RFR: 8351927: Change VirtualThread implementation to use use FJP delayed task handling

[Viktor Klang]

On Thu, 13 Mar 2025 10:48:14 GMT, Alan Bateman <alanb at openjdk.org> wrote:

> Follow up to JDK-8319447 to change the VirtualThread implementation to use FJP's delayed task handling.
> 
> The SPTE based implementation is not removed. It will continue to be used by tests. If custom schedulers are exposed in the future then they will use this implementation.
> 
> For timed-Object.wait, waitTimeoutExpired is changed to use lazySubmit to avoid signalling and increase the chance that the unparked virtual thread will continue on the current carrier. For timed-park, the timeout task is changed to reduced form of unpark that also uses lazySubmit, for the same reason.
> 
> `jcmd Thread.vthread_scheduler` is changed to no longer print the delay schedulers. Instead, the delayed task count will appear in the default scheduler output.

src/java.base/share/classes/java/lang/VirtualThread.java line 889:

> 887:     private void parkTimeoutExpired() {
> 888:         assert !VirtualThread.currentThread().isVirtual();
> 889:         if (!getAndSetParkPermit(true)

@AlanBateman Would it make sense to test whether the park-permit is false before the LOCK XCHG?

src/java.base/share/classes/java/lang/VirtualThread.java line 1455:

> 1453:             return pool.schedule(command, delay, unit);
> 1454:         } else {
> 1455:             return DelayedTaskSchedulers.schedule(command, delay, unit);

@AlanBateman Would it make sense to test if the Scheduler implements ScheduledExecutorService?

src/java.base/share/classes/java/lang/VirtualThread.java line 1462:

> 1460:      * Supports scheduling a runnable task to run after a delay. It uses a number
> 1461:      * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
> 1462:      * work queue used. This class is used when using a custom scheduler.

@AlanBateman It might make sense to instead require a custom Scheduler to implement ScheduledExecutorService? 🤔

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PR Review Comment: https://git.openjdk.org/jdk/pull/24030#discussion_r2033942539
PR Review Comment: https://git.openjdk.org/jdk/pull/24030#discussion_r2033943663
PR Review Comment: https://git.openjdk.org/jdk/pull/24030#discussion_r2033944680