Add A/B throughput benchmark for comparing two refs

The existing test_benchmark asserts a single request completes within
5ms, which catches gross connection-setup regressions but cannot see
throughput changes: the effects we care about are tens of microseconds
per request, a hundredth of that resolution. There was no way to answer
"does this patch make the server faster" other than measuring by hand.

Absolute req/s is not usable for that. Running the same binary five
times on an idle 8-core machine gave 53.9k to 74.6k req/s, and shared CI
runners are noisier still, so a number printed per push says nothing.

Build both refs and measure them alternately in one session, flipping
the order each round to cancel ordering bias, then report only the ratio
of the medians. Whether that ratio means anything is decided by an exact
permutation test rather than by comparing the change against the min/max
spread - a single slow round is enough to make a spread check give up,
while the rank test rides it out.

Validated against a patch that removes a redundant poll() per request:
individual measurements ranged 41.7k-93.9k req/s, yet nine rounds
resolved a 1.244x speedup at p = 0.019. The same data truncated to five
rounds was inconclusive, so the workflow defaults to nine.

Manual dispatch only, Linux and non-SSL for now, and it never fails the
build - this is a measurement, not a test.

.github/workflows/benchmark_ab.yaml

@@ -0,0 +1,62 @@
+name: benchmark-ab
+
+# Manual A/B throughput comparison between two refs.
+#
+# This is a measurement, not a test: it never fails the build on a slow result.
+# Absolute req/s from a shared runner is meaningless on its own, so both refs
+# are built and measured alternately in the same job and only the ratio of the
+# medians is reported, with a permutation test to say whether the difference
+# stands out from the run-to-run noise.
+#
+# Non-SSL and Linux only for now.
+
+on:
+  workflow_dispatch:
+    inputs:
+      base:
+        description: "Baseline ref"
+        required: false
+        default: "origin/master"
+      head:
+        description: "Ref to compare (defaults to the ref this run was started on)"
+        required: false
+        default: ""
+      rounds:
+        description: "Measurement rounds per ref (9+ recommended; below 4 the test can never reach significance)"
+        required: false
+        default: "9"
+      duration:
+        description: "Load duration per measurement"
+        required: false
+        default: "5s"
+      connections:
+        description: "Concurrent connections"
+        required: false
+        default: "10"
+
+permissions:
+  contents: read
+
+jobs:
+  ubuntu:
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: install bombardier
+        run: go install github.com/codesenberg/bombardier@latest
+
+      - name: run A/B benchmark
+        run: |
+          export PATH="$(go env GOPATH)/bin:$PATH"
+          HEAD_REF="${{ inputs.head }}"
+          if [ -z "$HEAD_REF" ]; then HEAD_REF="${{ github.sha }}"; fi
+          ./benchmark/ab.sh \
+            --base "${{ inputs.base }}" \
+            --head "$HEAD_REF" \
+            --rounds "${{ inputs.rounds }}" \
+            --duration "${{ inputs.duration }}" \
+            --connections "${{ inputs.connections }}"

