V1 采样器：新增 reduced top-k/top-p 采样路径

新增环境变量 VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER 用于开关控制
扩展 SamplingMetadata，增加 max_top_k 与 has_any_no_top_k
在 InputBatch 侧计算 top-k 的主机端汇总信息，避免 device 同步
更新 Sampler/TopKTopPSampler 传递并使用新参数以启用优化采样

vllm/envs.py

@@ -196,6 +196,7 @@ if TYPE_CHECKING:
     VLLM_USE_LIGHTOP_RMS_ROPE_CONCAT: bool = False
     VLLM_USE_FUSED_RMS_ROPE: bool = False
     VLLM_USE_MARLIN_W16A16_MOE:bool = False
+    VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER: bool = False
     VLLM_USE_FUSED_FILL_RMS_CAT:bool = False
 
 def get_default_cache_root():
@@ -1280,6 +1281,11 @@ environment_variables: dict[str, Callable[[], Any]] = {
     "VLLM_USE_FUSED_FILL_RMS_CAT":
         lambda: (os.environ.get("VLLM_USE_FUSED_FILL_RMS_CAT", "False").lower() in
                  ("true", "1")),  
+    # If set to 1/True, enable the reduced top-k/top-p sampling path in the
+    # V1 PyTorch-native sampler.
+    "VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER":
+        lambda: (os.getenv("VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER",
+                           "0").lower() in ("true", "1")),
 }
 
 # --8<-- [end:env-vars-definition]

vllm/v1/sample/metadata.py

@@ -41,3 +41,10 @@ class SamplingMetadata:
 
     # Loaded logits processors
     logitsprocs: LogitsProcessorManager
+
+    # Optional host-side summaries to avoid device sync in fast paths.
+    # When `top_k` is provided, `max_top_k` is the maximum top-k value across
+    # the batch on the host (Python int).
+    max_top_k: Optional[int] = None
+    # True if any request in the batch has top_k == vocab_size (i.e. no top-k).
+    has_any_no_top_k: bool = False

vllm/v1/sample/ops/topk_topp_sampler.py

@@ -72,12 +72,34 @@ class TopKTopPSampler(nn.Module):
         generators: dict[int, torch.Generator],
         k: Optional[torch.Tensor],
         p: Optional[torch.Tensor],
+        *,
+        max_top_k: Optional[int] = None,
+        has_any_no_top_k: bool = False,
     ) -> torch.Tensor:
         """
         PyTorch-native implementation of top-k and top-p sampling.
 
         The logits tensor may be updated in-place.
         """
+        # Fast path: when top-k is enabled, avoid full-vocab sort/softmax by
+        # sampling only from the top-k candidates (and applying top-p within
+        # that set). This is especially important on ROCm where the PyTorch
+        # native sort path can be very expensive.
+        #
+        # NOTE: Do not branch on device tensors here; doing so triggers
+        # `aten::is_nonzero` and synchronizes the CPU with GPU.
+        if (envs.VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER and k is not None
+                and max_top_k is not None and not has_any_no_top_k
+                and max_top_k <= 4096):
+            try:
+                return sample_top_k_top_p_reduced(logits,
+                                                  generators,
+                                                  k,
+                                                  p,
+                                                  max_top_k=max_top_k)
+            except Exception:
+                # Fall back to the reference implementation for safety.
+                pass
         logits = apply_top_k_top_p(logits, k, p)
         probs = logits.softmax(dim=-1, dtype=torch.float32)
         return random_sample(probs, generators)
@@ -88,6 +110,9 @@ class TopKTopPSampler(nn.Module):
         generators: dict[int, torch.Generator],
         k: Optional[torch.Tensor],
         p: Optional[torch.Tensor],
+        *,
+        max_top_k: Optional[int] = None,
+        has_any_no_top_k: bool = False,
     ) -> torch.Tensor:
         """More optimized implementation for top-k and top-p sampling."""
         if k is None and p is None:
@@ -100,7 +125,12 @@ class TopKTopPSampler(nn.Module):
             logger.warning("FlashInfer 0.2.3+ does not support "
                            "per-request generators. Falling back to "
                            "PyTorch-native implementation.")
-            return self.forward_native(logits, generators, k, p)
+            return self.forward_native(logits,
+                                       generators,
+                                       k,
+                                       p,
+                                       max_top_k=max_top_k,
+                                       has_any_no_top_k=has_any_no_top_k)
         # flashinfer sampling functions expect contiguous logits.
         # In flex_attn/triton_attn fp32 inference, logits can be non-contiguous
         # because of slicing operation in logits_processor.
@@ -257,6 +287,56 @@ def random_sample(
     return probs.div_(q).argmax(dim=-1).view(-1)
 
