EventLoop: store Timers in min Pairing Heap #15206
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The main advantage is the ability to grow on demand and avoid a large single contiguous mmap of memory that needs to preallocates most of the memory. We also don't need to remember the maximum reached index anymore, which is replaced with a `#each_index` method (only needed to allow `fork`).
Co-authored-by: Johannes Müller <straightshoota@gmail.com>
ysbaddaden
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Replaces the
Dequeused in #14996 for a min Pairing Heap which is a kind of Mergeable Heap and is one of the best performing heap in practical tests when arbitrary deletions are required (think cancelling a timeout), otherwise a D-ary Heap (e.g. 4-heap) will usually perform better. See the A Nearly-Tight Analysis of Multipass Pairing Heaps paper or the Wikipedia page for more details.The implementation itself is based on the Pairing Heaps: Experiments and Analysis paper, and merely implements a recursive twopass algorithm (the auxiliary twopass might perform even better). The
Crystal::PointerPairingList(T)type is generic and relies on intrusive nodes (the links are intoT) to avoid extra allocations for the nodes (same asCrystal::PointerLinkedList(T)). It also requires aT#heap_comparemethod, so we can use the same type for a min or max heap, or to build a more complex comparison.Note: I also tried a 4-heap, and while it performs very well and only needs a flat array, the arbitrary deletion (e.g. cancelling timeout) needs a linear scan and its performance quickly plummets, even at low occupancy, and becomes painfully slow at higher occupancy (tens of microseconds on each delete, while the pairing heap does it in tens of nanoseconds).
Follow up to #14996
Builds on top of #15205, you should open the last commit for just the pairing heap.