Add Automatic Prefix Caching (#2762)

Co-authored-by: ElizaWszola <eliza@neuralmagic.com>
Co-authored-by: Michael Goin <michael@neuralmagic.com>

benchmarks/benchmark_throughput.py

@@ -73,21 +73,21 @@ def run_vllm(
     enforce_eager: bool,
     kv_cache_dtype: str,
     device: str,
+    enable_prefix_caching: bool,
 ) -> float:
     from vllm import LLM, SamplingParams
-    llm = LLM(
-        model=model,
-        tokenizer=tokenizer,
-        quantization=quantization,
-        tensor_parallel_size=tensor_parallel_size,
-        seed=seed,
-        trust_remote_code=trust_remote_code,
-        dtype=dtype,
-        max_model_len=max_model_len,
-        enforce_eager=enforce_eager,
-        kv_cache_dtype=kv_cache_dtype,
-        device=device,
-    )
+    llm = LLM(model=model,
+              tokenizer=tokenizer,
+              quantization=quantization,
+              tensor_parallel_size=tensor_parallel_size,
+              seed=seed,
+              trust_remote_code=trust_remote_code,
+              dtype=dtype,
+              max_model_len=max_model_len,
+              enforce_eager=enforce_eager,
+              kv_cache_dtype=kv_cache_dtype,
+              device=device,
+              enable_prefix_caching=enable_prefix_caching)
 
     # Add the requests to the engine.
     for prompt, _, output_len in requests:
@@ -211,7 +211,8 @@ def main(args: argparse.Namespace):
                                 args.seed, args.n, args.use_beam_search,
                                 args.trust_remote_code, args.dtype,
                                 args.max_model_len, args.enforce_eager,
-                                args.kv_cache_dtype, args.device)
+                                args.kv_cache_dtype, args.device,
+                                args.enable_prefix_caching)
     elif args.backend == "hf":
         assert args.tensor_parallel_size == 1
         elapsed_time = run_hf(requests, args.model, tokenizer, args.n,
@@ -302,6 +303,7 @@ if __name__ == "__main__":
         default="cuda",
         choices=["cuda"],
         help='device type for vLLM execution, supporting CUDA only currently.')
+    parser.add_argument("--enable_prefix_caching", action='store_true')
     args = parser.parse_args()
     if args.tokenizer is None:
         args.tokenizer = args.model

docs/source/models/engine_args.rst

@@ -81,6 +81,10 @@ Below, you can find an explanation of every engine argument for vLLM:
 
     Token block size for contiguous chunks of tokens.
 
+.. option:: --enable-prefix-caching
+
+    Enables automatic prefix caching
+
 .. option:: --seed <seed>
 
     Random seed for operations.

examples/offline_inference_with_prefix.py

@@ -37,20 +37,13 @@ for output in outputs:
 
 print("-" * 80)
 
-# -1 since the last token can change when concatenating prompts.
-prefix_pos = len(llm.llm_engine.tokenizer.encode(prefix)) - 1
-
 # The llm.generate call will batch all prompts and send the batch at once if resources allow.
 # The prefix will only be cached after the first batch is processed, so we need to call generate once
 # to calculate the prefix and cache it.
-outputs = llm.generate(generating_prompts[0],
-                       sampling_params,
-                       prefix_pos=[prefix_pos])
+outputs = llm.generate(generating_prompts[0], sampling_params)
 
 # Subsequent batches can leverage the cached prefix
-outputs = llm.generate(generating_prompts,
-                       sampling_params,
-                       prefix_pos=[prefix_pos] * len(generating_prompts))
+outputs = llm.generate(generating_prompts, sampling_params)
 
 # Print the outputs. You should see the same outputs as before
 for output in outputs:

tests/prefix_caching/test_prefix_caching.py

@@ -4,38 +4,73 @@ Run `pytest tests/prefix_caching/test_prefix_caching.py`.
 """
 import pytest
 
-from vllm import LLM, SamplingParams
-
-prefix = (
-    "You are an expert school principal, skilled in effectively managing "
-    "faculty and staff. Draft 10-15 questions for a potential first grade "
-    "Head Teacher for my K-12, all-girls', independent school that emphasizes "
-    "community, joyful discovery, and life-long learning. The candidate is "
-    "coming in for a first-round panel interview for a 8th grade Math "
-    "teaching role. They have 5 years of previous teaching experience "
-    "as an assistant teacher at a co-ed, public school with experience "
-    "in middle school math teaching. Based on these information, fulfill "
-    "the following paragraph: ")
-
-
-@pytest.mark.parametrize("model", ["facebook/opt-125m"])
-@pytest.mark.parametrize("max_tokens", [16])
-def test_prefix_caching(
-    example_prompts,
-    model: str,
-    max_tokens: int,
+from vllm.core.block_manager import BlockAllocator
+from vllm.utils import Device
+
+
+@pytest.mark.parametrize("block_size", [16])
+@pytest.mark.parametrize("num_blocks", [16])
+def test_block_allocator(
+    block_size: int,
+    num_blocks: int,
 ):
-    llm = LLM(model=model)
-    # -1 since the last token can change when concatenating prompts.
-    prefix_pos = len(llm.llm_engine.tokenizer.encode(prefix)) - 1
-    prompts = [prefix + prompt for prompt in example_prompts]
-    sampling_params = SamplingParams(temperature=0.0, max_tokens=max_tokens)
-    outputs_without_prefix = llm.generate(prompts, sampling_params)
-    outputs_with_prefix = llm.generate(prompts,
-                                       sampling_params,
-                                       prefix_pos=[prefix_pos] * len(prompts))
-    for output_without_prefix, output_with_prefix in zip(
-            outputs_without_prefix, outputs_with_prefix):
-        assert (output_without_prefix.outputs[0].token_ids ==
-                output_with_prefix.outputs[0].token_ids)
-    assert len(llm.llm_engine.scheduler.prefix_pool.prefixes) == 1
+    block_hash = 1
+    block_allocator = BlockAllocator(Device.CPU,
+                                     block_size,
+                                     num_blocks,
+                                     enable_caching=True)
+
+    # Allocate two PysicalTokenBlocks with the same hash and check that they are the same PhysicalTokenBlock
+    first_block = block_allocator.allocate(block_hash, 0)
+    second_block = block_allocator.allocate(block_hash, 0)
+    assert (first_block == second_block)
+    assert (second_block.ref_count == 2)
+
+    # Free the first_block and confirm that the ref_count is correctly decremented on the second block
+    block_allocator.free(first_block)
+    assert (second_block.ref_count == 1)
+
+    # Free the second block
+    block_allocator.free(second_block)
+
+    # Reallocate the first block and confirm that, even after the block had its ref_count go to 0, we still get the same block back
+    first_block = block_allocator.allocate(block_hash, 0)
+    assert (first_block == second_block)
+    assert (first_block.block_hash == block_hash)
+
+
+@pytest.mark.parametrize("num_blocks", [16])
+def test_eviction(num_blocks: int, ):
+    block_size = 16
+    block_allocator = BlockAllocator(Device.CPU,
+                                     block_size,
+                                     num_blocks,
+                                     enable_caching=True)
+    blocks = []
+
+    for i in range(num_blocks):
+        # use i as the block_hash
+        blocks.append(block_allocator.allocate(i, 0))
+
+    #Free all blocks
+    for block in blocks:
+        block_allocator.free(block)
+
+    # Allocate a new block and confirm that it's the first block freed. I.E The Least Recently Used block
+    new_block_hash = block_size
+    new_block = block_allocator.allocate(new_block_hash, 0)
+    assert (new_block == blocks[0])
+    assert (new_block.block_hash == new_block_hash)
+
+    # Reallocate the second in blocks to remove it from the free list
+    realloc_block_hash = 1
+    realloc_block = block_allocator.allocate(realloc_block_hash, 0)
+    assert (realloc_block == blocks[realloc_block_hash])
+    assert (realloc_block.block_hash == realloc_block_hash)
+
+    # Allocate a new block and confirm that it's not the realloc_block, since the realloc_block shouldn't be in the free list
+    new_block_hash = block_size + 1
+    new_block = block_allocator.allocate(new_block_hash, 0)
+    assert (realloc_block != new_block)
+    assert (new_block.block_hash == new_block_hash)
+    assert (new_block.block_number == 2)

