On Thu, 20 Feb 2025 22:35:07 GMT, Sunmisc Unsafe <d...@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... > > Is it possible to use getAndSet instead of cas-loop on #pend? > > final Task t = tail.getAndSet(task); > t.next = task; > > but that would require a new head field to bypass, probably not worth the > gamble if the footprint increases a lot @sunmisc You are right that it would be nice if there were a way to efficiently use getAndSet here because a failed reference CAS hits slow paths that vary across GCs. But all of the ways I know to do this are much worse. ------------- PR Comment: https://git.openjdk.org/jdk/pull/23702#issuecomment-2674519257
