RFR: 8319447: Improve performance of delayed task handling [v2]

[Viktor Klang]

On Fri, 21 Feb 2025 13:16:10 GMT, Doug Lea <dl at openjdk.org> wrote:

>> (Copied from https://bugs.openjdk.org/browse/JDK-8319447)
>> 
>> The problems addressed by this CR/PR are that ScheduledThreadPoolExecutor is both ill-suited for many (if not most) of its applications, and is a performance bottleneck (as seen especially in Loom and CompletableFuture usages). After considering many options over the years, the approach taken here is to connect (lazily, only if used) a form of ScheduledExecutorService (DelayScheduler) to any ForkJoinPool (including the commonPool), which can then use more efficient and scalable techniques to request and trigger delayed actions, periodic actions, and cancellations, as well as coordinate shutdown and termination mechanics (see the internal documentation in DelayScheduler.java for algotihmic details). This speeds up some Loom operations by almost an order of magnitude (and similarly for CompletableFuture). Further incremental improvements may be possible, but delay scheduling overhead is now unlikely to be a common performance concern.
>> 
>> We also introduce method submitWithTimeout to schedule a timeout that cancels or otherwise completes a submitted task that takes too long. Support for this very common usage was missing from the ScheduledExecutorService API, and workarounds that users have tried are wasteful, often leaky, and error-prone. This cannot be added to the ScheduledExecutorService interface because it relies on ForkJoinTask methods (such as completeExceptionally) to be available in user-supplied timeout actions. The need to allow a pluggable handler reflects experience with the similar CompletableFuture.orTimeout, which users have found not to be flexible enough, so might be subject of future improvements.
>> 
>> A DelayScheduler is optionally (on first use of a scheduling method) constructed and started as part of a ForkJoinPool, not any other kind of ExecutorService. It doesn't make sense to do so with the other j.u.c pool implementation ThreadPoolExecutor. ScheduledThreadPoolExecutor already extends it in incompatible ways (which is why we can't just improve or replace STPE internals). However, as discussed in internal documentation, the implementation isolates calls and callbacks in a way that could be extracted out into (package-private) interfaces if another j.u.c pool type is introduced.
>> 
>> Only one of the policy controls in ScheduledThreadPoolExecutor applies to ForkJoinPools with DelaySchedulers: new method cancelDelayedTasksOnShutdown controls whether quiescent shutdown sh...
>
> Doug Lea has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address feedback

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java line 3526:

> 3524:         if (((ds = delayScheduler) == null &&
> 3525:              (ds = startDelayScheduler()) == null) ||
> 3526:             task == null || (runState & SHUTDOWN) != 0L)

Might not make sense to start the delay scheduler if the task is null.

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java line 3534:

> 3532:     /**
> 3533:      * Submits a one-shot task that becomes enabled after the given
> 3534:      * delay, At which point it will execute unless explicitly

Suggestion:

     * delay, at which point it will execute unless explicitly

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java line 3557:

> 3555:         return scheduleDelayedTask(
> 3556:             new ScheduledForkJoinTask<Void>(
> 3557:                 unit.toNanos(delay), 0L, false, command, null, this));

Suggestion:

                unit.toNanos(delay), 0L, false, command, null, this)); // Implicit null-check of unit

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

PR Review Comment: https://git.openjdk.org/jdk/pull/23702#discussion_r1965928320
PR Review Comment: https://git.openjdk.org/jdk/pull/23702#discussion_r1965928717
PR Review Comment: https://git.openjdk.org/jdk/pull/23702#discussion_r1965929483