tests/test_cache_block_hashing.py

+"""Test hashing of cache blocks.
+
+Run `pytest tests/test_cache_block_hashing.py`.
+"""
+import pytest
+
+from vllm.transformers_utils.tokenizer import TokenizerGroup
+from vllm.sequence import Sequence
+
+# Make two prefixes with different first blocks.
+prefix_start = [("You are an expert"), ("You are a")]
+prefix_common = (
+    " school principal, skilled in effectively managing "
+    "faculty and staff. Draft 10-15 questions for a potential first grade "
+    "Head Teacher for my K-12, all-girls', independent school that emphasizes "
+    "community, joyful discovery, and life-long learning. The candidate is "
+    "coming in for a first-round panel interview for a 8th grade Math "
+    "teaching role. They have 5 years of previous teaching experience "
+    "as an assistant teacher at a co-ed, public school with experience "
+    "in middle school math teaching. Based on this, fulfill "
+    "the following: ")
+prefixes = [start + prefix_common for start in prefix_start]
+
+# Sample prompts.
+sample_prompts = [
+    "Hello, my name is", "The president of the United States is",
+    "The capital of France is", "The future of AI is"
+]
+
+
+# Helper function.
+def flatten_2d(li):
+    return [lss for ls in li for lss in ls]
+
+
+@pytest.mark.parametrize("model", ["facebook/opt-125m"])
+@pytest.mark.parametrize("block_size", [16])
+@pytest.mark.parametrize("max_num_seqs", [256])
+def test_auto_prefix_caching(model: str, block_size: int, max_num_seqs: int):
+
+    tokenizer = TokenizerGroup(
+        tokenizer_id="facebook/opt-125m",
+        enable_lora=False,
+        max_num_seqs=max_num_seqs,
+        max_input_length=None,
+    )
+
+    hashes = []
+
+    for prefix in prefixes:
+        hashes.append([])
+        prompts = [prefix + prompt for prompt in sample_prompts]
+        seq_id = 0
+        for prompt in prompts:
+            hashes[-1].append([])
+            prompt_token_ids = tokenizer.encode(prompt)
+            seq = Sequence(seq_id, prompt, prompt_token_ids, block_size)
+
+            num_blocks = len(prompt_token_ids) // block_size
+            for idx in range(num_blocks):
+                hashes[-1][-1].append(seq.hash_of_block(idx))
+
+            seq_id += 1
+
+    # Check that hashes made with two prefixes with different first blocks are
+    # different everywhere.
+    for hash0, hash1 in zip(flatten_2d(hashes[0]), flatten_2d(hashes[1])):
+        assert (hash0 != hash1)
+
+    # Check that hashes of different prompts made with the same prefix are the
+    # same until the hashes that contain the prompt.
+    for hash_pref in hashes:
+        same_hashes = [tuple(h[:-1]) for h in hash_pref]
+        different_hashes = [h[-1] for h in hash_pref]
+        assert (len(set(same_hashes)) == 1)
+        assert (len(set(different_hashes)) == len(different_hashes))

vllm/block.py

@@ -5,6 +5,8 @@ from vllm.utils import Device
 
 _BLANK_TOKEN_ID = -1
 
+DEFAULT_LAST_ACCESSED_TIME = -1
+
 
 class LogicalTokenBlock:
     """A block that stores a contiguous chunk of tokens from left to right.
@@ -55,17 +57,27 @@ class PhysicalTokenBlock:
         device: Device,
         block_number: int,
         block_size: int,
+        block_hash: int,
+        num_hashed_tokens: int,
     ) -> None:
         self.device = device
         self.block_number = block_number
         self.block_size = block_size
+        self.block_hash = block_hash
+        self.num_hashed_tokens = num_hashed_tokens
 
         self.ref_count = 0
+        self.last_accessed = DEFAULT_LAST_ACCESSED_TIME
+
+        self.computed = False
 
     def __repr__(self) -> str:
         return (f'PhysicalTokenBlock(device={self.device}, '
                 f'block_number={self.block_number}, '
-                f'ref_count={self.ref_count})')
+                f'num_hashed_tokens={self.num_hashed_tokens}, '
+                f'ref_count={self.ref_count}, '
+                f'last_accessed={self.last_accessed}, '
+                f'computed={self.computed})')
 
 
 # Mapping: logical block number -> physical block.

vllm/config.py

@@ -303,12 +303,14 @@ class CacheConfig:
         swap_space: int,
         cache_dtype: str,
         sliding_window: Optional[int] = None,
+        enable_prefix_caching: bool = False,
     ) -> None:
         self.block_size = block_size
         self.gpu_memory_utilization = gpu_memory_utilization
         self.swap_space_bytes = swap_space * _GB
         self.cache_dtype = cache_dtype
         self.sliding_window = sliding_window
+        self.enable_prefix_caching = enable_prefix_caching
         self._verify_args()
         self._verify_cache_dtype()
 

vllm/core/block_manager.py

 """A block manager that manages token blocks."""
 import enum
+from itertools import count
+from os.path import commonprefix
 from typing import Dict, List, Optional, Set, Tuple
 
 from vllm.block import BlockTable, PhysicalTokenBlock
 from vllm.sequence import Sequence, SequenceGroup, SequenceStatus
 from vllm.utils import Device
+from vllm.core.evictor import Evictor, EvictionPolicy, make_evictor
 
 
 class BlockAllocator:
@@ -15,29 +18,68 @@ class BlockAllocator:
     the reference count becomes zero, the block is added back to the free list.
     """
 
