Code review turned a 3x speedup into 8.9x (off-heap HdrHistogram in QuestDB)

QuestDB, an open-source time-series database, excels in handling demanding workloads by offering ultra-low latency and high ingestion throughput with its multi-tier storage engine. A recent community-driven enhancement focused on optimizing QuestDB's integration with HdrHistogram by rewriting it as an off-heap, flyweight class. This change aimed to improve the approx_percentile() function's scalability across worker threads without incurring garbage collection or allocator costs on the data path. Initial benchmarks showed a parallel speedup of 3x but revealed a single-threaded regression. After a review and optimization, which involved hoisting configuration-dependent computations out of the hot path, the rewrite achieved a significant improvement, with the single-threaded regression eliminated and parallel performance reaching up to 8.9x. This effort highlighted the importance of community contributions, the trade-offs of off-heap memory management, and the value of collaborative reviews in optimizing performance-critical components.

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