[V1] Move KV block hashes from Request to KVCacheManager (#12922)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

tests/v1/core/test_prefix_caching.py

@@ -51,7 +51,7 @@ def test_prefill():
     all_token_ids = common_token_ids + unique_token_ids
     req0 = make_request("0", all_token_ids)
     computed_blocks, num_computed_tokens = manager.get_computed_blocks(req0)
-    assert len(req0.kv_block_hashes) == 3
+    assert len(manager.req_to_block_hashes[req0.request_id]) == 3
     assert not computed_blocks
     assert num_computed_tokens == 0
     blocks = manager.allocate_slots(req0, 55, computed_blocks)
@@ -76,7 +76,7 @@ def test_prefill():
     unique_token_ids = [3] * 5
     req1 = make_request("1", common_token_ids + unique_token_ids)
     computed_blocks, num_computed_tokens = manager.get_computed_blocks(req1)
-    assert len(req1.kv_block_hashes) == 3
+    assert len(manager.req_to_block_hashes[req1.request_id]) == 3
     assert [b.block_id for b in computed_blocks] == [0, 1, 2]
     assert num_computed_tokens == 3 * 16
     num_new_tokens = 53 - 3 * 16
@@ -107,7 +107,7 @@ def test_prefill():
     unique_token_ids = [3] * 6
     req2 = make_request("2", common_token_ids + unique_token_ids)
     computed_blocks, num_computed_tokens = manager.get_computed_blocks(req2)
-    assert len(req2.kv_block_hashes) == 3
+    assert len(manager.req_to_block_hashes[req2.request_id]) == 3
     assert [b.block_id for b in computed_blocks] == [0, 1, 2]
     assert num_computed_tokens == 3 * 16
     num_new_tokens = 53 - 3 * 16
@@ -494,10 +494,11 @@ def test_mm_prefix_caching():
     # Completed block should have hashes with extra keys.
     assert not computed_blocks
     assert num_computed_tokens == 0
-    assert len(req0.kv_block_hashes) == 3
-    assert req0.kv_block_hashes[0].extra_keys == ("aaa", )
-    assert req0.kv_block_hashes[1].extra_keys == ("aaa", "bbb")
-    assert req0.kv_block_hashes[2].extra_keys == ("bbb", )
+    block_hashes = manager.req_to_block_hashes[req0.request_id]
+    assert len(block_hashes) == 3
+    assert block_hashes[0].extra_keys == ("aaa", )
+    assert block_hashes[1].extra_keys == ("aaa", "bbb")
+    assert block_hashes[2].extra_keys == ("bbb", )
 
     blocks = manager.allocate_slots(req0, 59, computed_blocks)
     assert [b.block_id for b in blocks] == [0, 1, 2, 3, 4]
@@ -510,8 +511,8 @@ def test_mm_prefix_caching():
     assert new_blocks is not None and len(new_blocks) == 0
 
     # The just completed block should have hashes with extra keys.
-    assert len(req0.kv_block_hashes) == 4
-    assert req0.kv_block_hashes[3].extra_keys == ("ccc", )
+    assert len(block_hashes) == 4
+    assert block_hashes[3].extra_keys == ("ccc", )
 
     # Cache hit.
     unique_token_ids = [-1] * 7 + [200] * 5
@@ -613,7 +614,7 @@ def test_reset_prefix_cache():
     all_token_ids = full_block_token_ids + unique_token_ids
     req1 = make_request("1", all_token_ids)
     computed_blocks, _ = manager.get_computed_blocks(req1)
-    assert len(req1.kv_block_hashes) == 3
+    assert len(manager.req_to_block_hashes[req1.request_id]) == 3
     assert len(computed_blocks) == 3
     blocks = manager.allocate_slots(req1, 7, computed_blocks)
     assert [b.block_id for b in blocks] == [4]

vllm/v1/core/kv_cache_manager.py

@@ -72,6 +72,12 @@ class KVCacheManager:
         self.req_to_blocks: DefaultDict[str,
                                         List[KVCacheBlock]] = defaultdict(list)
 
+        # Mapping from request ID to kv block hashes.
+        # This is to avoid recomputing the block hashes for each call of
+        # `get_computed_blocks` or `allocate_slots`.
+        self.req_to_block_hashes: DefaultDict[
+            str, List[BlockHashType]] = defaultdict(list)
+
     @property
     def usage(self) -> float:
         return 1.0 - (self.free_block_queue.num_free_blocks /
@@ -97,11 +103,11 @@ class KVCacheManager:
         computed_blocks = []
 