-    def __init__(
-        self,
-        device: Device,
-        block_size: int,
-        num_blocks: int,
-    ) -> None:
+    def __init__(self,
+                 device: Device,
+                 block_size: int,
+                 num_blocks: int,
+                 eviction_policy: EvictionPolicy = EvictionPolicy.LRU,
+                 enable_caching: bool = False) -> None:
         self.device = device
         self.block_size = block_size
         self.num_blocks = num_blocks
+        self.enable_caching = enable_caching
+
+        self.current_num_blocks = 0
+        self.cached_blocks: Dict[int, PhysicalTokenBlock] = {}
+
+        # Switch over to FIFO eviction when caching is disabled
+        if not self.enable_caching:
+            eviction_policy = EvictionPolicy.FIFO
+        self.evictor: Evictor = make_evictor(eviction_policy)
+
+        self.default_hash_ctr = count()
+
+    def allocate_block(self, block_hash: int,
+                       num_hashed_tokens: int) -> PhysicalTokenBlock:
+        if self.current_num_blocks == self.num_blocks:
+            block = self.evictor.evict()
+            block.block_hash = block_hash
+            block.num_hashed_tokens = num_hashed_tokens
+            return block
+        block = PhysicalTokenBlock(device=self.device,
+                                   block_number=self.current_num_blocks,
+                                   block_size=self.block_size,
+                                   block_hash=block_hash,
+                                   num_hashed_tokens=num_hashed_tokens)
+        self.current_num_blocks += 1
+        return block
 
-        # Initialize the free blocks.
-        self.free_blocks: BlockTable = []
-        for i in range(num_blocks):
-            block = PhysicalTokenBlock(device=device,
-                                       block_number=i,
-                                       block_size=block_size)
-            self.free_blocks.append(block)
-
-    def allocate(self) -> PhysicalTokenBlock:
-        if not self.free_blocks:
-            raise ValueError("Out of memory! No free blocks are available.")
-        block = self.free_blocks.pop()
-        block.ref_count = 1
+    def allocate(self,
+                 block_hash: Optional[int] = None,
+                 num_hashed_tokens: int = 0) -> PhysicalTokenBlock:
+        # If caching is disabled, just allocate a new block and return it
+        if not self.enable_caching:
+            block = self.allocate_block(next(self.default_hash_ctr),
+                                        num_hashed_tokens)
+            block.ref_count += 1
+            return block
+
+        if block_hash is None:
+            block_hash = next(self.default_hash_ctr)
+        if block_hash in self.evictor:
+            assert block_hash not in self.cached_blocks
+            block = self.evictor.remove(block_hash)
+            assert block.ref_count == 0
+            self.cached_blocks[block_hash] = block
+            block.ref_count += 1
+            assert block.block_hash == block_hash
+            return block
+        if block_hash not in self.cached_blocks:
+            self.cached_blocks[block_hash] = self.allocate_block(
+                block_hash, num_hashed_tokens)
+        block = self.cached_blocks[block_hash]
+        assert block.block_hash == block_hash
+        block.ref_count += 1
         return block
 
     def free(self, block: PhysicalTokenBlock) -> None:
@@ -45,10 +87,27 @@ class BlockAllocator:
             raise ValueError(f"Double free! {block} is already freed.")
         block.ref_count -= 1
         if block.ref_count == 0:
-            self.free_blocks.append(block)
+            assert block.block_hash not in self.evictor
+            self.evictor.add(block)
+
+            # If caching is enabled, remove the block from the cached_blocks
+            if self.enable_caching:
+                del self.cached_blocks[block.block_hash]
 
     def get_num_free_blocks(self) -> int:
-        return len(self.free_blocks)
+        return self.num_blocks - self.current_num_blocks + self.evictor.num_blocks
+
+    def contains_block(self, block_hash: int) -> bool:
+        return block_hash in self.cached_blocks or block_hash in self.evictor
+
+    def update_hash(self, block_hash: int, block: PhysicalTokenBlock):
+        # If caching is enabled, update the hash of block and the cached_blocks dictionary.
+        if self.enable_caching:
+            assert not self.contains_block(block_hash)
+            old_hash = block.block_hash
+            block.block_hash = block_hash
+            del self.cached_blocks[old_hash]
+            self.cached_blocks[block_hash] = block
 
 
 class AllocStatus(enum.Enum):
@@ -75,6 +134,7 @@ class BlockSpaceManager:
         num_cpu_blocks: int,
         watermark: float = 0.01,
         sliding_window: Optional[int] = None,
+        enable_caching: bool = False,
     ) -> None:
         self.block_size = block_size
         self.num_total_gpu_blocks = num_gpu_blocks
@@ -89,11 +149,17 @@ class BlockSpaceManager:
         self.watermark = watermark
         assert watermark >= 0.0
 
+        self.enable_caching = enable_caching
+
         self.watermark_blocks = int(watermark * num_gpu_blocks)
-        self.gpu_allocator = BlockAllocator(Device.GPU, block_size,
-                                            num_gpu_blocks)
-        self.cpu_allocator = BlockAllocator(Device.CPU, block_size,
-                                            num_cpu_blocks)
+        self.gpu_allocator = BlockAllocator(Device.GPU,
+                                            block_size,
+                                            num_gpu_blocks,
+                                            enable_caching=enable_caching)
+        self.cpu_allocator = BlockAllocator(Device.CPU,
+                                            block_size,
+                                            num_cpu_blocks,
+                                            enable_caching=enable_caching)
         # Mapping: seq_id -> BlockTable.
         self.block_tables: Dict[int, BlockTable] = {}
 
@@ -103,9 +169,6 @@ class BlockSpaceManager:
         seq = seq_group.get_seqs(status=SequenceStatus.WAITING)[0]
         num_required_blocks = len(seq.logical_token_blocks)
 
-        if seq_group.prefix is not None and seq_group.prefix.allocated:
-            num_required_blocks -= seq_group.prefix.get_num_blocks()
-
         if self.block_sliding_window is not None:
             num_required_blocks = min(num_required_blocks,
                                       self.block_sliding_window)
@@ -129,36 +192,16 @@ class BlockSpaceManager:
         num_prompt_blocks = len(seq.logical_token_blocks)
 
         block_table: BlockTable = []
-        prefix_block_table: BlockTable = []
-        num_prefix_blocks = 0
-
-        prefix = seq_group.prefix
-        if prefix is not None and prefix.allocated:
-            # Prefix has already been allocated. Use the existing block table.
-            num_prompt_blocks -= prefix.get_num_blocks()
-            for block in prefix.block_table:
-                block.ref_count += seq_group.num_seqs()
-                block_table.append(block)
-
         for logical_idx in range(num_prompt_blocks):
             if (self.block_sliding_window is not None
                     and logical_idx >= self.block_sliding_window):
                 block = block_table[logical_idx % self.block_sliding_window]
             else:
-                block = self.gpu_allocator.allocate()
-            # Set the reference counts of the token blocks.
-            block.ref_count = seq_group.num_seqs()
+                block = self.gpu_allocator.allocate(
+                    seq.hash_of_block(logical_idx),
+                    seq.num_hashed_tokens_of_block(logical_idx))
             block_table.append(block)
 
