is not enough. Reserving thread 
must also get information about when all the pending unreservations have 
been performed so that it can either:
- trigger System.gc() to discover fresh pending unreservations, or
- finally give up with OOME

In the absence of this information, all reserving thread can do is 
speculate about this information by observing the timings of 
unreservations happening or not happening - to time out on waiting for 
unreservations to happen. This works (as shown in webrev.10.part2), but 
is not very robust or agile - it introduces unnecessary delays. This 
information is hidden in ReferenceHandler thread (have all pending 
References been enqueued?) and in the Cleaner (has the queue drained out?).

I moved the retrial and helping logic to ExtendedCleaner because I think 
it is reusable for other situations. But if you think it doesn't belong 
to ExtendedCleaner, I can move it back to Bits. We don't strictly need 
to help the Cleaner thread with cleanups. I did it that way because this 
seemed an easy way to communicate the information about the drained 
queue back to allocator thread and to retry reservations at appropriate 
intervals. But let me think about a way to just get this information 
without helping - similarly to what I've done it in ReferenceHandler...

> Having an arbitrary thread (the one trying to allocate a DirectBuffer) 
> help with the Cleaning
> puts an unknown thread perhaps with limited stack or 
> AccessControlContext in place to call the
> cleaning functions is unappealing at best.  The cleaning functions are 
> less predictable than
> the Reference enqueuing functions already discussed but are not much 
> more complex.
> In most cases they are about the complexity of the Deallocator in 
> Direct-X-Buffer, etc.

Allocating thread could be "conditioned" before calling the cleanup 
action by:
- saving and clearing thread-locals
- saving and setting AccessControlContext to unprivileged.
...and restoring these back after the action

The problem with unsufficient stack is more difficult solve though. 
Isn't there a new annotation designed to help with that (mainly intended 
for critical sections of java.util.concurrent classes). The problem with 
using it here would be in that we don't know what cleanup action(s) 
might be executed since Cleaner is a general-purpose API and this 
annotation is only designed for parts of code that are known in advance...

> Can the pieces be disentangled and still pass the DirectBufferAllocTest?

If we want to get that additional piece of information from 
ReferenceHandler and Cleaner, they must be entangled with Bits, but I 
might be able to loosen this entanglement a bit. Will try these ideas 
tomorrow. Stay tuned.

