unified_v2.1: relax gates + add unified_kv_both isolation control

v2.0 ran on B3 and triggered PD-sep only 2 / 1214 times (0.2%). The
gates were too conservative; the v2-vs-v1 latency gap (TTFT p90
7.35 -> 8.96 s) is therefore probably attributable to kv_both
always-on overhead, not to the PD-sep mechanism itself. v2.1 has two
fixes plus an isolation control.

Bug fix:
- The "chosen has live decodes worth protecting" gate combined
  num_requests and ongoing_decode_tokens with AND, falling through
  when EITHER was small. Under agentic workloads each worker rarely
  stacks more than 1-2 concurrent requests, so the gate killed 84%
  of v2.0 candidates that reached it. Replace with a pure
  ongoing_decode_tokens == 0 check ("chosen_no_active_decode") —
  same semantic, much higher recall.

Threshold relaxation (B2 microbench is the calibration source):
- pd_sep_min_new_tokens: 16000 -> 8000 (B2 TPOT idx 1.9x already
  at 8k, TTFT idx 12x — strictly worth migrating)
- pd_sep_min_decodes_protected: 2 -> 1
- pd_sep_min_src_cache_tokens: 8000 -> 4000
- pd_sep_min_extra_cache_tokens: 4000 -> 2000

Isolation control:
- New --policy unified_kv_both option. Uses the exact same picker as
  --policy unified but the vLLMs are launched in kv_role=kv_both
  (the same launch mode unified_v2 requires). PD-sep never fires.
  Compares against unified_v2 to attribute any v2 effect to the
  PD-sep branch alone, not the kv_both always-on overhead.
- Both unified_kv_both and unified_v2 auto-enable kv_both launch in
  b3_isolated_policy.sh.

Tests:
- Updated the existing "chosen has no decodes" test for the new
  gate name and semantic.
- All 24 proxy tests pass.

Refs: window_1_results/v2_breakdown analysis (88.7% of candidates
caught by old new_local_below_threshold; 84% of the remainder
caught by the old few_decodes gate).

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

scripts/cache_aware_proxy.py

@@ -70,11 +70,14 @@ class Settings:
     # 2 × 48 × 4 × 128 × 2 = 98304 bytes per token.
     kv_bytes_per_token: int = 98304
 
-    # --- unified_v2 gating knobs ---
-    pd_sep_min_new_tokens: int = 16000          # B2 idx ≥ 3× starts here
-    pd_sep_min_decodes_protected: int = 2       # require src has live decodes to protect
-    pd_sep_min_src_cache_tokens: int = 8000     # require non-trivial cache to transfer
-    pd_sep_min_extra_cache_tokens: int = 4000   # src must have meaningfully more cache than chosen
+    # --- unified_v2 gating knobs (relaxed in v2.1 after the v1 0.2% trigger rate) ---
+    # B2 microbench shows TPOT idx 1.9x already at new_tokens=8k and TTFT
+    # idx ~12x; the previous 16k threshold was too conservative and
+    # rejected 88.7% of candidates (window_1_results/v2_breakdown).
+    pd_sep_min_new_tokens: int = 8000
+    pd_sep_min_decodes_protected: int = 1       # any in-flight work on chosen counts
+    pd_sep_min_src_cache_tokens: int = 4000     # half a block; was 8000
+    pd_sep_min_extra_cache_tokens: int = 2000   # half a block; was 4000
     pd_sep_margin_s: float = 0.2                # require cost gap > 0.2 s before migrating
     # Patch 6.6: per-request KV-xfer wall-clock timeout (proxy side).
     pd_sep_xfer_timeout_s: float = 60.0
@@ -461,12 +464,18 @@ def pick_instance_unified_v2(
         decision["v2_reason"] = f"new_local_below_threshold ({new_local} < {SETTINGS.pd_sep_min_new_tokens})"
         return chosen, chosen_idx, decision, None
 
-    # Hard gate 2: chosen must have live decodes worth protecting.
-    if chosen.ongoing_decode_tokens // max(1, SETTINGS.heavy_threshold) < 1 \
-            and chosen.num_requests < SETTINGS.pd_sep_min_decodes_protected:
-        # Heuristic for "num_decodes": prefer num_requests as an upper
-        # bound since we don't track decode count separately at route time.
-        decision["v2_reason"] = f"chosen_few_decodes ({chosen.num_requests})"
+    # Hard gate 2: chosen must have live decoding work to protect.
+    # v2.1 simplification: pure ongoing_decode_tokens check. The previous
+    # gate combined num_requests and decode_tokens with AND, but
+    # num_requests includes requests still in prefill — adding a prefill
+    # to a chosen that has only its own prefill running doesn't disrupt
+    # any decode, so skipping makes sense. The right semantic is "skip
+    # iff no decode is currently happening on chosen".
+    if chosen.ongoing_decode_tokens == 0:
+        decision["v2_reason"] = (
+            f"chosen_no_active_decode "
+            f"(num_req={chosen.num_requests} decode_tok={chosen.ongoing_decode_tokens})"
+        )
         return chosen, chosen_idx, decision, None
 
     # Find best alternative cache source.
@@ -684,7 +693,10 @@ async def lifespan(app: FastAPI):
 
         # Bootstrap combined instances for offload (need engine_ids for KV transfer)
         policy = getattr(global_args, 'policy', 'linear')
-        needs_bootstrap = global_args.offload or policy == "unified_v2"
+        needs_bootstrap = (
+            global_args.offload
+            or policy in ("unified_v2", "unified_kv_both")
+        )
         if needs_bootstrap and bp_list:
             await init_prefill_bootstrap(combined_instances, app.state.ready)
         elif needs_bootstrap and not bp_list:
@@ -846,7 +858,11 @@ async def _handle_combined(api, req_data, token_ids, input_length, session_id, h
         chosen, best_idx = pick_instance_sticky(
             combined_instances, token_ids, session_id, input_length,
             session_affinity_combined)
-    elif policy == "unified":
+    elif policy == "unified" or policy == "unified_kv_both":
+        # unified_kv_both: same picker as `unified`, but the vLLMs are
+        # launched in kv_role=kv_both. Use this as an isolation control
+        # for `unified_v2` so the v2-vs-v1 gap reflects only the PD-sep
+        # branch, not the kv_both always-on overhead.
         chosen, best_idx, decision = pick_instance_unified_hybrid(
             combined_instances, token_ids, session_id, input_length,
             session_affinity_combined)
@@ -1206,11 +1222,13 @@ def parse_args():
                    help="Comma-separated bootstrap ports for combined instances (for offload mode)")
     p.add_argument("--policy", type=str, default="linear",
                    choices=["linear", "lmetric", "load_only", "sticky",
-                             "unified", "unified_v2"],
+                             "unified", "unified_kv_both", "unified_v2"],
                    help="Routing policy: linear (cache-aware), lmetric (P_tokens × BS), "
                         "load_only (B3 control: pure min-num_requests), "
                         "sticky (B3 control: hard session affinity), "
                         "unified (hybrid affinity + LMetric fallback), "
+                        "unified_kv_both (unified on kv_both vLLMs; isolation "
+                        "control for unified_v2; PD-sep never triggers), "
                         "or unified_v2 (unified + selective per-request PD-sep "
                         "via Mooncake; requires --bootstrap-ports and "
                         "kv_role=kv_both vLLM launch)")