feat(sampler): 增加 reduced topk+topp 采样快速路径以降低全词表 softmax 开销

新增 VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER 开关并补充适用场景说明

在 V1 GPU 输入批预计算 max_top_k/has_any_no_top_k，native sampler 满足条件时走 reduced fast path，异常自动回退

vllm/envs.py

@@ -301,6 +301,7 @@ if TYPE_CHECKING:
     VLLM_REJECT_SAMPLE_OPT: bool = False
     VLLM_USE_MOE_W16A16_TRITON: bool = False
     VLLM_V1_FAST_TOKEN_ID_COPY: bool = False
+    VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER: bool = False
 
 
 def get_default_cache_root():
@@ -1883,6 +1884,19 @@ environment_variables: dict[str, Callable[[], Any]] = {
     "VLLM_V1_FAST_TOKEN_ID_COPY":
         lambda: (os.environ.get("VLLM_V1_FAST_TOKEN_ID_COPY", "False").lower() in
                     ("true", "1")),
+    # If set to 1/True, enable reduced top-k/top-p sampling fast path in the
+    # V1 PyTorch-native sampler path.
+    #
+    # Recommended when both top_k is enabled and top_p < 1.0 (nucleus
+    # sampling). Not recommended for top-k only (top_p == 1.0) due to
+    # potential behavior differences when the k-th logit is tied.
+    "VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER":
+        lambda: (
+            os.environ.get(
+                "VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER", "False"
+            ).lower()
+            in ("true", "1")
+        ),
 }
 
 # --8<-- [end:env-vars-definition]

vllm/v1/sample/metadata.py

@@ -40,5 +40,9 @@ class SamplingMetadata:
     # Loaded logits processors
     logitsprocs: LogitsProcessors
 
+    # Optional host-side summaries for top-k fast paths.
+    max_top_k: int | None = None
+    has_any_no_top_k: bool = False
+
     # Speculative token ids
     spec_token_ids: list[list[int]] | None = None

vllm/v1/sample/ops/topk_topp_sampler.py

@@ -95,12 +95,44 @@ class TopKTopPSampler(nn.Module):
         generators: dict[int, torch.Generator],
         k: torch.Tensor | None,
         p: torch.Tensor | None,
+        *,
+        max_top_k: int | None = None,
+        has_any_no_top_k: bool = False,
     ) -> tuple[torch.Tensor, torch.Tensor | None]:
         """
         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 from only the reduced candidate set.
+        if (
+            self.logprobs_mode not in ("processed_logits", "processed_logprobs")
+            and envs.VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER
+            and k is not None
+            and p 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,
+                    ),
+                    None,
+                )
+            except Exception:
+                # Fall back to the reference implementation for safety.
+                logger.debug_once(
+                    "Reduced top-k/top-p sampler failed; falling back to the "
+                    "reference implementation."
+                )
+
         logits = self.apply_top_k_top_p(logits, k, p)
         logits_to_return = None
         if self.logprobs_mode == "processed_logits":
@@ -332,6 +364,47 @@ 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: torch.Tensor | None,
+    *,
+    max_top_k: int,
+) -> torch.Tensor:
+    """Sample logits from only the top-k candidate set."""
+    vocab_size = logits.shape[-1]
+    # Guard for extreme values that can defeat the purpose of this fast 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 on the reduced candidate set.
+    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 in descending-logit order within the reduced set.
+        cumprob = torch.cumsum(probs, dim=-1)
+        cumprob_prev = cumprob - probs
+        keep_p = cumprob_prev < p.unsqueeze(1)
+        probs = probs * keep_p
+
+    # Sample position in reduced set, then map 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: torch.Tensor | None,

vllm/v1/sample/sampler.py

@@ -5,6 +5,7 @@
 import torch
 import torch.nn as nn
 
+from vllm import envs
 from vllm.config.model import LogprobsMode
 from vllm.utils.platform_utils import is_pin_memory_available
 from vllm.v1.outputs import LogprobsTensors, SamplerOutput
@@ -184,12 +185,29 @@ class Sampler(nn.Module):
             logits = processor.apply(logits)
 
         # Apply top_k and/or top_p.
-        random_sampled, processed_logprobs = self.topk_topp_sampler(
-            logits,
-            sampling_metadata.generators,
-            sampling_metadata.top_k,
-            sampling_metadata.top_p,
-        )
+        if (
+            envs.VLLM_V1_USE_REDUCED_TOPK_TOPP_SAMPLER
+            and sampling_metadata.top_k is not None
+            and sampling_metadata.top_p is not None
+            and sampling_metadata.max_top_k is not None
+            and not sampling_metadata.has_any_no_top_k
+            and self.topk_topp_sampler.forward.__name__ == "forward_native"
+        ):
+            random_sampled, processed_logprobs = self.topk_topp_sampler(
+                logits,
+                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,
+            )
+        else:
+            random_sampled, processed_logprobs = self.topk_topp_sampler(
+                logits,
+                sampling_metadata.generators,
+                sampling_metadata.top_k,
+                sampling_metadata.top_p,
+            )
 
         if greedy_sampled is None:
             return random_sampled, processed_logprobs

vllm/v1/worker/gpu_input_batch.py

@@ -812,6 +812,16 @@ class InputBatch:
 
     def _make_sampling_metadata(self, repeat_counts: Optional[torch.Tensor] = None) -> SamplingMetadata:
         num_reqs = self.num_reqs
+        # Host-side summaries for reduced top-k/top-p sampling.
+        # Compute before copy_slice(top_k), which may rewrite top_k_cpu_tensor
+        # when repeat_counts is provided.
+        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())
+
         if not self.all_greedy:
             temperature = copy_slice(
                 self.temperature_cpu_tensor, self.temperature, 
@@ -889,6 +899,8 @@ 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 get_pooling_params(self) -> list[PoolingParams]: