On Tue, 16 Apr 2024 11:47:23 GMT, Per Minborg <pminb...@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? ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/18794#discussion_r1568940281
