RFR: 8344332: (bf) Migrate DirectByteBuffer away from jdk.internal.ref.Cleaner [v2]

[ExE Boss]

On Sun, 18 May 2025 20:55:48 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> This change makes java.nio no longer use jdk.internal.ref.Cleaner to manage
>> native memory for Direct-X-Buffers. Instead it uses bespoke PhantomReferences
>> and a dedicated ReferenceQueue. This differs from PR 22165, which proposed to
>> use java.lang.ref.Cleaner.
>> 
>> This change is algorithmically similar to the two previous versions:
>> JDK-6857566 and JDK-8156500 (current mainline). The critical function is
>> Bits::reserveMemory(). For both of those versions and this change, a thread
>> calls that function and tries to reserve some space. If it fails, then it
>> keeps trying until all cleaners deactivated (cleared) by prior GCs have been
>> cleaned. If reservation still fails, then it invokes the GC to try to
>> deactivate more cleaners for cleaning. After that GC it keeps trying the
>> reservation and waiting for cleaning, with sleeps to avoid a spin loop,
>> eventually either succeeding or giving up and throwing OOME.
>> 
>> Retaining that algorithmic approach is one of the goals of this change, since
>> it has been successfully in use since JDK 9 (and was originally developed and
>> extensively tested in JDK 8).
>> 
>> The key to this approach is having a way to determine that deactivated
>> cleaners have been cleaned. JDK-6857566 accomplished this by having waiting
>> threads help the reference processor until there was no available work.
>> JDK-8156500 waits for the reference processor to quiesce, relying on its
>> immediate processing of cleaners. java.lang.ref.Cleaner doesn't provide a way
>> to do this, which is why this change rolls its own Cleaner-like mechanism from
>> the underlying primitives. Like JDK-6857566, this change has waiting threads
>> help with cleaning references. This was a potentially undesirable feature of
>> JDK-6857566, as arbitrary allocating threads were invoking arbitrary cleaners.
>> (Though by the time of JDK-6857566 the cleaners were only used by DBB, and
>> became internal-only somewhere around that time as well.) That's not a concern
>> here, as the cleaners involved are only from DBB, and we know what they look
>> like.
>> 
>> As noted in the discussion of JDK-6857566, it's good to have DBB cleaning
>> being done off the reference processing thread, as it may be expensive and
>> slow down enqueuing other pending references. JDK-6857566 only did some of
>> that, and JDK-8156500 lost that feature. This change moves all of the DBB
>> cleaning off of the reference processing thread. (So does PR 22165.)
>> 
>> Neither JDK-6857566 nor this change are...
>
> Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:
> 
>   move jdk.internal.nio.Cleaner to sun.nio

src/java.base/share/classes/java/nio/Bits.java line 145:

> 143:                         // Increment with overflow to 0, so the value can
> 144:                         // never equal the initial/reset cleanedEpoch value.
> 145:                         RESERVE_GC_EPOCH = Integer.max(0, RESERVE_GC_EPOCH + 1);

Could also do the following which avoids the branch in `Integer.max`:
Suggestion:

                        RESERVE_GC_EPOCH = (RESERVE_GC_EPOCH + 1) & Integer.MAX_VALUE;

src/java.base/share/classes/java/nio/Direct-X-Buffer.java.template line 209:

> 207:         super(-1, 0, cap, cap, fd, isSync, segment);
> 208:         address = addr;
> 209: 	cleaner = (unmapper == null) ? null : BufferCleaner.register(this, unmapper);

**OpenJDK** unfortunately uses the less accessible spaces[^1]:
Suggestion:

        cleaner = (unmapper == null) ? null : BufferCleaner.register(this, unmapper);


[^1]:	- https://github.com/prettier/prettier/issues/7475
	- https://alexandersandberg.com/tabs-for-accessibility/

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/25289#discussion_r2096283888
PR Review Comment: https://git.openjdk.org/jdk/pull/25289#discussion_r2096296130