[V1][Spec Decode] KV cache slots for eagle heads (vllm-project#16370)

Signed-off-by: LiuXiaoxuanPKU <[email protected]>
Signed-off-by: Yang Wang <[email protected]>

Author: LiuXiaoxuanPKU

tests/v1/core/test_kv_cache_utils.py

@@ -7,6 +7,7 @@
 from vllm.multimodal.inputs import MultiModalKwargs, PlaceholderRange
 from vllm.sampling_params import SamplingParams
 from vllm.utils import GiB_bytes, sha256
+from vllm.v1.core.kv_cache_manager import KVCacheManager
 # disable yapf here as it formats differently than isort such that both fail
 # yapf: disable
 from vllm.v1.core.kv_cache_utils import (NONE_HASH, BlockHashType,
@@ -48,6 +49,18 @@ def make_request(request_id,
     )
 
 
+def new_kv_cache_spec(block_size=16,
+                      num_kv_heads=2,
+                      head_size=64,
+                      dtype=torch.float32,
+                      use_mla=False):
+    return FullAttentionSpec(block_size=block_size,
+                             num_kv_heads=num_kv_heads,
+                             head_size=head_size,
+                             dtype=dtype,
+                             use_mla=use_mla)
+
+
 def test_none_hash():
     assert NONE_HASH is not None
     assert isinstance(NONE_HASH, int)
@@ -327,18 +340,6 @@ def stats(requests, queries, hits):
 
 
 def test_unify_kv_cache_configs():
-
-    def new_kv_cache_spec(block_size=16,
-                          num_kv_heads=2,
-                          head_size=64,
-                          dtype=torch.float32,
-                          use_mla=False):
-        return FullAttentionSpec(block_size=block_size,
-                                 num_kv_heads=num_kv_heads,
-                                 head_size=head_size,
-                                 dtype=dtype,
-                                 use_mla=use_mla)
-
     same_kv_cache_config = [
         KVCacheConfig(
             num_blocks=10,
@@ -470,3 +471,64 @@ def test_estimate_max_model_len(model_id, max_model_len,
     estimated_max_len = estimate_max_model_len(vllm_config, kv_cache_spec,
                                                8 * GiB_bytes)
     assert estimated_max_len == want_estimated_max_len
+
+
+def test_allocate_with_lookahead():
+    """Verify that lookahead tokens correctly affect block allocation"""
+    block_size = 4
+    config = KVCacheConfig(
+        num_blocks=10,
+        tensors={
+            "layer1": KVCacheTensor(100),
+        },
+        kv_cache_groups=[
+            KVCacheGroupSpec(["layer1"],
+                             new_kv_cache_spec(block_size=block_size)),
+        ],
+    )
+
+    request = make_request(
+        request_id=0,
+        prompt_token_ids=[],
+        mm_positions=None,
+        mm_hashes=None,
+    )
+
+    # Test case 1: Requires additional lookahead tokens
+    kv_cache_manager = KVCacheManager(kv_cache_config=config,
+                                      max_model_len=100,
+                                      num_preallocate_tokens=0)
+    blocks = kv_cache_manager.allocate_slots(
+        request,
+        num_tokens=3,
+        num_lookahead_tokens=2,  # Total required: 3+2=5 tokens
+    )
+    assert len(blocks) == 2  # ceil(5/4)=2 blocks
+
+    # Test case 2: With precomputed blocks
+    kv_cache_manager = KVCacheManager(kv_cache_config=config,
+                                      max_model_len=100,
+                                      num_preallocate_tokens=4)
+    # num_preallocate_blocks = 4 // 4 - 2 // 4 = 1
+    # required_blocks = ceil((3 + 2) /4) = 2
+    # total_blocks = 1 + 2 = 3
+    blocks = kv_cache_manager.allocate_slots(
+        request,
+        num_tokens=3,
+        num_lookahead_tokens=2,
+    )
+    assert len(blocks) == 3
+
+    # Test case 3: With precomputed blocks
+    # num_preallocate_blocks = 4 // 4 - 4 // 4 = 0
+    # required_blocks = ceil((3 + 4) / 4) = 2
+    # total_blocks = 0 + 2 = 2
+    kv_cache_manager = KVCacheManager(kv_cache_config=config,
+                                      max_model_len=100,
+                                      num_preallocate_tokens=4)
+    blocks = kv_cache_manager.allocate_slots(
+        request,
+        num_tokens=3,
+        num_lookahead_tokens=4,
+    )
+    assert len(blocks) == 2

vllm/v1/core/kv_cache_manager.py

@@ -164,7 +164,8 @@ def allocate_slots(
         self,
         request: Request,
         num_tokens: int,
-        new_computed_blocks: Optional[list[KVCacheBlock]] = None
+        new_computed_blocks: Optional[list[KVCacheBlock]] = None,
+        num_lookahead_tokens: int = 0,
     ) -> Optional[list[KVCacheBlock]]:
         """Add slots for a request with new tokens to append.
 
