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## Summary Flips the Linux default for `--udp-recvmmsg` from **off** to **on**. Operators opt out with `--udp-recvmmsg=false` (or `=0`). > **Stacked on #1929.** This depends on the recvmmsg-scoping change in #1929 and is based on that branch, so the diff shows only the default-on change. GitHub will auto-retarget the base to `master` once #1929 merges. Merge #1929 first. ## Why this is now safe The original objection to default-on (recorded in `docs/PerformanceIterationLog.md`) was the **per-session-relay-socket prealloc tax**: `--udp-recvmmsg` applied the 16-buffer batch path to every connected relay socket, which only ever carries one flow, so the churn ate the listener-side win. #1929 scoped recvmmsg to **shared fan-in sockets only** (`udp_recvmmsg_eligible`: the client listener, plus the per-thread shared relay socket under `--multiplex-peer`). Per-session relay sockets now stay on the single-recv path regardless of the flag, so that tax is gone. The one socket touched by default — the client listener — is a genuine fan-in point: - batches whenever client concurrency is non-trivial (measured `avg_batch ≈ 16` under load), and - costs little when idle (few packets ⇒ few prealloc cycles). ## What changed - `mainrelay.c`: `turn_params.udp_recvmmsg` default `false → true` (Linux only). - Removed the now-dead `--multiplex-peer` auto-enable block and the `udp_recvmmsg_set_explicitly` tracking it relied on; multiplex-peer gets its recvmmsg window from the default. The opt-out flows through the normal `get_bool_value` path. - Help text, `man/man1/turnserver.1`, `examples/etc/turnserver.conf`, `CLAUDE.md`, and `docs/PerformanceIterationLog.md` updated for the new default + opt-out. Per-session relay sockets and DTLS session sockets are unchanged. ## Validation - **Format:** clang-format 15.0.7 clean. - **macOS:** build + ctest 6/6 + `run_tests.sh` pass. - **Linux (Docker, clean build):** ctest 5/5; `run_tests.sh`, `run_tests_conf.sh`, `run_tests_multiplex_peer.sh` all pass (no FAIL). - **Runtime proof (loopback, `--udp-recvmmsg-log`):** - Default, no flag: recvmmsg active, `calls=13714 packets=219306 avg_batch=15.99`. - `--udp-recvmmsg=false`: zero recvmmsg activity — opt-out confirmed. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>