On Wed, 17 Apr 2024 14:24:59 GMT, Maurizio Cimadamore <mcimadam...@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/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? This is partially done but we could pull more on this string and unify the implementations. Incidentally, it is also possible to unify the two implementation classes of `StableValue` so it becomes monomorphic. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/18794#discussion_r1570837425
