Maybe because the main scenario dubbo faces is persistent connection. For persistent connections the most frequent thing is ticking.
ScheduledThreadPoolExecutor is O(1)*connections compared to O(C) in HashedWheelTimer when ticking. Considering connections is far less then C ScheduledThreadPoolExecutor maybe more efficient in my opinion. best regards, Jason > On Jan 24, 2019, at 07:05, 华 钟明 <[email protected]> wrote: > > Thank you for your question, your question has been placed in the issue, > https://github.com/apache/incubator-dubbo/issues/3324 > please continue to pay attention > Best Regards！ > Zhongming Hua > > > 在 2019/1/24 上午12:31，“[email protected]”<[email protected]> 写入: > > Hi Folks: > > 最近在研究 Dubbo 2.7 代码时，我发现了一个有意思的改动，Dubbo 2.6 中大量使用了 > ScheduledThreadPoolExecutor，例如实现定时发送心跳，开启定时器用于判断异步请求超时等场景，在 Dubbo 2.7 之中替换为了由 > Netty 实现的 HashedWheelTimer。 > > 我对这一改动比较好奇，于是对定时器的实现做了一些调研，探讨目的是想为 2.7 选用一个最合适的定时器实现。 > > JDK 的 Timer 存在设计问题，不予讨论，我想主要对比 HashedWheelTimer 和 > ScheduledThreadPoolExecutor 的区别。 > > Netty 在 3.x 中有大量使用 HashedWheelTimer，但是在 4.1 中，我们可以发现，Netty 保留了 > HashedWheelTimer，但在其源码中并未使用它，而是选择了 ScheduledThreadPoolExecutor，在 Stack > Overflow > 中也有人提出这样的疑问：https://stackoverflow.com/questions/867621/efficient-timer-algorithm。 > > Netty 提供的 HashedWheelTimer 正如其 java doc 中所言：A Timer optimized for > approximated I/O timeout scheduling. > 是适用于固定场景的，如果有深入研究过时间轮算法，会发现，其并没有实现多级时间轮，时间轮算法可以参考这篇论文：http://www.cs.columbia.edu/~nahum/w6998/papers/sosp87-timing-wheels.pdf > > > 我并没有找到 2.7 有关于这个改动的邮件，不太清楚去除 ScheduledThreadPoolExecutor, 使用 > HashedWheelTimer 的原因，不知道我的检查是否有所遗漏。 > > 下面是四个讨论点，索引一下，方便大家讨论： > > 1. 2.7 使用 Netty 的 HashedWheelTimer 是为了解决什么现有问题？ > 2. 使用 ScheduledThreadPoolExecutor 可以更好的控制多线程执行定时任务（虽然现在的 HashWheelTimer > 也可以做到），尽量保持了和 2.6 的风格一致。 > 3. Netty4.1 没有使用 HashedWheelTimer，我没有搜索到相关的原因，但我觉得 Netty > 这款优秀的框架必然考虑到了什么，希望得到大家的讨论。由于 Netty 的这一点改动，我更倾向于在 Dubbo 中也使用 > ScheduledThreadPoolExecutor > 4. 在心跳这样的场景，我们可以把定时器交给 netty 的 eventLoop 去做 > Schedule，这样并不是耦合，可以做成可配置的项来选择性的重载，可以保证 Timer 线程和请求线程的线程安全（这一点是参考了 > IdleStateHandler 的设计）。在 Dubbo 2.7 中，Timer reput 的逻辑耦合在了抽象类中，这样导致了 Timer > 并不具备很高的重用性。 > 5. HashedWheelTimer 和 ScheduledThreadPoolExecutor 分别适用的场景 > > 期待大家的讨论 > > Recently, when I was researching Dubbo 2.7 code, I found an interesting > change. Dubbo 2.6 used the ScheduledThreadPoolExecutor extensively. For > example, Dubbo implemented a timed heartbeat and started the timer to judge > the asynchronous request timeout. It is replaced in Dubbo 2.7 using > HashedWheelTimer implemented by Netty. > > I was curious about this change, so I did some research on the > implementation of the timer. The purpose of the discussion was to choose the > most suitable timer for 2.7. > > JDK's Timer has design issues that is no use to be discussed. I want to > compare the differences between HashedWheelTimer and > ScheduledThreadPoolExecutor. > > Netty has a lot of use of HashedWheelTimer in 3.x, but in 4.1, we can find > that Netty retains HashedWheelTimer, but it is not used in its source code, > but chose ScheduledThreadPoolExecutor, which was also proposed in Stack > Overflow. Question: > https://stackoverflow.com/questions/867621/efficient-timer-algorithm. > > The HashedWheelTimer provided by Netty is as described in its java doc: A > Timer optimized for approximated I/O timeout scheduling. It is suitable for > fixed scenes. If you have studied the time wheel algorithm in depth, you will > find that it does not implement multi-level time wheel. The time wheel > algorithm can refer to this paper: > http://www.cs.columbia.edu/~nahum/w6998/papers/sosp87-timing-wheels.pdf > > I didn't find the 2.7 email about this change. I don't know why I removed > the ScheduledThreadPoolExecutor and used HashedWheelTimer. I don't know if my > check is missing. > > Here are four discussion points, indexing, for your convenience: > > 1. 2.7 Is Netty's HashedWheelTimer used to solve any existing problems? > 2. Use ScheduledThreadPoolExecutor to better control multi-threaded > execution timing tasks (although HashWheelTimer can do it now), and try to > keep the style consistent with 2.6. > 3. Netty4.1 didn't use HashedWheelTimer, I didn't search for related > reasons, but I think Netty's excellent framework must take into account what > I want, and I hope to get everyone's discussion. Due to this change in Netty, > I prefer to use ScheduledThreadPoolExecutor in Dubbo as well. > 4. In a scenario like heartbeat, we can pass the timer to netty's > eventLoop to do the Schedule. This is not a coupling. It can be made into a > configurable item for selective overloading. It can guarantee the thread of > the Timer thread and the request thread. Security (this is a reference to the > design of the IdleStateHandler). In Dubbo 2.7, the logic of Timer reput is > coupled in an abstract class, which results in a Timer that is not highly > reusable. > 5. The scenarios for HashedWheelTimer and ScheduledThreadPoolExecutor > respectively > > Look forward to your discussions. > >
