Why Skip Lists Are the Wrong Default for Matchmaking Queues: A Fenwick Tree Case Study
The ongoing quest for efficient matchmaking algorithms has significant implications for the gaming industry, where player experience and fairness depend on timely and accurate matchmaking. The default choice of skip lists in many popular matchmaking engines may be due to their widespread adoption and familiarity, but this case study reveals a potentially more optimal solution. By switching to Fenwick trees, game developers can potentially reduce latency and improve overall performance.
The Fenwick tree's superior cache locality and reduced memory usage make it an attractive alternative to skip lists, especially in scenarios where range-count queries and global rank lookups are critical. As game developers consider adopting this new data structure, they should also keep an eye on future research into other potential optimizations, such as the use of Bloom filters or more advanced indexing techniques.
Key Takeaways
Game developers can potentially reduce matchmaking latency by switching to Fenwick trees.
The study suggests that cache locality is a critical factor in matchmaking performance.
The use of Fenwick trees may have broader implications for other applications that rely on efficient range-count queries and global rank lookups.
About the Source
This analysis is based on reporting by HackerNoon. Here is a short excerpt for context:
Matchmaking queues need three things from their core data structure: range-count queries as the skill window widens, global rank lookups for leaderboards, and constant add/remove updates. The usual default is a skip-list-backed sorted set, the kind Redis ships and OpenMatch used, but benchmarked on the same host, it runs about 35x slower on rank queries and 38x slower on range counts than a Fenwick tree, and uses roughly 3x the memory. The cause is cache locality: a Fenwick tree is a single ~40 KB array that stays L2-resident, while a skip list chases pointers across scattered heap nodes. When MMR is bounded and quantizes naturally, a Fenwick tree with per-bucket player lists is the better default, and the article includes the Go code, reproducible numbers, and the cases where a skip list still wins.Read the original at HackerNoon
