THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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A major drawback of a simple timing wheel is that it assumes that a timer request is within the time interval of n * u from the current time. If a timer request is out of this interval, it is an overflow. A hierarchical timing wheel deals with such overflows. It is a hierarchically organized timing wheels. The lowest level has the finest time resolution. As moving up the hierarchy, time resolutions become coarser. If the resolution of a wheel at one level is u and the size is n, the resolution of the next level should be n * u. At each level overflows are delegated to the wheel in one level higher. When the wheel in the higher level ticks, it reinsert timer tasks to the lower level. A overflow An overflow wheel can be created on-demand. When a bucket in a overflow an overflow bucket expires, all tasks in it are reinserted into the timer recursively. The tasks are then moved the to the finer grain wheels or be executed. The insert (start-timer) cost is O(m) where m is the number of wheels, which is usually very small compared to the number of requests in the system, and the delete (stop-timer) cost is still O(1).
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