-        if prefix is not None and not prefix.allocated:
-            # Allocate blocks for the prefix, we will compute the prefix's
-            # KV cache in this run.
-            num_prefix_blocks = prefix.get_num_blocks()
-            prefix_block_table = block_table[:num_prefix_blocks]
-            for block in prefix_block_table:
-                block.ref_count += 1
-            prefix.set_block_table(prefix_block_table)
-
         # Assign the block table for each sequence.
         for seq in seq_group.get_seqs(status=SequenceStatus.WAITING):
             self.block_tables[seq.seq_id] = block_table.copy()
@@ -170,12 +213,72 @@ class BlockSpaceManager:
         num_seqs = seq_group.num_seqs(status=SequenceStatus.RUNNING)
         return num_seqs <= num_free_gpu_blocks
 
-    def append_slot(self, seq: Sequence) -> Optional[Tuple[int, int]]:
+    def _promote_last_block(
+        self,
+        seq: Sequence,
+        last_block: PhysicalTokenBlock,
+    ) -> PhysicalTokenBlock:
+        # Compute a new hash for the block so that it can be shared by other Sequences
+        new_hash = seq.hash_of_block(len(seq.logical_token_blocks) - 1)
+
+        # if new_hash is already in the cached table, then free last_block and return the cached version
+        if self.gpu_allocator.contains_block(new_hash):
+            self.gpu_allocator.free(last_block)
+            return self.gpu_allocator.allocate(new_hash)
+        else:
+            self.gpu_allocator.update_hash(new_hash, last_block)
+            return last_block
+
+    def _is_last_block_full(
+        self,
+        seq: Sequence,
+    ) -> bool:
+        token_ids_len = len(seq.data.get_token_ids())
+        return token_ids_len > 0 and token_ids_len % seq.block_size == 0
+
+    def _is_last_block(
+        self,
+        seq: Sequence,
+        index: int,
+    ) -> bool:
+        return index == len(seq.logical_token_blocks) - 1
+
+    def _maybe_promote_last_block(
+        self,
+        seq: Sequence,
+        last_block: PhysicalTokenBlock,
+    ) -> PhysicalTokenBlock:
+        if self._is_last_block_full(seq):
+            return self._promote_last_block(seq, last_block)
+        else:
+            return last_block
+
+    def _allocate_last_physical_block(
+        self,
+        seq: Sequence,
+    ) -> PhysicalTokenBlock:
+        block_hash: Optional[int] = None
+        if (self._is_last_block_full(seq)):
+            block_hash = seq.hash_of_block(len(seq.logical_token_blocks) - 1)
+        num_hashed_tokens = seq.num_hashed_tokens_of_block(
+            len(seq.logical_token_blocks) - 1)
+        new_block = self.gpu_allocator.allocate(block_hash, num_hashed_tokens)
+        if block_hash is None:
+            assert new_block.ref_count == 1
+        return new_block
+
+    def append_slot(
+        self,
+        seq: Sequence,
+    ) -> Optional[Tuple[int, int]]:
         """Allocate a physical slot for a new token."""
         logical_blocks = seq.logical_token_blocks
         block_table = self.block_tables[seq.seq_id]
-
+        # If we need to allocate a new physical block
         if len(block_table) < len(logical_blocks):
+            # Currently this code only supports adding one physical block
+            assert len(block_table) == len(logical_blocks) - 1
+
             if (self.block_sliding_window
                     and len(block_table) >= self.block_sliding_window):
                 # reuse a block
@@ -184,8 +287,8 @@ class BlockSpaceManager:
             else:
                 # The sequence has a new logical block.
                 # Allocate a new physical block.
-                block = self.gpu_allocator.allocate()
-                block_table.append(block)
+                new_block = self._allocate_last_physical_block(seq)
+                block_table.append(new_block)
                 return None
 
         # We want to append the token to the last physical block.
@@ -193,11 +296,15 @@ class BlockSpaceManager:
         assert last_block.device == Device.GPU
         if last_block.ref_count == 1:
             # Not shared with other sequences. Appendable.
+            # If the last block is now complete, promote it to a full block so that it can be shared
+            new_block = self._maybe_promote_last_block(seq, last_block)
+            block_table[-1] = new_block
             return None
         else:
             # The last block is shared with other sequences.
             # Copy on Write: Allocate a new block and copy the tokens.
-            new_block = self.gpu_allocator.allocate()
+            new_block = self._allocate_last_physical_block(seq)
+
             block_table[-1] = new_block
             self.gpu_allocator.free(last_block)
             return last_block.block_number, new_block.block_number
@@ -233,25 +340,18 @@ class BlockSpaceManager:
 
     def swap_in(self, seq_group: SequenceGroup) -> Dict[int, int]:
         # CPU block -> GPU block.
-        if seq_group.prefix is not None:
-            # make sure to swap in the prefix first
-            assert seq_group.prefix.allocated and seq_group.prefix.computed
-
         mapping: Dict[PhysicalTokenBlock, PhysicalTokenBlock] = {}
         for seq in seq_group.get_seqs(status=SequenceStatus.SWAPPED):
             new_block_table: BlockTable = []
             block_table = self.block_tables[seq.seq_id]
-            if seq_group.prefix is not None:
-                for block in seq_group.prefix.block_table:
-                    new_block_table.append(block)
-                    block.ref_count += 1
 
             for cpu_block in block_table:
                 if cpu_block in mapping:
                     gpu_block = mapping[cpu_block]
                     gpu_block.ref_count += 1
                 else:
-                    gpu_block = self.gpu_allocator.allocate()
+                    gpu_block = self.gpu_allocator.allocate(
+                        cpu_block.block_hash, cpu_block.num_hashed_tokens)
                     mapping[cpu_block] = gpu_block
                 new_block_table.append(gpu_block)
                 # Free the CPU block swapped in to GPU.
@@ -276,17 +376,12 @@ class BlockSpaceManager:
             block_table = self.block_tables[seq.seq_id]
 
             for gpu_block in block_table:
-                if (seq_group.prefix is not None
-                        and gpu_block in seq_group.prefix.block_table):
-                    # NOTE: We do not swap out the prefix blocks for now.
-                    self.gpu_allocator.free(gpu_block)
-                    continue
-
                 if gpu_block in mapping:
                     cpu_block = mapping[gpu_block]
                     cpu_block.ref_count += 1
                 else:
-                    cpu_block = self.cpu_allocator.allocate()
+                    cpu_block = self.cpu_allocator.allocate(
+                        gpu_block.block_hash, gpu_block.num_hashed_tokens)
                     mapping[gpu_block] = cpu_block
                 new_block_table.append(cpu_block)
                 # Free the GPU block swapped out to CPU.
@@ -328,3 +423,49 @@ class BlockSpaceManager:
 