 
+def sample_top_k_top_p_reduced(
+    logits: torch.Tensor,
+    generators: dict[int, torch.Generator],
+    k: torch.Tensor,
+    p: Optional[torch.Tensor],
+    *,
+    max_top_k: int,
+) -> torch.Tensor:
+    """Sample from logits using only the top-k candidates.
+
+    This avoids full-vocab sorting and full-vocab softmax/exponential kernels.
+    Semantics match applying top-k then top-p (if provided) and sampling from
+    the resulting distribution.
+    """
+    vocab_size = logits.shape[-1]
+    # Cap for safety; very large top-k values may be expensive or defeat the
+    # purpose of the reduced path.
+    if max_top_k <= 0 or max_top_k >= vocab_size:
+        masked_logits = apply_top_k_top_p(logits, k, p)
+        probs = masked_logits.softmax(dim=-1, dtype=torch.float32)
+        return random_sample(probs, generators)
+
+    topk = logits.topk(max_top_k, dim=-1)
+    topk_logits = topk.values
+    topk_indices = topk.indices
+
+    # Apply per-row top-k (some rows may have smaller k).
+    # topk_logits is sorted descending by default.
+    k = k.to(torch.long)
+    arange_k = torch.arange(max_top_k, device=logits.device).unsqueeze(0)
+    keep_k = arange_k < k.unsqueeze(1)
+    topk_logits = topk_logits.masked_fill(~keep_k, -float("inf"))
+
+    # Convert to probabilities over the reduced candidate set.
+    probs = topk_logits.softmax(dim=-1, dtype=torch.float32)
+
+    if p is not None:
+        # Apply top-p within the reduced set. Since candidates are already
+        # sorted by descending logit, we can do cumulative top-p on this order.
+        # Keep tokens until cumprob exceeds p, inclusive of the boundary token.
+        cumprob = torch.cumsum(probs, dim=-1)
+        cumprob_prev = cumprob - probs
+        keep_p = cumprob_prev <= p.unsqueeze(1)
+        probs = probs * keep_p
+
+    # Sample a position within the reduced set and map it back to vocab ids.
+    pos = random_sample(probs, generators)
+    return topk_indices.gather(1, pos.unsqueeze(1)).squeeze(1)
+
+
 def flashinfer_sample(
     logits: torch.Tensor,
     k: Optional[torch.Tensor],

vllm/v1/sample/sampler.py

@@ -123,6 +123,8 @@ class Sampler(nn.Module):
             sampling_metadata.generators,
             sampling_metadata.top_k,
             sampling_metadata.top_p,
+            max_top_k=sampling_metadata.max_top_k,
+            has_any_no_top_k=sampling_metadata.has_any_no_top_k,
         )
 
         if greedy_sampled is None:

vllm/v1/worker/gpu_input_batch.py

@@ -660,6 +660,15 @@ class InputBatch:
                        self.allowed_token_ids_mask, num_reqs)
             allowed_token_ids_mask = self.allowed_token_ids_mask[:num_reqs]
 
+        # Host-side summaries to avoid device synchronization in sampling
+        # fast paths (e.g. reduced top-k/top-p sampling).
+        max_top_k = None
+        has_any_no_top_k = False
+        if not self.no_top_k and num_reqs > 0:
+            top_k_cpu = self.top_k_cpu[:num_reqs]
+            max_top_k = int(top_k_cpu.max())
+            has_any_no_top_k = bool((top_k_cpu == self.vocab_size).any())
+
         return SamplingMetadata(
             temperature=temperature,
             all_greedy=self.all_greedy,
@@ -677,6 +686,8 @@ class InputBatch:
             allowed_token_ids_mask=allowed_token_ids_mask,
             bad_words_token_ids=self.bad_words_token_ids,
             logitsprocs=self.logitsprocs,
+            max_top_k=max_top_k,
+            has_any_no_top_k=has_any_no_top_k,
         )
 
     def _make_sampling_metadata_expanded(
@@ -751,6 +762,13 @@ class InputBatch:
                     expanded_generators[row_idx] = generator
                 row_idx += 1
 
+        max_top_k = None
+        has_any_no_top_k = False
+        if not self.no_top_k and num_reqs > 0:
+            top_k_cpu = self.top_k_cpu[:num_reqs]
+            max_top_k = int(top_k_cpu.max())
+            has_any_no_top_k = bool((top_k_cpu == self.vocab_size).any())
+
         return SamplingMetadata(
             temperature=_expand_cpu_to_gpu(
                 None if all_greedy else self.temperature_cpu_tensor),
@@ -776,6 +794,8 @@ class InputBatch:
                 allowed_token_ids_mask, dtype=torch.bool),
             bad_words_token_ids=expanded_bad_words_token_ids,
             logitsprocs=self.logitsprocs,
+            max_top_k=max_top_k,
+            has_any_no_top_k=has_any_no_top_k,
         )
 
     @property