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EXPERIMENTS.md

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libevent-agent-workspace — Experiments Log

Append-only. One ## EXP-NNN section per experiment, newest at the bottom. Use experiments/TEMPLATE.md for the entry format. Failures are as valuable as wins — log every rejection with its reason.

Metric reminder: never MB/s. Report microseconds per run_once (lower=better), events/sec, and syscall count.

EXT-PR-1866 — 2026-06-01 — Track & improve upstream io_uring fast path — TRACKING

External PR libevent/libevent#1866 (widgetii): opt-in io_uring multishot-recv fast path for socket bufferevents. Full analysis, reproduced baseline, and improvement attempts in EXT-PR-1866/README.md.

  • Baseline reproduced (c4, kernel 6.14): io_uring +35% (64p) / +47% (128p) at 64 KiB.
  • Perf tune V1 (buffer pool 128→1024, depth 256→512): REJECT −11% @128p — small L3-resident pool beats a 16 MiB cache-thrashing one.
  • Perf tune V2 (IORING_SETUP_SINGLE_ISSUER): reject (~0%, noise).
  • DEFER/COOP_TASKRUN avoided — incompatible with the CQ-poll-via-epoll wakeup.
  • Conclusion: perf config near-optimal; the real improvement is the documented timeout limitation (IORING_OP_LINK_TIMEOUT on each multishot recv) — a feature, not a tune. Identified as next step.