mosquitto: set_tcp_nodelay, raw CAN throughput measured

Nagle on the broker held small messages to subscribers ~40 ms at CAN rates.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

docs/impl/reports/MOSQUITTO.md

@@ -18,6 +18,7 @@ sudo apt-get install mosquitto mosquitto-clients     # 2.0.11-1ubuntu1.2 on Ubun
 # Yukart bus. Local only: displays reach it through svc-gw, never directly.
 listener 1883 127.0.0.1
 allow_anonymous true
+set_tcp_nodelay true
 ```
 
 `systemctl enable --now mosquitto`. It listens on `127.0.0.1:1883` only, so
@@ -30,6 +31,9 @@ reach it.
   `persistence true` (`/var/lib/mosquitto/`), so after a reboot the broker
   hands out the last values from before it. svc-gw therefore treats a value
   known only from a retained copy as stale until a live one arrives.
+- **`set_tcp_nodelay true`** turns Nagle's algorithm off for the broker's
+  sockets. Without it, small messages to subscribers wait for the previous
+  one's ACK: a 40 ms tail at raw CAN rates (svc-core `BUS-BENCH.md`).
 - Units that use the broker order themselves `After=mosquitto.service` and
   nothing else, and reconnect by themselves when it restarts.
 - Startup tier 1 (project-yukart `docs/ORIN-RUNTIME.md`).
@@ -43,6 +47,8 @@ once a second; `svc-gw` subscribes and serves it to the tablet. Watching it:
 mosquitto_sub -v -t 'yukart/#'
 ```
 
-## Not checked yet
+## Throughput
 
-- Throughput at raw CAN rates (`yukart/can/raw`, ~1000–3000 frames/s).
+Raw CAN rates are fine: a saturated 500 kbit/s bus (~4,400 frames/s) lost no
+frames, per frame or batched; batched it costs the broker about 1 % of a core.
+Measured 2026-10-12, svc-core `docs/impl/reports/BUS-BENCH.md`.