     def get_num_free_cpu_blocks(self) -> int:
         return self.cpu_allocator.get_num_free_blocks()
+
+    def access_all_blocks_in_seq(
+        self,
+        seq: Sequence,
+        access_time: float,
+    ) -> None:
+        block_table = self.block_tables[seq.seq_id]
+        for block in block_table:
+            block.last_accessed = access_time
+
+    def compute_last_full_block_in_seq(self, seq: Sequence):
+        if seq.seq_id not in self.block_tables:
+            return
+        max_full_block = seq.get_len() // seq.block_size - 1
+        block_table = self.block_tables[seq.seq_id]
+        if max_full_block == -1:
+            return
+        block_table[max_full_block].computed = True
+
+    def get_all_block_ids_till_computed(self, seq: Sequence) -> List[int]:
+        if seq.seq_id not in self.block_tables:
+            return []
+        block_table = self.block_tables[seq.seq_id]
+        for block_idx in reversed(range(len(block_table))):
+            if block_table[block_idx].computed:
+                return [b.block_number for b in block_table[:block_idx + 1]]
+        return []
+
+    # Can return non-empty result only with prefix caching enabled.
+    def get_common_computed_block_ids(self,
+                                      seq_group: SequenceGroup) -> List[int]:
+        if not self.enable_caching:
+            return []
+
+        ids_list = [
+            self.get_all_block_ids_till_computed(seq)
+            for seq in iter(seq_group.seqs_dict.values())
+        ]
+        return commonprefix([ids for ids in ids_list if ids != []])
+
+    # We only mark the last full block because with prefix caching,
+    # all blocks until the marked one are guaranteed to be computed.
+    def mark_blocks_as_computed(self, seq_group: SequenceGroup):
+        if self.enable_caching:
+            for seq in seq_group.seqs_dict.values():
+                self.compute_last_full_block_in_seq(seq)

vllm/core/evictor.py

+import enum
+from typing import Dict, List, Optional
+from abc import ABC, abstractmethod, abstractproperty
+
+from vllm.block import PhysicalTokenBlock
+
+
+class EvictionPolicy(enum.Enum):
+    """Enum for eviction policy used by make_evictor to instantiate the correct
+       Evictor subclass.
+    """
+    LRU = enum.auto()
+    FIFO = enum.auto()
+
+
+class Evictor(ABC):
+    """The Evictor subclasses should be used by the BlockAllocator class to
+    handle eviction of freed PhysicalTokenBlocks.
+    """
+
+    @abstractmethod
+    def __init__(self):
+        pass
+
+    @abstractmethod
+    def __contains__(self, block_hash: int) -> bool:
+        pass
+
+    @abstractmethod
+    def evict(self) -> PhysicalTokenBlock:
+        """Runs the eviction algorithm and returns the evicted block"""
+        pass
+
+    @abstractmethod
+    def add(self, block: PhysicalTokenBlock):
+        """Adds block to the evictor, making it a candidate for eviction"""
+        pass
+
+    @abstractmethod
+    def remove(self, block_hash: int) -> PhysicalTokenBlock:
+        """Simply removes the block with the hash value block_hash from the
+        evictor. Caller is responsible for making sure that block_hash is contained
+        in the evictor before calling remove. Should be used to "bring back" blocks
+        that have been freed but not evicted yet.
+        """
+        pass
+
+    @abstractproperty
+    def num_blocks(self) -> int:
+        pass
+
+
+class LRUEvictor(Evictor):
+    """Evicts in a least-recently-used order using the last_accessed timestamp
+    that's recorded in the PhysicalTokenBlock. If there are multiple blocks with
+    the same last_accessed time, then the one with the largest num_hashed_tokens
+    will be evicted. If two blocks each have the lowest last_accessed time and
+    highest num_hashed_tokens value, then one will be chose arbitrarily
+    """
+
+    def __init__(self):
+        self.free_table: Dict[int, PhysicalTokenBlock] = {}
+
+    def __contains__(self, block_hash: int) -> bool:
+        return block_hash in self.free_table
+
+    # TODO: The performance of this evict function can be optimized further.
+    def evict(self) -> PhysicalTokenBlock:
+        free_blocks: List[PhysicalTokenBlock] = list(self.free_table.values())
+        if len(free_blocks) == 0:
+            raise ValueError("No usable cache memory left")
+
+        # Find lowest timestamp
+        lowest_timestamp = free_blocks[0].last_accessed
+        for block in free_blocks:
+            if block.last_accessed < lowest_timestamp:
+                lowest_timestamp = block.last_accessed
+
+        # Find all blocks with the lowest timestamp
+        least_recent: List[PhysicalTokenBlock] = []
+        for block in free_blocks:
+            if block.last_accessed == lowest_timestamp:
+                least_recent.append(block)
+
+        # Find highest prefix count per block
+        highest_num_hashed_tokens = 0
+        for block in least_recent:
+            if block.num_hashed_tokens > highest_num_hashed_tokens:
+                highest_num_hashed_tokens = block.num_hashed_tokens
+
+        evicted_block: Optional[PhysicalTokenBlock] = None
+
+        # Find the first block with the lowest timestamp
+        for block in least_recent:
+            if block.num_hashed_tokens == highest_num_hashed_tokens:
+                evicted_block = block
+                break
+
+        assert evicted_block is not None
+
+        del self.free_table[evicted_block.block_hash]
+
+        evicted_block.computed = False
+        return evicted_block
+
+    def add(self, block: PhysicalTokenBlock):
+        self.free_table[block.block_hash] = block
+
+    def remove(self, block_hash: int) -> PhysicalTokenBlock:
+        if block_hash not in self.free_table:
+            raise ValueError(
+                "Attempting to remove block that's not in the evictor")
+        block: PhysicalTokenBlock = self.free_table[block_hash]
+        del self.free_table[block_hash]
+        return block
+
+    @property
+    def num_blocks(self) -> int:
+        return len(self.free_table)
+
+
+class RandomEvictor(Evictor):
+    """Evicts in a first-in-first-out order"""
+
+    def __init__(self):
+        self.free_table: Dict[int, PhysicalTokenBlock] = {}
+
+    def __contains__(self, block_hash: int) -> bool:
+        return block_hash in self.free_table
+
+    def evict(self) -> PhysicalTokenBlock:
+        if len(self.free_table) == 0:
+            raise ValueError("No usable cache memory left")
+        evicted_block = next(iter(self.free_table.values()))
+        evicted_block.computed = False
+        del self.free_table[evicted_block.block_hash]
+        return evicted_block
+
+    def add(self, block: PhysicalTokenBlock):
+        self.free_table[block.block_hash] = block
+
+    def remove(self, block_hash: int) -> PhysicalTokenBlock:
+        if block_hash not in self.free_table:
+            raise ValueError(
+                "Attempting to remove block that's not in the evictor")
+        block: PhysicalTokenBlock = self.free_table[block_hash]
+        del self.free_table[block_hash]
+        return block
+
+    @property
+    def num_blocks(self) -> int:
+        return len(self.free_table)
+
+
+def make_evictor(eviction_policy: EvictionPolicy) -> Evictor:
+    if eviction_policy == EvictionPolicy.LRU:
+        return LRUEvictor()
+    elif eviction_policy == EvictionPolicy.FIFO:
+        return RandomEvictor()
+    else:
+        raise ValueError(f"Unknown cache eviction policy: {eviction_policy}")

vllm/core/scheduler.py

@@ -10,7 +10,6 @@ from vllm.lora.request import LoRARequest
 from vllm.logger import init_logger
 from vllm.sequence import (Sequence, SequenceData, SequenceGroup,
                            SequenceGroupMetadata, SequenceStatus)
-from vllm.prefix import PrefixPool
 
 logger = init_logger(__name__)
 
