RFR: 8319447: Improve performance of delayed task handling [v4]

[Doug Lea]

On Tue, 25 Feb 2025 14:51:29 GMT, Viktor Klang <vklang at openjdk.org> wrote:

>> Doug Lea has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Standardize parameter checking
>
> src/java.base/share/classes/java/util/concurrent/DelayScheduler.java line 455:
> 
>> 453:         final long nextDelay;       // 0: once; <0: fixedDelay; >0: fixedRate
>> 454:         int heapIndex;              // if non-negative, index on heap
>> 455:         final boolean isImmediate;  // run by scheduler vs submitted when ready
> 
> If desired, it would be possible to create a bitset for `isImmediate` and `isCallable` such that we could get rid of a reference field per ScheduledForkJoinTask.
> I suspect it would also be possible to use the same field for `pool` and `result` as there shouldn't be a result available for as long as `pool` is non-null.
> 
> Might not be worth doing at this point, but saving 2 reference fields for something which might get allocated millions of times might be worth it?

I tried a few variants and didn't see improvements. But I added a sentence to doc with my guess why:
     *  (We use the same structure for both
     * Runnable and Callable versions, since including an extra field
     * in either case doesn't hurt -- it seems mildly preferable
     * for these objects to be larger than other kinds of tasks to
     * reduce false sharing during possibly frequent bookkeeping
     * updates.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23702#discussion_r1970539084