RFR: 8330465: Stable Values and Collections (Internal) [v5]

[Per Minborg]

On Wed, 15 May 2024 15:27:34 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 two `AtomicReference` and two protected by double-checked locking under concurrent access by all threads:
>> 
>> 
>> Benchmark                                      Mode  Cnt      Score      Error   Units
>> StableBenchmark.atomic                        thrpt   10    259.478 ?   36.809  ops/us
>> StableBenchmark.dcl                           thrpt   10    225.710 ?   26.638  ops/us
>> StableBenchmark.stable                        thrpt   10   4382.478 ? 1151.472  ops/us <- StableValue significantly faster
>> 
>> 
>> Performance compared to static variables protected by `AtomicReference`, class-holder idiom holder, and double-checked locking (all threads):
>> 
>> 
>> Benchmark                                      Mode  Cnt      Score      Error   Units
>> StableStaticBenchmark.atomic                  thrpt   10   6487.835 ?  385.639  ops/us
>> StableStaticBenchmark.dcl                     thrpt   10   6605.239 ?  210.610  ops/us
>> StableStaticBenchmark.stable                  thrpt   10  14338.239 ? 1426.874  ops/us
>> StableStaticBenchmark.staticCHI               thrpt   10  13780.341 ? 1839.651  ops/us
>> 
>> 
>> Performance for stable lists (thread safe) in both instance and static contexts whereby we access a single value compared to `ArrayList` instances (which are not thread-safe) (all threads):
>> 
>> 
>> Benchmark                                      Mode  Cnt      Score      Error   Units
>> StableListElementBenchmark.instanceArrayList  thrpt   10   5812.992 ? 1169.730  ops/us
>> StableListElementBenchmark.instanceList       thrpt   10   4818.643 ?  704.893  ops/us
>> StableListElementBenchmark...
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Switch to monomorphic StableValue and use lazy arrays

I have reworked the stable collections so that we create StableValues on demand and store them in a lazily populated backing array. This improved performance significantly as well as gave us improved startup times (only one array needs to be created upfront). Also, StableValue is now monomorphic. On the flip side is the fact that slightly more memory is needed.

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PR Comment: https://git.openjdk.org/jdk/pull/18794#issuecomment-2112886060