@@ -95,10 +94,8 @@ class Scheduler:
             block_size=self.cache_config.block_size,
             num_gpu_blocks=self.cache_config.num_gpu_blocks,
             num_cpu_blocks=self.cache_config.num_cpu_blocks,
-            sliding_window=self.cache_config.sliding_window)
-
-        # Create the prefix pool to cache the prefixes.
-        self.prefix_pool = PrefixPool(self.cache_config.block_size)
+            sliding_window=self.cache_config.sliding_window,
+            enable_caching=self.cache_config.enable_prefix_caching)
 
         # Sequence groups in the WAITING state.
         self.waiting: Deque[SequenceGroup] = deque()
@@ -374,10 +371,12 @@ class Scheduler:
 
             seq_data: Dict[int, SequenceData] = {}
             block_tables: Dict[int, List[int]] = {}
+
             for seq in seq_group.get_seqs(status=SequenceStatus.RUNNING):
                 seq_id = seq.seq_id
                 seq_data[seq_id] = seq.data
                 block_tables[seq_id] = self.block_manager.get_block_table(seq)
+                self.block_manager.access_all_blocks_in_seq(seq, now)
 
             seq_group_metadata = SequenceGroupMetadata(
                 request_id=seq_group.request_id,
@@ -386,7 +385,8 @@ class Scheduler:
                 sampling_params=seq_group.sampling_params,
                 block_tables=block_tables,
                 lora_request=seq_group.lora_request,
-                prefix=seq_group.prefix,
+                computed_block_nums=self.block_manager.
+                get_common_computed_block_ids(seq_group),
                 state=seq_group.state,
             )
             seq_group_metadata_list.append(seq_group_metadata)
@@ -496,3 +496,6 @@ class Scheduler:
         blocks_to_swap_out.update(mapping)
         for seq in seq_group.get_seqs(status=SequenceStatus.RUNNING):
             seq.status = SequenceStatus.SWAPPED
+
+    def mark_blocks_as_computed(self, seq_group: SequenceGroup):
+        self.block_manager.mark_blocks_as_computed(seq_group)

vllm/engine/arg_utils.py

@@ -25,6 +25,7 @@ class EngineArgs:
     tensor_parallel_size: int = 1
     max_parallel_loading_workers: Optional[int] = None
     block_size: int = 16
+    enable_prefix_caching: bool = False
     swap_space: int = 4  # GiB
     gpu_memory_utilization: float = 0.90
     max_num_batched_tokens: Optional[int] = None
@@ -173,6 +174,11 @@ class EngineArgs:
                             default=EngineArgs.block_size,
                             choices=[8, 16, 32, 128],
                             help='token block size')
+
+        parser.add_argument('--enable-prefix-caching',
+                            action='store_true',
+                            help='Enables automatic prefix caching')
+
         parser.add_argument('--seed',
                             type=int,
                             default=EngineArgs.seed,
@@ -293,7 +299,8 @@ class EngineArgs:
         cache_config = CacheConfig(self.block_size,
                                    self.gpu_memory_utilization,
                                    self.swap_space, self.kv_cache_dtype,
-                                   model_config.get_sliding_window())
+                                   model_config.get_sliding_window(),
+                                   self.enable_prefix_caching)
         parallel_config = ParallelConfig(self.pipeline_parallel_size,
                                          self.tensor_parallel_size,
                                          self.worker_use_ray,

vllm/engine/async_llm_engine.py

@@ -225,7 +225,6 @@ class _AsyncLLMEngine(LLMEngine):
         prompt_token_ids: Optional[List[int]] = None,
         arrival_time: Optional[float] = None,
         lora_request: Optional[LoRARequest] = None,
-        prefix_pos: Optional[int] = None,
     ) -> None:
         if lora_request is not None and not self.lora_config:
             raise ValueError(f"Got lora_request {lora_request} but LoRA is "
@@ -245,7 +244,6 @@ class _AsyncLLMEngine(LLMEngine):
             sampling_params=sampling_params,
             arrival_time=arrival_time,
             lora_request=lora_request,
-            prefix_pos=prefix_pos,
         )
 
     async def _run_workers_async(
@@ -422,7 +420,6 @@ class AsyncLLMEngine:
         prompt_token_ids: Optional[List[int]] = None,
         arrival_time: Optional[float] = None,
         lora_request: Optional[LoRARequest] = None,
-        prefix_pos: Optional[int] = None,
     ) -> AsyncStream:
         if self.log_requests:
             shortened_prompt = prompt
@@ -435,7 +432,6 @@ class AsyncLLMEngine:
                                                               max_log_len]
             logger.info(f"Received request {request_id}: "
                         f"prompt: {shortened_prompt!r}, "
-                        f"prefix_pos: {prefix_pos},"
                         f"sampling_params: {sampling_params}, "
                         f"prompt_token_ids: {shortened_token_ids}, "
                         f"lora_request: {lora_request}.")
@@ -472,8 +468,7 @@ class AsyncLLMEngine:
             sampling_params=sampling_params,
             prompt_token_ids=prompt_token_ids,
             arrival_time=arrival_time,
-            lora_request=lora_request,
-            prefix_pos=prefix_pos)
+            lora_request=lora_request)
 
         return stream
 
@@ -484,7 +479,6 @@ class AsyncLLMEngine:
         request_id: str,
         prompt_token_ids: Optional[List[int]] = None,
         lora_request: Optional[LoRARequest] = None,
-        prefix_pos: Optional[int] = None,
     ) -> AsyncIterator[RequestOutput]:
         """Generate outputs for a request.
 
@@ -500,11 +494,6 @@ class AsyncLLMEngine:
             prompt_token_ids: The token IDs of the prompt. If None, we
                 use the tokenizer to convert the prompts to token IDs.
             lora_request: LoRA request to use for generation, if any.
-            prefix_pos: If not None, we use the given position as the prefix
-                position for each prompt. We will cache the prefix's KV
-                cache and reuse it for the next request with the same prefix.
-                This is an experimental feature, and may be replaced with
-                automatic prefix caching in the future.
 
         Yields:
             The output `RequestOutput` objects from the LLMEngine for the
@@ -565,7 +554,6 @@ class AsyncLLMEngine:
                 prompt_token_ids=prompt_token_ids,
                 arrival_time=arrival_time,
                 lora_request=lora_request,
-                prefix_pos=prefix_pos,
             )
 
             async for request_output in stream:

vllm/engine/llm_engine.py

@@ -415,7 +415,6 @@ class LLMEngine:
         prompt_token_ids: Optional[List[int]] = None,
         arrival_time: Optional[float] = None,
         lora_request: Optional[LoRARequest] = None,
-        prefix_pos: Optional[int] = None,
     ) -> None:
         """Add a request to the engine's request pool.
 
@@ -432,11 +431,6 @@ class LLMEngine:
                 use the tokenizer to convert the prompts to token IDs.
             arrival_time: The arrival time of the request. If None, we use
                 the current monotonic time.
-            prefix_pos: If not None, we use the given position as the prefix
-                position for each prompt. We will cache the prefix's KV
-                cache and reuse it for the next request with the same prefix.
-                This is an experimental feature, and may be replaced with
-                automatic prefix caching in the future.
 