Regards, Peter

> Roger
> On 3/28/2016 1:18 PM, Peter Levart wrote:
>> Hi Mandy, Kim, Per and Roger
>> I'd like to continue the discussion about the 2nd part of removing 
>> jdk.internal.ref.Cleaner in this discussion thread.
>> There was some discussion about whether to synchronize with 
>> ReferenceHandler thread and wait for it to enqueue the Reference(s) 
>> or simply detect that there are no more pending Reference(s) by 
>> timing out on waiting for cleanup actions in discussion thread: "Re: 
>> Analysis on JDK-8022321 java/lang/ref/OOMEInReferenceHandler.java 
>> fails intermittently". Based on that discussion, I have prepared a 
>> webrev that uses an approach where the detection is performed using 
>> timeout:
>> http://cr.openjdk.java.net/~plevart/jdk9-dev/removeInternalCleaner/webrev.10.part2/
>> While this webrev passes the DirectBufferAllocTest, I don't have a 
>> good feeling about this approach since it is not very robust. I can 
>> imagine situations where it would not behave optimally - it would 
>> either trigger reference discovery (System.gc()) more frequently that 
>> necessary or it would cause delays in execution. So I still prefer 
>> the approach where allocating thread(s) explicitly synchronize with 
>> ReferenceHandler thread and wait for it to enqueue pending 
>> Reference(s). Luckily this can be performed in an easy way (as I will 
>> show you shortly). Waiting on discovery of pending references by 
>> ReferenceHandler thread and handing them to it could be moved to 
>> native code so that no notification would have to be performed in 
>> native code from the ReferenceHandler thread to the allocating thread(s).
>> But first, let me reply to Mandy's comments...
>> On 03/25/2016 11:20 PM, Mandy Chung wrote:
>>>> On Mar 19, 2016, at 7:00 AM, Peter Levart<peter.levart at gmail.com>  wrote:
>>>> Here's the webrev:
>>>>      http://cr.openjdk.java.net/~plevart/jdk9-dev/removeInternalCleaner/webrev.08.part2/
>>>>> On 03/07/2016 07:35 PM, Mandy Chung wrote:
>>>>>> I studied webrev.06priv and the history of JDK-6857566.
>>>>>> I’m not comfortable for any arbitrary thread to handle the enqueuing of the pending references (this change is more about the fix for JDK-6857566).
>>>>> Why? A Thread is a Thread is a Thread... When legacy Cleaner is removed, ReferenceHandler thread will be left with swapping pointers only - no custom code will be involved. The only things I can think of against using arbitrary thread are:
>>>>> :
>>> My uncomfort was the fix for JDK-6857566 - both enqueuing pending ref and invoking the cleaning code in an arbitrary thread.
>>> Looking at it again - enqueuing the pending reference is not so much of a concern (simply updating the link) but the common cleaner could be used by other code that may only expect to be invoked in system thread that’s still my concern (thinking of thread locals).
>> As you'll see in the webrev below, enqueueing is performed solely be 
>> ReferenceHandler thread. Allocating thread(s) just wait for it to do 
>> its job. There's a little synchronization action performed at the end 
>> of enqueueing a chunk of pending references that notifies waiters 
>> (allocating threads) so that they can continue. This actually 
>> improves throughput (compared to helping enqueue Reference(s) one by 
>> one) because there's not much actual work to be done (just swapping 
>> pointers) so synchronization dominates. The goal here is to minimize 
>> synchronization among threads and by executing enqueuing of the whole 
>> bunch of pending references in private by a single thread achieves a 
>> reduction in synchronization when lots of Reference(s) are discovered 
>> at once - precisely the situation when it matters.
>> OTOH helping the Cleaner thread is beneficial as cleanup actions take 
>> time to execute and this is the easiest way to retry allocation while 
>> there's still chance it will succeed. As the common Cleaner is using 
>> InnocuousThread, cleanup actions can't rely on any thread locals to 
>> be preserved from invocation to invocation anyway - they are cleared 
>> after each cleanup action so each action gets empty thread locals. We 
>> could simulate this in threads that help execute cleanup actions by 
>> saving thread-locals to local variables, clearing thread-locals, 
>> executing cleanup action and then restoring thread-locals from local 
>> variables. Mandy, if you think this is important I'll add such 
>> save/clear/restore code to appropriate place.
>>>    On the other hand, invoking Deallocator::run (deallocating the native memory) in arbitrary threads has no problem.  Consider me being paranoid of the fix for JDK-6857566.  The current list of clients using CleanerFactory::cleaner may be safe being called from arbitrary threads but I can’t say what will be added in the future.
>> Right, save/clear/restore thread locals then (left for next webrev)...
>>>>>> The allocating thread may do a System.gc() that may discover phantom reachable references.  All it’s interested is only the direct byte buffer ones so that it can deallocate the native memory.  What is the downside of having a dedicated Cleaner for direct byte buffer that could special case for it?
>>>>> A dedicated Cleaner for direct buffers might be a good idea if other uses of shared Cleaner in JDK become heavy. So that helping process Cleanable(s) does not involve other unrelated Cleanable(s). But it comes with a price of another dedicated background thread.
>>> Perhaps provide one Cleaner specific for native memory deallocation or anything safe to be called in arbitrary thread.  It could provide the entry point for the allocating thread to assist the cleaning (i.e. Bits::reserveMemory could call it).  That will make it explicit that this cleaner provides explicit control for other threads to assist the cleaning action (and JavaLangRefAccess would only be used by this special cleaner and not in NIO).
>>> All clients of Unsafe.freeMemory could use that special cleaner for native memory deallocation use such as IOVecWrapper, DirectByteBuffer, Marlin’s OffHeapArray.
>>> The common cleaner would be kept for other things to use and it should be lazily created to avoid another thread.
>>> Does this sound reasonable?
>>> Mandy
>> Of course. Having specialized Cleaner(s) with additional capability 
>> requires extension to the Cleaner API for some cleaners. 
>> Unfortunately java.lang.ref.Cleaner is a final class.
>> Here's what I propose: by transforming java.lang.ref.Cleaner into an 
>> interface implemented by a class in a concealed package 
>> (jdk.internal.ref.CleanerImpl) the public API can be left unchanged 
>> while the implementation is actually simplified (there's no injection 
>> of Cleaner.impl access function into CleanerImpl class needed any 
>> more). The result of that transformation is also the ability to 
>> specify an extension interface (ExtendedCleaner)  located in a 
>> concealed package so it can only be used by system code (java.base 
>> and modules to which jdk.internal.ref is explicitly exported) and the 
>> ability to extend the functionality of implementation by subclassing 
>> it (CleanerImpl.ExtendedImpl). The guts of previous CleanerImpl are 
>> simply moved into a private nested class CleanerImpl.Task:
>> http://cr.openjdk.java.net/~plevart/jdk9-dev/removeInternalCleaner/webrev.11.part2/
>> I'm interested in what Roger has to say about this transformation. It 
>> is source compatible, but not binary compatible (invokevirtual vs. 
>> invokeinterface). So it can be safely performed only before JDK 9 ships.
>> I packed the entire retry-while-helping mechanics into the 
>> implementation of this ExtendedCleaner interface. java.nio.Bits is 
>> consequently much simplified. The common cleaner is now 
>> ExtendedCleaner as other usages besides handling deallocation of 
>> native memory are minor and are not problematic from the standpoint 
>> of arbitrary threads helping with cleanup, especially when 
>> saving/clearing/restoring of thread-locals is implemented. It would 
>> not be a problem to provide another instance, simple 
>> java.lang.ref.Cleaner this time, for other usages if needed.
>> And now a few words about ReferenceHandler thread and synchronization 
>> with it (for Kim and Per mostly). I think it should not be a problem 
>> to move the following two java.lang.ref.Reference methods to native 
>> code if desired:
>>     static Reference<?> getPendingReferences(int[] discoveryPhaseHolder)
>>     static int getDiscoveryPhase()
>> The 1st one is only invoked by a ReferenceHandler thread while the 
>> 2nd is invoked by arbitrary thread. The difference between this and 
>> webrev.09.part2 is that there's no need any more for ReferenceHandler 
>> thread to notify the thread executing the 2nd method and that there's 
>> no need for the 2nd method to perform any waiting. It just needs to 
>> obtain the lock briefly so that it can read the consistent state of 
>> two fields.  Those two fields are Java static fields currently: 
>> Reference.pending & Reference.discoveryPhase and those two methods 
>> are Java methods, but they could be moved to native code if desired 
>> to make the protocol between VM and Java code more robust.
>> So Kim, Per, what do you think of supporting those 2 methods in 
>> native code? Would that present any problem?
>> With webrev.11.part2 I get a 40% improvement in throughput vs. 
>> webrev.10.part2 executing DirectBufferAllocTest in 16 allocating 
>> threads on a 4-core i7 CPU.
>> Regards, Peter

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