RFR: 8330465: Stable Values and Collections (Internal)

[Maurizio Cimadamore]

On Tue, 16 Apr 2024 11:47:23 GMT, Per Minborg <pminborg 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 ? 0.002  ns/op
> StableListSumBenchmark.staticList             avgt   10  0.356 ? 0.00...

src/java.base/share/classes/java/util/ImmutableCollections.java line 1505:

> 1503:     }
> 1504: 
> 1505:     static final class StableMap<K, V>

Question: can stable maps be implemented in terms of stable lists? After all, you need a stable backing array - and then you need to have a way to go from a key to an index in the stable array. The logic that does key->index conversion belongs to Map, but after that we should be able to just "delegate" to StableList?

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PR Review Comment: https://git.openjdk.org/jdk/pull/18794#discussion_r1568940281