The exact implementation strategy is a hierarchical timer wheel. A timer wheel is effectively a ring buffer of linked lists of events, and a pointer to the ring buffer. As the time advances, the pointer moves forward, and any events in the ring buffer slots that the pointer passed will get executed. A hierarchical timer wheel layers multiple timer wheels running at different resolutions on top of each other. When an event is scheduled so far in the future than it does not fit the innermost (core) wheel, it instead gets scheduled on one of the outer wheels. On each rotation of the inner wheel, one slot’s worth of events are promoted from the second wheel to the core. On each rotation of the second wheel, one slot’s worth of events is promoted from the third wheel to the second, and so on.