bus-bench: raw CAN over MQTT, measured on the car Orin

SocketCAN to yukart/can/raw and back, per frame or batched, with loss and
latency from the kernel timestamp. No loss at a saturated 500 kbit/s bus;
batched costs the broker ~1 % of a core. Report in docs/impl/reports.

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

scripts/bus-bench.sh

@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+# Raw CAN over MQTT, measured on the car Orin against a vcan0 (project-yukart decision 4).
+# Runs on the Orin; sudo once, to create vcan0. Prints one JSON line per run: the
+# sink's and the bridge's reports, and mosquitto's CPU over the run.
+#
+#   usage: bus-bench.sh BUS_BENCH_BINARY CAPTURE.log [secs]
+#   env:   BATCHES  batch windows in ms, 0 = one message per frame   ("0 10")
+#          FLAGS    extra options for both roles, e.g. --nagle
+#          GAP_MS   cangen gap for the full-bus run (0.15: ~4500 frames/s here, past what
+#                   500 kbit/s can carry with 8-byte frames)
+set -euo pipefail
+
+BIN="$1"; CAPTURE="$2"; SECS="${3:-20}"
+BATCHES="${BATCHES:-0 10}"; FLAGS="${FLAGS:-}"
+
+if ! ip link show vcan0 >/dev/null 2>&1; then
+  sudo modprobe vcan
+  sudo ip link add vcan0 type vcan
+fi
+sudo ip link set vcan0 up txqueuelen 10000
+
+ticks() { awk '{print $14 + $15}' "/proc/$1/stat"; }
+HZ=$(getconf CLK_TCK)
+OUT=$(mktemp -d)
+
+# run NAME BATCH_MS GENERATOR...: the generator feeds vcan0 for a little over SECS seconds.
+run() {
+  local name="$1" batch="$2"; shift 2
+  # shellcheck disable=SC2086
+  "$BIN" sink --secs "$SECS" $FLAGS > "$OUT/sink" 2>/dev/null &
+  local sink=$!
+  sleep 0.5
+  # shellcheck disable=SC2086
+  "$BIN" bridge --batch-ms "$batch" --secs $((SECS + 3)) $FLAGS > "$OUT/bridge" 2>/dev/null &
+  local bridge=$!
+  sleep 0.5
+  local broker m0 m1 t0 t1
+  broker=$(pgrep -x mosquitto); m0=$(ticks "$broker"); t0=$(date +%s.%N)
+  timeout "$((SECS + 1))" "$@" >/dev/null 2>&1 || true
+  m1=$(ticks "$broker"); t1=$(date +%s.%N)
+  wait "$sink" "$bridge"
+  python3 - "$name" "$batch" "$FLAGS" "$OUT" "$((m1 - m0))" "$HZ" "$t0" "$t1" <<'PY'
+import json, sys
+name, batch, flags, out, ticks, hz, t0, t1 = sys.argv[1:]
+r = {"run": name, "batch_ms": int(batch), "flags": flags,
+     "sink": json.load(open(f"{out}/sink")), "bridge": json.load(open(f"{out}/bridge")),
+     "mosquitto_cpu_pct": round(100 * int(ticks) / int(hz) / (float(t1) - float(t0)), 1)}
+for k in ("role",):
+    r["sink"].pop(k); r["bridge"].pop(k)
+print(json.dumps(r))
+PY
+}
+
+LOOPS=$(( (SECS + 9) / 10 + 1 ))    # the capture is 10 s; loop past the run's end
+for batch in $BATCHES; do
+  run "car replay"  "$batch" canplayer -l "$LOOPS" -I "$CAPTURE" vcan0=can_pcan
+  run "full bus"    "$batch" cangen vcan0 -g "${GAP_MS:-0.15}" -L 8
+  run "flood"       "$batch" cangen vcan0 -g 0 -L 8
+done