         Details:
             - Set arrival_time to the current time if it is None.
@@ -479,18 +473,13 @@ class LLMEngine:
         seq = Sequence(seq_id, prompt, prompt_token_ids, block_size,
                        lora_request)
 
-        # Check whether the input specifies prefix
-        prefix = self.scheduler.prefix_pool.add_or_get_prefix(
-            prompt_token_ids[:prefix_pos], lora_request.lora_int_id
-            if lora_request else 0) if prefix_pos is not None else None
-
         # Defensive copy of SamplingParams, which are used by the sampler,
         # this doesn't deep-copy LogitsProcessor objects
         sampling_params = sampling_params.clone()
 
         # Create the sequence group.
         seq_group = SequenceGroup(request_id, [seq], sampling_params,
-                                  arrival_time, lora_request, prefix)
+                                  arrival_time, lora_request)
 
         # Add the sequence group to the scheduler.
         self.scheduler.add_seq_group(seq_group)
@@ -752,6 +741,13 @@ class LLMEngine:
         now = time.time()
         # Update the scheduled sequence groups with the model outputs.
         scheduled_seq_groups = scheduler_outputs.scheduled_seq_groups
+
+        # If prefix caching is enabled, mark all blocks in the sequence groups
+        # as completed so that future requests don't attempt to recompute them
+        if self.cache_config.enable_prefix_caching:
+            for seq_group in scheduled_seq_groups:
+                self.scheduler.mark_blocks_as_computed(seq_group)
+
         for seq_group, outputs in zip(scheduled_seq_groups, output):
             self._process_sequence_group_outputs(seq_group, outputs)
 
@@ -768,12 +764,6 @@ class LLMEngine:
             request_output = RequestOutput.from_seq_group(seq_group)
             request_outputs.append(request_output)
 
-        # Update prefix state, now all the uncomputed prefixes are computed.
-        for seq_group in scheduled_seq_groups:
-            if (seq_group.prefix is not None and seq_group.prefix.allocated
-                    and not seq_group.prefix.computed):
-                seq_group.prefix.computed = True
-
         # Log stats.
         if self.log_stats:
             self.stat_logger.log(self._get_stats(scheduler_outputs))

vllm/entrypoints/api_server.py

@@ -39,15 +39,11 @@ async def generate(request: Request) -> Response:
     """
     request_dict = await request.json()
     prompt = request_dict.pop("prompt")
-    prefix_pos = request_dict.pop("prefix_pos", None)
     stream = request_dict.pop("stream", False)
     sampling_params = SamplingParams(**request_dict)
     request_id = random_uuid()
 
-    results_generator = engine.generate(prompt,
-                                        sampling_params,
-                                        request_id,
-                                        prefix_pos=prefix_pos)
+    results_generator = engine.generate(prompt, sampling_params, request_id)
 
     # Streaming case
     async def stream_results() -> AsyncGenerator[bytes, None]:

vllm/entrypoints/llm.py

@@ -124,7 +124,6 @@ class LLM:
         prompts: Optional[Union[str, List[str]]] = None,
         sampling_params: Optional[SamplingParams] = None,
         prompt_token_ids: Optional[List[List[int]]] = None,
-        prefix_pos: Optional[Union[int, List[int]]] = None,
         use_tqdm: bool = True,
         lora_request: Optional[LoRARequest] = None,
     ) -> List[RequestOutput]:
@@ -140,11 +139,6 @@ class LLM:
                 None, we use the default sampling parameters.
             prompt_token_ids: A list of token IDs for the prompts. If None, we
                 use the tokenizer to convert the prompts to token IDs.
-            prefix_pos: If not None, we use the given position as the prefix
-                position for each prompt. We will cache the prefix's KV
-                cache and reuse it for the next request with the same prefix.
-                This is an experimental feature, and may be replaced with
-                automatic prefix caching in the future.
             use_tqdm: Whether to use tqdm to display the progress bar.
             lora_request: LoRA request to use for generation, if any.
 
@@ -171,14 +165,12 @@ class LLM:
             prompt_token_ids)
         for i in range(num_requests):
             prompt = prompts[i] if prompts is not None else None
-            prefix_pos_i = prefix_pos[i] if prefix_pos is not None else None
             token_ids = None if prompt_token_ids is None else prompt_token_ids[
                 i]
             self._add_request(prompt,
                               sampling_params,
                               token_ids,
-                              lora_request=lora_request,
-                              prefix_pos=prefix_pos_i)
+                              lora_request=lora_request)
         return self._run_engine(use_tqdm)
 
     def _add_request(
@@ -187,15 +179,13 @@ class LLM:
         sampling_params: SamplingParams,
         prompt_token_ids: Optional[List[int]],
         lora_request: Optional[LoRARequest] = None,
-        prefix_pos: Optional[int] = None,
     ) -> None:
         request_id = str(next(self.request_counter))
         self.llm_engine.add_request(request_id,
                                     prompt,
                                     sampling_params,
                                     prompt_token_ids,
-                                    lora_request=lora_request,
-                                    prefix_pos=prefix_pos)
+                                    lora_request=lora_request)
 
     def _run_engine(self, use_tqdm: bool) -> List[RequestOutput]:
         # Initialize tqdm.

vllm/prefix.py

-from typing import Dict, List, Sequence, Tuple, Optional
-
-from vllm.block import BlockTable
-
-
-class Prefix:
-    """Data and states associated with a prefix of prompt tokens for multiple
-    sequence groups.
-
-    NOTE: This feature is experimental and may be replaced with automatic
-        prefix caching in the future.
-
-    Args:
-        token_ids: The token ids of the prefix.
-        block_size: The block size of the executed model.
-    """
-
-    def __init__(
-        self,
-        token_ids: Sequence[int],
-        block_size: int,
-    ) -> None:
-        self.token_ids = tuple(token_ids)
-        self.block_size = block_size
-        self.length = len(token_ids)
-        self.hash = hash(token_ids)
-        assert self.length % block_size == 0
-        self.block_table: Optional[BlockTable] = None
-        self.computed = False
-
-    @property
-    def allocated(self) -> bool:
-        return self.block_table is not None
-
-    def get_num_blocks(self) -> int:
-        return self.length // self.block_size
-
-    def get_block_numbers(self) -> List[int]:
-        return [block.block_number for block in self.block_table]
-
-    def get_length(self) -> int:
-        return self.length
-
-    def __hash__(self) -> int:
-        return self.hash
-
-    def set_block_table(self, block_table: BlockTable) -> None:
-        self.block_table = block_table.copy()
-
-
-class PrefixPool:
-    """Manages all the prompt prefixes.
-
-    NOTE: This feature is experimental and may be replaced with automatic
-        prefix caching in the future.
-
-    Args:
-        block_size: The block size of the executed model.
-
-    Attributes:
-        prefixes: A list of all the prefixes.
-        block_size: The block size of the executed model.
-    """
-
-    def __init__(
-        self,
-        block_size: int,
-    ) -> None:
-        # TODO(zhuohan): Add a capacity limit to the prefix pool.
-        self.prefixes: Dict[int, Prefix] = {}
-        self.block_size = block_size
-
-    def _truncate_token_ids(self, token_ids: Sequence[int]) -> Tuple[int]:
-        new_length = len(token_ids) // self.block_size * self.block_size
-        return tuple(token_ids[:new_length])
-
-    def add_or_get_prefix(self, token_ids: Sequence[int],
-                          lora_int_id: int) -> Optional[Prefix]:
-        token_ids = self._truncate_token_ids(token_ids)
-        if len(token_ids) == 0:
-            # Prefix is empty.
-            return None
-        prefix = Prefix(token_ids, self.block_size)
-        prefix_hash = hash((prefix, lora_int_id))
-        if prefix_hash not in self.prefixes:
-            self.prefixes[prefix_hash] = prefix
-        return self.prefixes[prefix_hash]