benchmark/ab.sh

@@ -0,0 +1,227 @@
+#!/usr/bin/env bash
+#
+# A/B throughput comparison between two git refs.
+#
+# Usage: ./ab.sh [--base REF] [--head REF] [--rounds N] [--duration S]
+#                [--connections N] [--threads N]
+#
+# Absolute numbers from a single run are meaningless: on a quiet 8-core laptop
+# the same binary varies by +/-20% run to run, and shared CI runners are worse.
+# So both refs are built and then measured alternately in the same session, and
+# only the ratio of the medians is reported.
+#
+# Requires: bombardier, python3, g++ (or $CXX), git.
+
+set -euo pipefail
+
+BASE_REF="master"
+HEAD_REF="HEAD"
+ROUNDS=5
+DURATION="5s"
+CONNECTIONS=10
+THREADS=""
+PORT=8080
+
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --base) BASE_REF="$2"; shift 2 ;;
+    --head) HEAD_REF="$2"; shift 2 ;;
+    --rounds) ROUNDS="$2"; shift 2 ;;
+    --duration) DURATION="$2"; shift 2 ;;
+    --connections) CONNECTIONS="$2"; shift 2 ;;
+    --threads) THREADS="$2"; shift 2 ;;
+    *) echo "Unknown option: $1" >&2; exit 1 ;;
+  esac
+done
+
+command -v bombardier >/dev/null || { echo "Error: bombardier not found" >&2; exit 1; }
+command -v python3 >/dev/null || { echo "Error: python3 not found" >&2; exit 1; }
+
+REPO_ROOT=$(git rev-parse --show-toplevel)
+CXX=${CXX:-g++}
+
+# Default the thread pool to the core count. The committed benchmark Makefile
+# hardcodes 16, which heavily oversubscribes a 2-4 vCPU CI runner and inflates
+# the variance we are trying to see through.
+if [ -z "$THREADS" ]; then
+  THREADS=$(python3 -c 'import os; print(os.cpu_count() or 4)')
+fi
+
+WORKDIR=$(mktemp -d)
+cleanup() {
+  pkill -f "$WORKDIR/.*/server-ab" 2>/dev/null || true
+  git -C "$REPO_ROOT" worktree remove --force "$WORKDIR/base" 2>/dev/null || true
+  git -C "$REPO_ROOT" worktree remove --force "$WORKDIR/head" 2>/dev/null || true
+  rm -rf "$WORKDIR"
+}
+trap cleanup EXIT
+
+BASE_SHA=$(git -C "$REPO_ROOT" rev-parse --short "$BASE_REF")
+HEAD_SHA=$(git -C "$REPO_ROOT" rev-parse --short "$HEAD_REF")
+
+echo "==> base: $BASE_REF ($BASE_SHA)"
+echo "==> head: $HEAD_REF ($HEAD_SHA)"
+echo "==> rounds=$ROUNDS duration=$DURATION connections=$CONNECTIONS threads=$THREADS"
+echo ""
+
+if [ "$BASE_SHA" = "$HEAD_SHA" ]; then
+  echo "Note: base and head are the same commit; this measures harness noise."
+  echo ""
+fi
+
+# --- Build both refs ---
+build() {
+  local name=$1 ref=$2
+  git -C "$REPO_ROOT" worktree add --detach --quiet "$WORKDIR/$name" "$ref"
+  if [ ! -f "$WORKDIR/$name/benchmark/cpp-httplib/main.cpp" ]; then
+    echo "Error: benchmark/cpp-httplib/main.cpp missing in $ref" >&2
+    exit 1
+  fi
+  "$CXX" -o "$WORKDIR/$name/server-ab" -O2 -std=c++11 \
+    -I"$WORKDIR/$name" \
+    -DCPPHTTPLIB_THREAD_POOL_COUNT="$THREADS" \
+    "$WORKDIR/$name/benchmark/cpp-httplib/main.cpp" -lpthread
+}
+
+echo "==> Building..."
+build base "$BASE_REF"
+build head "$HEAD_REF"
+
+# --- Measure one ref once, echo rps ---
+measure() {
+  local name=$1
+  local json rc
+
+  "$WORKDIR/$name/server-ab" >/dev/null 2>&1 &
+  local pid=$!
+
+  # Wait for the listener (no dependency on nc)
+  local i
+  for i in $(seq 1 200); do
+    if (exec 3<>/dev/tcp/127.0.0.1/$PORT) 2>/dev/null; then exec 3>&- 3<&-; break; fi
+    sleep 0.05
+  done
+
+  set +e
+  json=$(bombardier -c "$CONNECTIONS" -d "$DURATION" -o json -p r \
+    "http://127.0.0.1:$PORT/" 2>/dev/null)
+  rc=$?
+  set -e
+
+  kill "$pid" 2>/dev/null || true
+  wait "$pid" 2>/dev/null || true
+
+  # Wait for the port to be released before the next run
+  for i in $(seq 1 200); do
+    if ! (exec 3<>/dev/tcp/127.0.0.1/$PORT) 2>/dev/null; then break; fi
+    exec 3>&- 3<&-
+    sleep 0.05
+  done
+
+  if [ $rc -ne 0 ] || [ -z "$json" ]; then
+    echo "Error: bombardier failed for $name" >&2
