perf: add __slots__ to KVCacheBlock (#36164)

Signed-off-by: cong-or <conchubhar.gannon@gmail.com>

tests/v1/core/test_kv_cache_utils.py

@@ -202,6 +202,18 @@ def test_kv_cache_block():
     assert block.block_hash is None
 
 
+def test_kv_cache_block_uses_slots():
+    block = KVCacheBlock(block_id=0)
+
+    # Slots eliminate per-instance __dict__, saving ~264 bytes per block.
+    # At 100K+ blocks this avoids tens of MB of overhead and GC pressure.
+    assert not hasattr(block, "__dict__")
+
+    # Verify that slots actually prevent dynamic attribute assignment.
+    with pytest.raises(AttributeError):
+        block.unexpected_field = True
+
+
 def test_free_kv_cache_block_queue_initialization():
     # Test with a single block
     block = KVCacheBlock(block_id=0)

vllm/v1/core/kv_cache_utils.py

@@ -106,7 +106,7 @@ def init_none_hash(hash_fn: Callable[[Any], bytes]):
         NONE_HASH = BlockHash(hash_fn(hash_seed))
 
 
-@dataclass
+@dataclass(slots=True)
 class KVCacheBlock:
     """KV-cache block metadata."""