         # The block hashes for the request may already be computed
-        # if the request was preempted and resumed.
-        if not request.kv_block_hashes:
-            request.set_kv_block_hashes(
-                hash_request_tokens(self.block_size, request))
-        block_hashes = request.kv_block_hashes
+        # if the scheduler has tried to schedule the request before.
+        block_hashes = self.req_to_block_hashes[request.request_id]
+        if not block_hashes:
+            block_hashes = hash_request_tokens(self.block_size, request)
+            self.req_to_block_hashes[request.request_id] = block_hashes
 
         for block_hash in block_hashes:
             # block_hashes is a chain of block hashes. If a block hash is not
@@ -435,7 +441,8 @@ class KVCacheManager:
             full_blocks: The list of blocks to update hash metadata.
             prev_block: The previous block in the chain.
         """
-        num_cached_block_hashes = len(request.kv_block_hashes)
+        block_hashes = self.req_to_block_hashes[request.request_id]
+        num_cached_block_hashes = len(block_hashes)
 
         # Update the new blocks with the block hashes through the chain.
         prev_block_hash_value = None
@@ -468,7 +475,7 @@ class KVCacheManager:
                 # this request (either the prompt tokens or the previously
                 # generated tokens with preemption). In this case we simply
                 # reuse the block hash.
-                block_hash = request.kv_block_hashes[blk_idx]
+                block_hash = block_hashes[blk_idx]
             else:
                 # Otherwise compute the block hash and cache it in the request
                 # in case it will be preempted in the future.
@@ -490,9 +497,17 @@ class KVCacheManager:
                 # Compute the hash of the current block.
                 block_hash = hash_block_tokens(prev_block_hash_value,
                                                block_tokens, extra_keys)
-                request.append_kv_block_hashes(block_hash)
+                block_hashes.append(block_hash)
 
             # Update and added the full block to the cache.
             blk.block_hash = block_hash
             self.cached_block_hash_to_block[block_hash][blk.block_id] = blk
             prev_block_hash_value = block_hash.hash_value
+
+    def free_block_hashes(self, request: Request) -> None:
+        """Discard the block hashes for the request.
+
+        NOTE: Unlike `free`, this method should be called only when the request
+        is finished, not when it is preempted.
+        """
+        self.req_to_block_hashes.pop(request.request_id, None)

vllm/v1/core/scheduler.py

@@ -579,6 +579,7 @@ class Scheduler:
     def _free_request(self, request: Request) -> None:
         assert request.is_finished()
         self.kv_cache_manager.free(request)
+        self.kv_cache_manager.free_block_hashes(request)
         self.encoder_cache_manager.free(request)
         self._cached_reqs_data.pop(request.request_id, None)
         del self.requests[request.request_id]

vllm/v1/request.py

@@ -12,7 +12,6 @@ from vllm.v1.utils import ConstantList
 if TYPE_CHECKING:
     from vllm.multimodal import MultiModalKwargs
     from vllm.multimodal.inputs import PlaceholderRange
-    from vllm.v1.core.kv_cache_utils import BlockHashType
 
 
 class Request:
@@ -63,11 +62,6 @@ class Request:
         if self.mm_hashes:
             assert len(self.mm_inputs) == len(self.mm_hashes)
 
-        # Cache the computed kv block hashes of the request to avoid
-        # recomputing.
-        self._kv_block_hashes: List[BlockHashType] = []
-        self.kv_block_hashes = ConstantList(self._kv_block_hashes)
-
         # Read-only views
         # Prevent directly appending to the these lists since
         # they should also be updated simultaneously.
@@ -124,13 +118,6 @@ class Request:
         num_tokens = self.mm_positions[input_id]["length"]
         return num_tokens
 
-    def set_kv_block_hashes(self, value: List["BlockHashType"]) -> None:
-        self._kv_block_hashes = value
-        self.kv_block_hashes = ConstantList(self._kv_block_hashes)
-
-    def append_kv_block_hashes(self, block_hash: "BlockHashType") -> None:
-        self._kv_block_hashes.append(block_hash)
-
 
 class RequestStatus(enum.IntEnum):
     """Status of a request."""