vllm/sequence.py

@@ -5,7 +5,6 @@ from dataclasses import dataclass
 from typing import Dict, List, Optional, Union
 
 from vllm.block import LogicalTokenBlock
-from vllm.prefix import Prefix
 from vllm.sampling_params import SamplingParams
 from vllm.lora.request import LoRARequest
 
@@ -161,6 +160,16 @@ class Sequence:
     def lora_int_id(self) -> int:
         return self.lora_request.lora_int_id if self.lora_request else 0
 
+    # TODO The current hashing function is O(L^2). We should optimize this in
+    # the future.
+    def hash_of_block(self, logical_idx: int) -> int:
+        # Compute the number of tokens in the sequence
+        num_tokens = self.num_hashed_tokens_of_block(logical_idx)
+        return hash(tuple(self.data.get_token_ids()[0:num_tokens]))
+
+    def num_hashed_tokens_of_block(self, logical_idx: int):
+        return logical_idx * self.block_size + self.block_size
+
     def _append_logical_block(self) -> None:
         block = LogicalTokenBlock(
             block_number=len(self.logical_token_blocks),
@@ -265,7 +274,6 @@ class SequenceGroup:
         sampling_params: The sampling parameters used to generate the outputs.
         arrival_time: The arrival time of the request.
         lora_request: LoRA request.
-        prefix: The prefix of the prompt of the sequence group.
     """
 
     def __init__(
@@ -275,7 +283,6 @@ class SequenceGroup:
         sampling_params: SamplingParams,
         arrival_time: float,
         lora_request: Optional[LoRARequest] = None,
-        prefix: Optional[Prefix] = None,
     ) -> None:
         self.request_id = request_id
         self.seqs_dict = {seq.seq_id: seq for seq in seqs}
@@ -286,7 +293,6 @@ class SequenceGroup:
                                       first_token_time=None,
                                       time_in_queue=None)
         self.lora_request = lora_request
-        self.prefix: Optional[Prefix] = prefix
         self.prompt_logprobs: Optional[PromptLogprobs] = None
         self.state = SequenceGroupState()
 
@@ -302,6 +308,10 @@ class SequenceGroup:
         # We use the prompt of an arbitrary sequence.
         return next(iter(self.seqs_dict.values())).data.prompt_token_ids
 
+    @property
+    def block_size(self) -> int:
+        return next(iter(self.seqs_dict.values())).block_size
+
     @property
     def lora_int_id(self) -> int:
         return self.lora_request.lora_int_id if self.lora_request else 0
@@ -408,7 +418,6 @@ class SequenceGroupMetadata:
             numbers)
         state: Internal state tied to this sequence group.
         lora_request: LoRA request.
-        prefix: The prefix of the prompt of the sequence group.
     """
 
     def __init__(
@@ -419,7 +428,7 @@ class SequenceGroupMetadata:
         sampling_params: SamplingParams,
         block_tables: Dict[int, List[int]],
         lora_request: Optional[LoRARequest] = None,
-        prefix: Optional[Prefix] = None,
+        computed_block_nums: Optional[List[int]] = None,
         state: Optional[SequenceGroupState] = None,
     ) -> None:
         self.request_id = request_id
@@ -428,7 +437,7 @@ class SequenceGroupMetadata:
         self.sampling_params = sampling_params
         self.block_tables = block_tables
         self.lora_request = lora_request
-        self.prefix = prefix
+        self.computed_block_nums = computed_block_nums
         self.state = SequenceGroupState() if state is None else state
 
     @property

vllm/worker/model_runner.py

@@ -145,33 +145,37 @@ class ModelRunner:
             prompt_tokens = seq_data.get_token_ids()
             prompt_len = len(prompt_tokens)
             prompt_lens.append(prompt_len)
-            prefix_len = 0
-            prefix = seq_group_metadata.prefix
-            if prefix is not None and prefix.computed:
-                prefix_len = prefix.get_length()
-                prompt_tokens = prompt_tokens[prefix_len:]
-                prefix_block_tables.append(prefix.get_block_numbers())
+            computed_len = 0
+
+            # NOTE: This only works for oooooooxxx style attention.
+            computed_block_nums = seq_group_metadata.computed_block_nums
+            if computed_block_nums is not None and len(
+                    computed_block_nums) > 0 and self.sliding_window is None:
+                # Prefix is not supported with sliding_window
+                computed_len = len(computed_block_nums) * self.block_size
+                prompt_tokens = prompt_tokens[computed_len:]
+                prefix_block_tables.append(computed_block_nums)
             else:
                 prefix_block_tables.append([])
             # actual prompt lens
-            context_lens.append(prefix_len)
-            subquery_lens.append(prompt_len - prefix_len)
+            context_lens.append(computed_len)
+            subquery_lens.append(prompt_len - computed_len)
 
             input_tokens.append(prompt_tokens)
             # NOTE(woosuk): Here we assume that the first token in the prompt
             # is always the first token in the sequence.
             input_positions.append(
-                list(range(prefix_len, prefix_len + len(prompt_tokens))))
+                list(range(computed_len, computed_len + len(prompt_tokens))))
 
             lora_id = seq_group_metadata.lora_int_id
 
             if lora_id > 0:
                 lora_requests.add(seq_group_metadata.lora_request)
 
-            lora_index_mapping.append([lora_id] * (prompt_len - prefix_len))
+            lora_index_mapping.append([lora_id] * (prompt_len - computed_len))
             lora_prompt_mapping.extend(
                 [lora_id] *
-                (prompt_len - prefix_len
+                (prompt_len - computed_len
                  if seq_group_metadata.sampling_params.prompt_logprobs else 1))
 
             if seq_group_metadata.block_tables is None:
@@ -190,11 +194,11 @@ class ModelRunner:
             # mapping will be [-1, -1, 2, 3, 4, 5, 6, 7, 0, 1].
             start_idx = 0
             if self.sliding_window is not None:
-                assert prefix_len == 0, (
+                assert computed_len == 0, (
                     "Prefix caching is currently not supported with "
                     "sliding window attention")
                 start_idx = max(0, prompt_len - self.sliding_window)
-            for i in range(prefix_len, prompt_len):
+            for i in range(computed_len, prompt_len):
                 if i < start_idx:
                     slot_mapping[-1].append(_PAD_SLOT_ID)
                     continue