fix(prefill): isolate cache budget policy from MLX runtime

inference_engine/distributed/cache_budget.py

@@ -0,0 +1,15 @@
+"""Platform-neutral cache-budget policy for model-loaded offload workers."""
+
+
+def adaptive_cache_budget(
+    *,
+    total_bytes: int,
+    active_model_bytes: int,
+    ceiling_bytes: int,
+    minimum_bytes: int,
+    reserve_bytes: int,
+) -> int:
+    if min(total_bytes, ceiling_bytes, minimum_bytes) <= 0 or reserve_bytes < 0:
+        raise ValueError("adaptive cache budget inputs are invalid")
+    available = max(0, total_bytes - active_model_bytes - reserve_bytes)
+    return max(minimum_bytes, min(ceiling_bytes, available))

scripts/start_prefill_worker_node.py

@@ -33,6 +33,7 @@
     NodeEndpoint,
     PrefillWorkerCapability,
 )
+from inference_engine.distributed.cache_budget import adaptive_cache_budget
 from inference_engine.distributed.exchange import (
     add_capability_service,
     exchange_once,
@@ -68,20 +69,6 @@ def physical_memory_bytes() -> int:
         return 0
 
 
-def adaptive_cache_budget(
-    *,
-    total_bytes: int,
-    active_model_bytes: int,
-    ceiling_bytes: int,
-    minimum_bytes: int,
-    reserve_bytes: int,
-) -> int:
-    if min(total_bytes, ceiling_bytes, minimum_bytes) <= 0 or reserve_bytes < 0:
-        raise ValueError("adaptive cache budget inputs are invalid")
-    available = max(0, total_bytes - active_model_bytes - reserve_bytes)
-    return max(minimum_bytes, min(ceiling_bytes, available))
-
-
 def mlx_active_memory_bytes() -> int:
     try:
         import mlx.core as mx

tests/inference_engine/bridge/test_prefill_worker_memory.py

@@ -1,6 +1,6 @@
 import pytest
 
-from scripts.start_prefill_worker_node import adaptive_cache_budget
+from inference_engine.distributed.cache_budget import adaptive_cache_budget
 
 
 def test_adaptive_budget_uses_only_model_headroom():