Fix replay methodology: trace-driven dispatch, no artificial limits

The replayer was artificially limiting concurrency with --max-inflight-sessions
(semaphore) and --time-scale (time compression), producing unrealistically low
1 req/GPU load that masked prefill-decode interference.

Replayer changes:
- Remove session_sem and time_scale entirely
- Each request dispatched at its trace timestamp exactly
- Sessions still sequential (turn N+1 waits for turn N completion)
- If turn completes late, next turn fires immediately

Sampler changes:
- Add --sample-ratio for GPU-proportional session sampling
- Keep --target-requests for backwards compat
- No time compression (preserve original arrival pattern)

bench.sh: remove --time-scale and --max-inflight-sessions args

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/bench.sh

@@ -30,9 +30,7 @@ POLICY="linear"     # linear | lmetric
 N_INSTANCES=8
 BASE_PORT=8000
 PROXY_PORT=9090
-REQUESTS=200
-TIME_SCALE=20
-MAX_SESSIONS=8
+REQUESTS=""         # empty = all requests in trace
 HEAVY_THRESHOLD=20000
 NO_OFFLOAD=false
 OVERLOAD_FACTOR_ARG=""
@@ -46,8 +44,6 @@ while [[ $# -gt 0 ]]; do
         --policy) POLICY="$2"; shift 2 ;;
         --instances) N_INSTANCES="$2"; shift 2 ;;
         --requests) REQUESTS="$2"; shift 2 ;;
-        --time-scale) TIME_SCALE="$2"; shift 2 ;;
-        --sessions) MAX_SESSIONS="$2"; shift 2 ;;
         --heavy-threshold) HEAVY_THRESHOLD="$2"; shift 2 ;;
         --no-offload) NO_OFFLOAD=true; shift ;;
         --overload-factor) OVERLOAD_FACTOR_ARG="$2"; shift 2 ;;
@@ -58,6 +54,7 @@ done
 
 if [ -z "$TAG" ]; then
     echo "Usage: bench.sh --tag NAME --mode {baseline|elastic} [--instances N] [--policy {linear|lmetric}] [--requests N]"
+    echo "  Trace QPS is controlled by sample_trace.py --sample-ratio, not by bench.sh."
     exit 1
 fi
 
@@ -76,9 +73,7 @@ cat > "$OUTDIR/config.json" << CONF
     "policy": "$POLICY",
     "model": "$MODEL",
     "n_instances": $N_INSTANCES,
-    "requests": $REQUESTS,
-    "time_scale": $TIME_SCALE,
-    "max_sessions": $MAX_SESSIONS,
+    "requests": "${REQUESTS:-all}",
     "heavy_threshold": $HEAVY_THRESHOLD,
     "no_offload": "$NO_OFFLOAD",
     "overload_factor": "${OVERLOAD_FACTOR_ARG:-2.0}",
@@ -245,7 +240,13 @@ launch_proxy() {
 # ─── Run benchmark ─────────────────────────────────────────────────────────
 
 run_benchmark() {
-    echo "[bench] Running $REQUESTS requests (time_scale=$TIME_SCALE, sessions=$MAX_SESSIONS)..."
+    local request_args=""
+    if [ -n "$REQUESTS" ]; then
+        request_args="--request-limit $REQUESTS"
+        echo "[bench] Running $REQUESTS requests (trace-driven timing)..."
+    else
+        echo "[bench] Running all requests in trace (trace-driven timing)..."
+    fi
 
     # Start GPU monitor in background
     bash "$PROJECT_DIR/scripts/gpu_monitor.sh" "$OUTDIR/gpu_util.csv" 5 &
@@ -256,9 +257,7 @@ run_benchmark() {
         --output "$OUTDIR/metrics.jsonl" \
         --endpoint "http://localhost:$PROXY_PORT" \
         --model "$MODEL" \
-        --time-scale "$TIME_SCALE" \
-        --max-inflight-sessions "$MAX_SESSIONS" \
-        --request-limit "$REQUESTS" \
+        $request_args \
         -v 2>&1 | tee "$OUTDIR/replayer.log"
 
     # Stop GPU monitor
@@ -324,7 +323,7 @@ print('=' * 70)
 
 echo "================================================================"
 echo "  bench.sh: $TAG"
-echo "  mode=$MODE  policy=$POLICY  requests=$REQUESTS  overload_factor=${OVERLOAD_FACTOR_ARG:-2.0}  max_batched_tokens=${MAX_BATCHED_TOKENS:-default}"
+echo "  mode=$MODE  policy=$POLICY  requests=${REQUESTS:-all}  overload_factor=${OVERLOAD_FACTOR_ARG:-2.0}"
 echo "  $(date)"
 echo "================================================================"