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• the tick size (the minimum time unit)
• the wheel size (the number of buckets per wheel)

Summary

In the new design, we use Hierarchical Timing Wheels for the timeout timer, WeakReference based list for watcher lists, and DelayQueue of timer buckets to advance the clock on demand. The advantage is that a completed request is removed from the timer queue immediately and becomes GCable. When a request is completed we will immediately remove it from a timer bucket with O(1) cost. A strong reference to a request is cleared at this point. All remaining references are weak references from watcher lists, so completed requests become GCable right away. The buckets remain in the delay queue. However, the number of buckets is bounded. And, in a healthy system, most of the requests are satisfied before timeout. Many of the buckets become empty before pulled out of the delay queue. Also, the timer should rarely have the buckets of the lower interval. We expect a improvement in GC behavior and lower CPU usage.