RFR: 8360046: Scalability issue when submitting virtual threads with almost empty tasks [v5]

[Doug Lea]

On Thu, 14 Aug 2025 11:27:10 GMT, Viktor Klang <vklang at openjdk.org> wrote:

>> Doug Lea has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Avoid underutilization on resize
>
> src/java.base/share/classes/java/util/concurrent/Exchanger.java line 380:
> 
>> 378:                                   (!ForkJoinWorkerThread.hasKnownQueuedWork()))) {
>> 379:                             for (int j = SPINS; p.match == null && j > 0; --j)
>> 380:                                 Thread.onSpinWait();
> 
> It might not be worth it, but I wonder if something like this would make a positive impact:
> 
> Suggestion:
> 
>                         if (!(tryCancel = (deadline != 0L &&
>                              (ns = deadline - System.nanoTime()) <= 0L) ||
>                             Thread.currentThread().isInterrupted()))
>                             && ncpu > 1 &&
>                              (i != 0 || /* check for busy VTs */ (!ForkJoinWorkerThread.hasKnownQueuedWork()))) {
>                             for (int j = SPINS; p.match == null && j > 0; --j)
>                                 Thread.onSpinWait();

(No longer applicable -- moved to different PR)

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/26479#discussion_r2348828552