+    exit 1
+  fi
+
+  python3 -c '
+import json, sys
+r = json.load(sys.stdin)["result"]
+total = sum(r[k] for k in ("req1xx","req2xx","req3xx","req4xx","req5xx","others"))
+bad = total - r["req2xx"]
+if bad:
+    sys.stderr.write("Error: %d non-2xx/error responses\n" % bad)
+    sys.exit(1)
+print("%.1f" % (total / r["timeTakenSeconds"]))
+' <<<"$json"
+}
+
+# --- Alternate, flipping the order each round to cancel ordering bias ---
+BASE_RESULTS=()
+HEAD_RESULTS=()
+
+echo ""
+echo "==> Measuring..."
+for ((r = 1; r <= ROUNDS; r++)); do
+  if (( r % 2 == 1 )); then order=("base" "head"); else order=("head" "base"); fi
+  line="    round $r:"
+  for name in "${order[@]}"; do
+    rps=$(measure "$name")
+    if [ "$name" = "base" ]; then BASE_RESULTS+=("$rps"); else HEAD_RESULTS+=("$rps"); fi
+    line="$line  $name=$rps"
+  done
+  echo "$line"
+done
+
+# --- Report ---
+SUMMARY=$(python3 -c '
+import statistics, sys
+from itertools import combinations
+
+base = [float(x) for x in sys.argv[1].split()]
+head = [float(x) for x in sys.argv[2].split()]
+bm, hm = statistics.median(base), statistics.median(head)
+
+def spread(v):
+    return (max(v) - min(v)) / statistics.median(v) * 100
+
+def u_stat(a, b):
+    """Mann-Whitney U: number of (a, b) pairs where a > b, ties count a half."""
+    return sum((x > y) + 0.5 * (x == y) for x in a for y in b)
+
+def exact_p(a, b):
+    """Two-sided permutation p-value. A single slow round cannot swing this
+    the way a min/max spread check can."""
+    n1, n2 = len(a), len(b)
+    pooled = a + b
+    observed = abs(u_stat(a, b) - n1 * n2 / 2)
+    total = extreme = 0
+    for idx in combinations(range(n1 + n2), n1):
+        s = set(idx)
+        ga = [pooled[i] for i in idx]
+        gb = [pooled[i] for i in range(n1 + n2) if i not in s]
+        total += 1
+        if abs(u_stat(ga, gb) - n1 * n2 / 2) >= observed:
+            extreme += 1
+    return extreme / total
+
+print("| | median req/s | min | max | spread |")
+print("|---|---|---|---|---|")
+print("| base | %.0f | %.0f | %.0f | %.1f%% |" % (bm, min(base), max(base), spread(base)))
+print("| head | %.0f | %.0f | %.0f | %.1f%% |" % (hm, min(head), max(head), spread(head)))
+print("")
+print("**ratio: %.3fx** (%+.1f%%)" % (hm / bm, (hm / bm - 1) * 100))
+print("")
+
+if len(base) + len(head) > 20:
+    print("> %d rounds: skipping the permutation test (too many combinations)."
+          % len(base))
+else:
+    p = exact_p(base, head)
+    if p <= 0.05:
+        print("> Separation is consistent across rounds (permutation p = %.3f)." % p)
+    else:
+        print("> Not separated from noise (permutation p = %.3f). Inconclusive;" % p)
+        print("> raise --rounds or --duration, or run on a quieter machine.")
+    min_p = exact_p(list(range(len(base))),
+                    list(range(len(base), len(base) + len(head))))
+    if min_p > 0.05:
+        print(">")
+        print("> With %d rounds even perfect separation only reaches p = %.3f,"
+              % (len(base), min_p))
+        print("> so this test can never call a win. Use --rounds 4 or more.")
+' "${BASE_RESULTS[*]}" "${HEAD_RESULTS[*]}")
+
+echo ""
+echo "$SUMMARY"
+
+if [ -n "${GITHUB_STEP_SUMMARY:-}" ]; then
+  {
+    echo "## Benchmark A/B"
+    echo ""
+    echo "- base: \`$BASE_REF\` ($BASE_SHA)"
+    echo "- head: \`$HEAD_REF\` ($HEAD_SHA)"
+    echo "- rounds=$ROUNDS duration=$DURATION connections=$CONNECTIONS threads=$THREADS"
+    echo ""
+    echo "$SUMMARY"
+  } >> "$GITHUB_STEP_SUMMARY"
+fi

justfile

@@ -46,6 +46,10 @@ build:
 bench:
     @(cd benchmark && make bench-all)
 
+# A/B throughput comparison between two refs (see benchmark/ab.sh --help)
+bench-ab *args:
+    @./benchmark/ab.sh {{args}}
+
 docs-serve:
     -@docs-gen serve docs-src --open