kumomta/docs/reference/kumo/make_queue_config/strategy.md

strategy

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Optional string to specify the scheduled queue strategy. There are two possible values:

• "TimerWheel" - the default. The timer wheel has O(1) insertion and O(1) pop costs, making it good for very large scheduled queues, but that comes in exchange for a flat periodic tick overhead. As the number of scheduled queues increases and/or the retry_interval decreases, so does the aggregate overhead of maintaining timerwheel queues.
• "SkipList" - A skiplist has O(log n) insertion and O(1) pop costs, making it a good general purpose queue, but becomes more expensive to insert into as the size of the queue increases. That higher insertion cost is in exchange for the overall maintenance being cheaper, as the queue can go to sleep until the time of the next due item. The ongoing and aggregate maintenance is therefore cheaper than a TimerWheel but the worst-case scenario where the destination is not accepting mail and causing the scheduled queue to grow is logarithmically more expensive as the queue grows.

Which should you use? Whichever works best for your environment! Make sure that you test normal healthy operation with a lot of queues as well as the worst case scenario where those queues are full and egress is blocked.

If you have very short retry_interval set for the majority of your queues you may wish to adopt SkipList for its lower idle CPU cost, or alternatively use TimerWheel and find a timerwheel_tick_interval that works for your typical number of queues.

!!! note Changing the strategy for a given queue requires that the queue either be aged out, or for kumod to be restarted, before it will take effect. This restriction may be removed in a future release.