MB2 inter-node scaffolding: per-host single-instance launcher + client host args

Adds the pieces needed to run the producer on dash1 and the consumer on
dash2 with the same shared cpfs venv:

start_vllm_single.sh
  INSTANCE / GPU / PORT / BP / MASTER / ROLE env vars; brings up ONE
  vLLM instance + applies the mooncake instrumentation patch (idempotent
  since the venv is cpfs-shared, so the first invocation applies and the
  second is a no-op). Per-instance MB2_LOG_DIR keeps producer/consumer
  events separate even though both directories live on the same cpfs
  path visible to both hosts.

mb2_kv_transfer.py
  New --src-host / --dst-host args. Defaults stay 127.0.0.1 for
  backward-compat with the intra-node sweep. /v1/completions URLs and
  /query URLs now use the supplied hosts. remote_bootstrap_addr is
  built as http://<src_host>:<src_bp> so the consumer's
  do_remote_prefill request carries a routable address.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

microbench/fresh_setup/start_vllm_single.sh

@@ -0,0 +1,99 @@
+#!/usr/bin/env bash
+# Start ONE vLLM instance with Mooncake kv_connector, for inter-node MB2.
+# Run separately on each host (dash1, dash2) before kicking off the bench.
+#
+# Usage on each host:
+#   INSTANCE=A GPU=0 PORT=8000 BP=8998 MASTER=29500 ROLE=kv_both \
+#     bash microbench/fresh_setup/start_vllm_single.sh start
+#   bash microbench/fresh_setup/start_vllm_single.sh status
+#   bash microbench/fresh_setup/start_vllm_single.sh stop
+#
+# All hosts share cpfs, so the venv at FRESH_ROOT/.venv is single-installed
+# and the instrumentation patch is global. Per-instance logs go under
+# FRESH_ROOT/mb2_transfer_logs/{INSTANCE}/ which is visible from any host.
+
+set -eo pipefail
+
+FRESH_ROOT="/home/admin/cpfs/wjh/agentic-kv-fresh"
+VENV="${FRESH_ROOT}/.venv"
+MODEL="${MODEL:-/home/admin/cpfs/wjh/models/Qwen/Qwen3-Coder-30B-A3B-Instruct}"
+LOGS_DIR="${LOGS_DIR:-${FRESH_ROOT}/mb2_logs}"
+MB2_LOG_ROOT="${FRESH_ROOT}/mb2_transfer_logs"
+
+INSTANCE="${INSTANCE:-A}"
+GPU="${GPU:-0}"
+PORT="${PORT:-8000}"
+BP="${BP:-8998}"
+MASTER="${MASTER:-29500}"
+ROLE="${ROLE:-kv_both}"
+
+mkdir -p "${LOGS_DIR}" "${MB2_LOG_ROOT}/${INSTANCE}"
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+INSTRUMENT="${SCRIPT_DIR}/instrument_mooncake.py"
+
+stop_local() {
+    pkill -9 -f "vllm serve.*--port ${PORT} " 2>/dev/null || true
+    pkill -9 -f "EngineCore" 2>/dev/null || true
+    sleep 2
+}
+
+case "${1:-start}" in
+    stop)
+        stop_local
+        # Patch revert is only safe to do when no other instance is using
+        # the venv — for a shared cpfs venv we leave it applied until all
+        # instances are stopped. Do it manually with:
+        #   python instrument_mooncake.py --revert
+        exit 0;;
+    status)
+        if curl -sf "http://127.0.0.1:${PORT}/health" >/dev/null 2>&1; then
+            echo "port ${PORT}: UP"
+        else
+            echo "port ${PORT}: DOWN"
+        fi
+        exit 0;;
+    start) ;;
+    *) echo "Unknown command: $1"; exit 1;;
+esac
+
+stop_local
+
+source "${VENV}/bin/activate"
+
+# Apply instrumentation on first launch (it's idempotent / safe to re-apply).
+if [[ -f "${INSTRUMENT}" ]]; then
+    python "${INSTRUMENT}" --apply --venv "${VENV}" 2>&1 || true
+fi
+
+cfg='{"kv_connector":"MooncakeConnector","kv_role":"'${ROLE}'"}'
+echo "[mb2-single] launching ${INSTANCE}: gpu=${GPU} port=${PORT} bp=${BP} role=${ROLE}"
+
+PYTHONHASHSEED=42 \
+VLLM_MOONCAKE_BOOTSTRAP_PORT="${BP}" \
+CUDA_VISIBLE_DEVICES="${GPU}" \
+MASTER_PORT="${MASTER}" \
+MB2_LOG_DIR="${MB2_LOG_ROOT}/${INSTANCE}" \
+nohup vllm serve "${MODEL}" \
+    --host 0.0.0.0 --port "${PORT}" \
+    --tensor-parallel-size 1 \
+    --trust-remote-code --enable-prefix-caching \
+    --dtype auto --gpu-memory-utilization 0.9 \
+    --max-model-len 200000 \
+    --kv-transfer-config "${cfg}" \
+    --enable-prompt-tokens-details \
+    > "${LOGS_DIR}/vllm_${INSTANCE}_$(hostname -s)_gpu${GPU}.log" 2>&1 &
+disown
+
+echo "[mb2-single] waiting for /health on port ${PORT}..."
+tries=0
+while ! curl -sf "http://127.0.0.1:${PORT}/health" >/dev/null 2>&1; do
+    tries=$((tries+1))
+    if [ ${tries} -gt 180 ]; then
+        echo "[mb2-single] FATAL port ${PORT} did not come up in 6 min"
+        tail -40 "${LOGS_DIR}/vllm_${INSTANCE}_"*"_gpu${GPU}.log" || true
+        exit 1
+    fi
+    sleep 2
+done
+echo "[mb2-single] ${INSTANCE} UP on $(hostname -s):${PORT} (bp ${BP}, gpu ${GPU})"