[V1][Spec Decode] Optimize N-gram matching with Numba (#13365)

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

Author: WoosukKwon

requirements-common.txt

@@ -1,6 +1,7 @@
 psutil
 sentencepiece  # Required for LLaMA tokenizer.
 numpy < 2.0.0
+numba == 0.60.0 # v0.61 doesn't support Python 3.9. Required for N-gram speculative decoding.
 requests >= 2.26.0
 tqdm
 blake3

vllm/v1/spec_decode/ngram_proposer.py

@@ -1,14 +1,12 @@
 # SPDX-License-Identifier: Apache-2.0
-from typing import List, Optional
+from typing import Optional
 
 import numpy as np
+from numba import jit
 
 
 class NgramProposer:
 
-    def __init__(self):
-        pass
-
     def propose(
         self,
         context_token_ids: np.ndarray,
@@ -21,7 +19,7 @@ def propose(
         that match.
         
         Args:
-            context_token_ids: List of token IDs representing the 
+            context_token_ids: Numpy array of token IDs representing the 
                                context sequence.
             n: Length of the n-gram to match.
             k: Number of tokens follow the match. If there are less 
@@ -41,66 +39,65 @@ def propose(
               followed that pattern. Here we will return [4,2,3] because 
               we only have three tokens after the match.
         """
-        # TODO: Use c++ to implement the _find_subarray_kmp to
-        # improve the efficiency
-        return self._find_subarray_kmp(context_token_ids, n, k)
+        return _find_subarray_kmp(context_token_ids, n, k)
 
-    @staticmethod
-    def _kmp_lps_array(pattern: List[int]) -> List[int]:
-        """
-        Build the lps (longest proper prefix which is also suffix) 
-        array for the pattern.
-        """
-        lps = [0] * len(pattern)
-        prev_lps = 0  # length of the previous longest prefix suffix
-        i = 1
 
-        while i < len(pattern):
-            if pattern[i] == pattern[prev_lps]:
-                prev_lps += 1
-                lps[i] = prev_lps
-                i += 1
+@jit(nopython=True)
+def _kmp_lps_array(pattern: np.ndarray) -> np.ndarray:
+    """
+    Build the lps (longest proper prefix which is also suffix) 
+    array for the pattern.
+    """
+    lps = np.zeros(len(pattern), dtype=np.int32)
+    prev_lps = 0  # length of the previous longest prefix suffix
+    i = 1
+
+    while i < len(pattern):
+        if pattern[i] == pattern[prev_lps]:
+            prev_lps += 1
+            lps[i] = prev_lps
+            i += 1
+        else:
+            if prev_lps != 0:
+                prev_lps = lps[prev_lps - 1]
             else:
-                if prev_lps != 0:
-                    prev_lps = lps[prev_lps - 1]
-                else:
-                    lps[i] = 0
-                    i += 1
+                lps[i] = 0
+                i += 1
+    return lps
 
-        return lps
 
-    @staticmethod
-    def _find_subarray_kmp(
-        context_token_ids: np.ndarray,
-        n: int,
-        k: int,
-    ) -> Optional[np.ndarray]:
-        context_len = context_token_ids.shape[0]
-        assert n > 0
+@jit(nopython=True)
+def _find_subarray_kmp(
+    context_token_ids: np.ndarray,
+    n: int,
+    k: int,
+) -> Optional[np.ndarray]:
+    context_len = context_token_ids.shape[0]
+    assert n > 0
 
-        pattern = context_token_ids[-n:]
-        # Precompute lps array for Y
-        lps = NgramProposer._kmp_lps_array(pattern)
+    pattern = context_token_ids[-n:]
+    # Precompute lps array for Y
+    lps = _kmp_lps_array(pattern)
 
-        i = 0
-        j = 0
-        # -n because the last n tokens are used as pattern
-        while i < context_len - n:
-            if context_token_ids[i] == pattern[j]:
-                i += 1
-                j += 1
+    i = 0
+    j = 0
+    # -n because the last n tokens are used as pattern
+    while i < context_len - n:
+        if context_token_ids[i] == pattern[j]:
+            i += 1
+            j += 1
 
-                # If we have matched the entire Y
-                if j == n:
-                    # Found pattern in context, gather the next K elements
-                    return context_token_ids[i:i + k]
+            # If we have matched the entire Y
+            if j == n:
+                # Found pattern in context, gather the next K elements
+                return context_token_ids[i:i + k]
+        else:
+            # Mismatch
+            if j != 0:
+                # Use the lps array to avoid re-checking elements
+                j = lps[j - 1]
             else:
-                # Mismatch
-                if j != 0:
-                    # Use the lps array to avoid re-checking elements
-                    j = lps[j - 1]
-                else:
-                    i += 1
+                i += 1
 
-        # Y not found
-        return None
+    # Y not found
+    return None

vllm/v1/worker/gpu_model_runner.py

@@ -120,11 +120,20 @@ def __init__(
         # Set up speculative decoding.
         self.use_spec_decode = False
         if self.speculative_config:
+            self.use_spec_decode = True
+
             # TODO: find a better way to check if we are using ngram.
             assert self.speculative_config.ngram_prompt_lookup_min, \
                     "Currently, only ngram spec decode is supported in V1."
-            self.drafter = NgramProposer()
-            self.use_spec_decode = True
+            if get_pp_group().is_last_rank:
+                self.drafter = NgramProposer()
+                # Trigger Numba JIT compilation for N-gram proposer.
+                # This usually takes less than 1 second.
+                self.drafter.propose(
+                    np.zeros(1024, dtype=np.int32),
+                    self.speculative_config.ngram_prompt_lookup_min,
+                    self.speculative_config.num_speculative_tokens,
+                )
 
         # Request states.
         self.requests: Dict[str, CachedRequestState] = {}