Apply LRU policy only to proper cache entries

vllm/v1/core/block_pool.py

@@ -413,13 +413,21 @@
             ordered_blocks: A list of blocks to free ordered by their eviction
                 priority.
         """
-        # Materialize the iterable to allow multiple passes.
-        blocks_list = list(ordered_blocks)
-        for block in blocks_list:
+        # Identify blocks with hash (LRU cache) and blocks without hash (will never match in APC)
+        blocks_with_hash = []
+        blocks_without_hash = []
+        for block in ordered_blocks:
             block.ref_cnt -= 1
-        self.free_block_queue.append_n(
-            [block for block in blocks_list if block.ref_cnt == 0 and not block.is_null]
-        )
+            if block.ref_cnt == 0 and not block.is_null:
+                if block.block_hash is None:
+                    blocks_without_hash.append(block)                    
+                else:
+                    blocks_with_hash.append(block)
+
+        # Allow immediate reallocation of blocks without hash
+        self.free_block_queue.prepend_n(blocks_without_hash)
+        # Append to LRU queue blocks for potential reuse
+        self.free_block_queue.append_n(blocks_with_hash)
 
     def evict_blocks(self, block_ids: set[int]) -> None:
         """evict blocks from the prefix cache by their block IDs.

vllm/v1/core/kv_cache_utils.py

@@ -349,6 +349,33 @@ def append_n(self, blocks: list[KVCacheBlock]) -> None:
 
         self.num_free_blocks += len(blocks)
 
+    def prepend_n(self, blocks: list[KVCacheBlock]) -> None:
+        """Put a list of blocks at the head of the free list (for immediate reuse).
+
+        Args:
+            blocks: The blocks to prepend.
+        """
+        if len(blocks) == 0:
+            return
+
+        first_block = self.fake_free_list_head.next_free_block
+        assert first_block is not None, (
+            "next_free_block of fake_free_list_head should always exist"
+        )
+
+        # Add inter-connections between consecutive blocks
+        prev_block = self.fake_free_list_head
+        for block in blocks:
+            block.prev_free_block = prev_block
+            prev_block.next_free_block = block
+            prev_block = block
+
+        # Connect the last block of <blocks> to the original first block
+        prev_block.next_free_block = first_block
+        first_block.prev_free_block = prev_block
+
+        self.num_free_blocks += len(blocks)
+
     def get_all_free_blocks(self) -> list[KVCacheBlock]:
         """Get all free blocks in the free list. Mainly used for testing.