@@ -174,6 +175,9 @@ def allocate_slots(
                 not include the tokens that have already been computed.
             new_computed_blocks: A list of new computed blocks just hitting the
                 prefix caching.
+            num_lookahead_tokens: The number of speculative tokens to allocate.
+                This is used by spec decode proposers with kv-cache such 
+                as eagle.
 
         Blocks layout:
         -----------------------------------------------------------------------
@@ -211,8 +215,9 @@ def allocate_slots(
         # the new prefix caching hits
         num_computed_tokens = (request.num_computed_tokens +
                                len(new_computed_blocks) * self.block_size)
-        num_required_blocks = cdiv(num_computed_tokens + num_tokens,
-                                   self.block_size)
+        num_required_blocks = cdiv(
+            num_computed_tokens + num_tokens + num_lookahead_tokens,
+            self.block_size)
         num_new_blocks = (num_required_blocks - len(req_blocks) -
                           len(new_computed_blocks))
 
@@ -246,8 +251,11 @@ def allocate_slots(
         else:
             # Get new blocks from the free block pool considering
             # preallocated blocks.
+            num_preallocate_blocks = max(
+                0, self.num_preallocate_blocks -
+                num_lookahead_tokens // self.block_size)
             num_new_blocks = min(
-                num_new_blocks + self.num_preallocate_blocks,
+                num_new_blocks + num_preallocate_blocks,
                 self.block_pool.get_num_free_blocks(),
                 # Should not exceed the maximum number of blocks per request.
                 # This is especially because the block table has the shape

vllm/v1/core/sched/scheduler.py

@@ -7,7 +7,8 @@
 from collections.abc import Iterable
 from typing import Optional, Union
 
-from vllm.config import CacheConfig, LoRAConfig, ModelConfig, SchedulerConfig
+from vllm.config import (CacheConfig, LoRAConfig, ModelConfig, SchedulerConfig,
+                         SpeculativeConfig)
 from vllm.logger import init_logger
 from vllm.multimodal import MULTIMODAL_REGISTRY, MultiModalRegistry
 from vllm.v1.core.encoder_cache_manager import (EncoderCacheManager,
@@ -39,6 +40,7 @@ def __init__(
         lora_config: Optional[LoRAConfig],
         kv_cache_config: KVCacheConfig,
         structured_output_manager: StructuredOutputManager,
+        speculative_config: SpeculativeConfig = None,
         mm_registry: MultiModalRegistry = MULTIMODAL_REGISTRY,
         include_finished_set: bool = False,
         log_stats: bool = False,
@@ -112,6 +114,11 @@ def __init__(
         self.encoder_cache_manager = EncoderCacheManager(
             cache_size=encoder_cache_size)
 
+        self.num_lookahead_tokens = 0
+        if speculative_config and speculative_config.method == "eagle":
+            self.num_lookahead_tokens = \
+                speculative_config.num_speculative_tokens
+
     def schedule(self) -> SchedulerOutput:
         # NOTE(woosuk) on the scheduling algorithm:
         # There's no "decoding phase" nor "prefill phase" in the scheduler.
@@ -188,7 +195,9 @@ def schedule(self) -> SchedulerOutput:
 
             while True:
                 new_blocks = self.kv_cache_manager.allocate_slots(
-                    request, num_new_tokens)
+                    request,
+                    num_new_tokens,
+                    num_lookahead_tokens=self.num_lookahead_tokens)
                 if new_blocks is None:
                     # The request cannot be scheduled.
                     # Preempt the lowest-priority request.

vllm/v1/engine/core.py

@@ -98,6 +98,7 @@ def __init__(
             cache_config=vllm_config.cache_config,
             lora_config=vllm_config.lora_config,
             kv_cache_config=kv_cache_config,
+            speculative_config=vllm_config.speculative_config,
             structured_output_manager=self.structured_output_manager,
             include_finished_set=vllm_config.parallel_config.data_parallel_size
             > 1,