RFR: 8330465: Stable Values and Collections (Internal)

[Chen Liang]

On Mon, 22 Apr 2024 16:16:39 GMT, Dan Heidinga <heidinga at openjdk.org> wrote:

>> # Stable Values & Collections (Internal)
>> 
>> ## Summary
>> This PR proposes to introduce an internal _Stable Values & Collections_ API, which provides immutable value holders where elements are initialized _at most once_. Stable Values & Collections offer the performance and safety benefits of final fields while offering greater flexibility as to the timing of initialization.
>> 
>> ## Goals
>>  * Provide an easy and intuitive API to describe value holders that can change at most once.
>>  * Decouple declaration from initialization without significant footprint or performance penalties.
>>  * Reduce the amount of static initializer and/or field initialization code.
>>  * Uphold integrity and consistency, even in a multi-threaded environment.
>>  
>> For more details, see the draft JEP: https://openjdk.org/jeps/8312611
>> 
>> ## Performance 
>> Performance compared to instance variables using an `AtomicReference` and one protected by double-checked locking under concurrent access by 8 threads:
>> 
>> 
>> Benchmark                       Mode  Cnt  Score   Error  Units
>> StableBenchmark.instanceAtomic  avgt   10  1.576 ? 0.052  ns/op
>> StableBenchmark.instanceDCL     avgt   10  1.608 ? 0.059  ns/op
>> StableBenchmark.instanceStable  avgt   10  0.979 ? 0.023  ns/op <- StableValue (~40% faster than DCL)
>> 
>> 
>> Performance compared to static variables protected by `AtomicReference`, class-holder idiom holder, and double-checked locking (8 threads):
>> 
>> 
>> Benchmark                       Mode  Cnt  Score   Error  Units
>> StableBenchmark.staticAtomic    avgt   10  1.335 ? 0.056  ns/op
>> StableBenchmark.staticCHI       avgt   10  0.623 ? 0.086  ns/op
>> StableBenchmark.staticDCL       avgt   10  1.418 ? 0.171  ns/op
>> StableBenchmark.staticList      avgt   10  0.617 ? 0.024  ns/op
>> StableBenchmark.staticStable    avgt   10  0.604 ? 0.022  ns/op <- StableValue ( > 2x faster than `AtomicInteger` and DCL)
>> 
>> 
>> Performance for stable lists in both instance and static contexts whereby the sum of random contents is calculated for stable lists (which are thread-safe) compared to `ArrayList` instances (which are not thread-safe) (under single thread access):
>> 
>> 
>> Benchmark                                     Mode  Cnt  Score   Error  Units
>> StableListSumBenchmark.instanceArrayList      avgt   10  0.356 ? 0.005  ns/op
>> StableListSumBenchmark.instanceList           avgt   10  0.373 ? 0.017  ns/op <- Stable list
>> StableListSumBenchmark.staticArrayList        avgt   10  0.352 ? ...
>
> src/java.base/share/classes/jdk/internal/lang/StableValue.java line 130:
> 
>> 128:      * } else {
>> 129:      *     V newValue = supplier.get();
>> 130:      *     stable.setOrThrow(newValue);
> 
> If ::computeIfUnset allows racy sets, then it isn't equivalent to this code as ::setOrThrow will throw on a race, correct?

Indeed, this if-else should be guarded by a synchronized block, except the lock is on the internal mutex which is not publicly exposed.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18794#discussion_r1575101750