## 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>
Pavel Punsky
8c7d8fcb86