On Tue, 31 Jan 2023 16:02:07 GMT, Per Minborg <pminb...@openjdk.org> wrote:
>> `ZoneOffset` instances are cached by the `ZoneOffset` class itself for
>> values in the range [-18h, 18h] for each second that is on an even quarter
>> of an hour (i.e. at most 2*18*4+1 = 145 values).
>>
>> Instead of using a `ConcurrentHashMap` for caching instanced, we could
>> instead use an `AtomicReferenceArray` with direct slot value access for said
>> even seconds. This will improve performance and reduce the number of object
>> even though the backing array will go from an initial 32 in the CHM to an
>> initial/final 145 in the ARA. The CHM will contain much more objects and
>> array slots for typical numbers of entries in the cache and will compute
>> hash/bucket/collision on the hot code path for each cache access.
>
> src/java.base/share/classes/java/time/ZoneOffset.java line 432:
>
>> 430: if (totalSeconds % (15 * SECONDS_PER_MINUTE) == 0) {
>> 431: int slot = cacheSlot(totalSeconds);
>> 432: ZoneOffset cached = SECONDS_CACHE.get(slot);
>
> I miss `AtomicReferenceArray::computeIfNull` that atomically will compute an
> element if the value at a certain index is `null`.
@minborg You could compareAndExchange in a CompletableFuture—if you succeed you
can complete it with the computation (bonus points since the computation can be
done async) and if you fail you get either a value or a CompletableFuture you
can decide if you want to block on it, and for how long?
-------------
PR: https://git.openjdk.org/jdk/pull/12346
