Support Min P Sampler (#1642)

Author: esmeetu

vllm/model_executor/layers/sampler.py

@@ -71,13 +71,18 @@ def forward(
             logits.div_(t.unsqueeze(dim=1))
 
         # Apply top-p and top-k truncation.
-        top_ps, top_ks = _get_top_p_top_k(input_metadata, self.vocab_size)
+        top_ps, top_ks, min_ps = _get_top_p_top_k_min_p(
+            input_metadata, self.vocab_size)
         assert len(top_ps) == len(top_ks) == logits.shape[0]
         do_top_p = any(p < 1.0 - _SAMPLING_EPS for p in top_ps)
         do_top_k = any(k != self.vocab_size for k in top_ks)
         if do_top_p or do_top_k:
             logits = _apply_top_p_top_k(logits, top_ps, top_ks)
 
+        do_min_p = any(mp > _SAMPLING_EPS for mp in min_ps)
+        if do_min_p:
+            logits = _apply_min_p(logits, min_ps)
+
         # We use float32 for probabilities and log probabilities.
         # Compute the probabilities.
         probs = torch.softmax(logits, dim=-1, dtype=torch.float)
@@ -261,15 +266,17 @@ def _get_temperatures(input_metadata: InputMetadata) -> List[float]:
     return temperatures
 
 
-def _get_top_p_top_k(
+def _get_top_p_top_k_min_p(
     input_metadata: InputMetadata,
     vocab_size: int,
-) -> Tuple[List[float], List[int]]:
+) -> Tuple[List[float], List[int], List[float]]:
     top_ps: List[float] = []
     top_ks: List[int] = []
+    min_ps: List[float] = []
     for i, seq_group in enumerate(input_metadata.seq_groups):
         seq_ids, sampling_params = seq_group
         top_p = sampling_params.top_p
+        min_p = sampling_params.min_p
         # k should not be greater than the vocab size.
         top_k = min(sampling_params.top_k, vocab_size)
         # k=-1 means no truncation.
@@ -279,9 +286,11 @@ def _get_top_p_top_k(
             prompt_len = input_metadata.prompt_lens[i]
             top_ps += [top_p] * (prompt_len - 1)
             top_ks += [top_k] * (prompt_len - 1)
+            min_ps += [min_p] * (prompt_len - 1)
         top_ps += [top_p] * len(seq_ids)
         top_ks += [top_k] * len(seq_ids)
-    return top_ps, top_ks
+        min_ps += [min_p] * len(seq_ids)
+    return top_ps, top_ks, min_ps
 
 
 def _apply_top_p_top_k(
@@ -313,6 +322,24 @@ def _apply_top_p_top_k(
     return logits
 
 
+def _apply_min_p(
+    logits: torch.Tensor,
+    min_ps: List[float],
+) -> torch.Tensor:
+    """
+    Adapted from
+    https://github.com/oobabooga/text-generation-webui/blob/3146124ec01f02c8fb1650a6517cf1b60b537aaf/modules/sampler_hijack.py#L16C17-L16C17
+    """
+    min_p = torch.tensor(min_ps, dtype=logits.dtype, device=logits.device)
+    probs = torch.softmax(logits, dim=-1)
+    top_probs, _ = probs.max(dim=-1, keepdim=True)
+    scaled_min_p = min_p.unsqueeze(dim=1) * top_probs
+    tokens_to_remove = probs < scaled_min_p
+    logits = logits.masked_fill(tokens_to_remove, -float("inf"))
+
+    return logits
+
+
 def _greedy_sample(
     selected_seq_groups: List[Tuple[List[int], SamplingParams]],
     logprobs: torch.Tensor,

vllm/sampling_params.py

@@ -52,6 +52,9 @@ class SamplingParams:
             to consider. Must be in (0, 1]. Set to 1 to consider all tokens.
         top_k: Integer that controls the number of top tokens to consider. Set
             to -1 to consider all tokens.
+        min_p: Float that represents the minimum probability for a token to be
+            considered, relative to the probability of the most likely token.
+            Must be in [0, 1]. Set to 0 to disable this.
         use_beam_search: Whether to use beam search instead of sampling.
         length_penalty: Float that penalizes sequences based on their length.
             Used in beam search.
@@ -94,6 +97,7 @@ def __init__(
         temperature: float = 1.0,
         top_p: float = 1.0,
         top_k: int = -1,
+        min_p: int = 0.0,
         use_beam_search: bool = False,
         length_penalty: float = 1.0,
         early_stopping: Union[bool, str] = False,
@@ -115,6 +119,7 @@ def __init__(
         self.temperature = temperature
         self.top_p = top_p
         self.top_k = top_k
+        self.min_p = min_p
         self.use_beam_search = use_beam_search
         self.length_penalty = length_penalty
         self.early_stopping = early_stopping
@@ -167,6 +172,9 @@ def _verify_args(self) -> None:
         if self.top_k < -1 or self.top_k == 0:
             raise ValueError(f"top_k must be -1 (disable), or at least 1, "
                              f"got {self.top_k}.")
+        if not 0.0 <= self.min_p <= 1.0:
+            raise ValueError("min_p must be in [0, 1], got "
+                             f"{self.min_p}.")
         if self.max_tokens < 1:
             raise ValueError(
                 f"max_tokens must be at least 1, got {self.max_tokens}.")
@@ -228,6 +236,7 @@ def __repr__(self) -> str:
                 f"temperature={self.temperature}, "
                 f"top_p={self.top_p}, "
                 f"top_k={self.top_k}, "
+                f"min_p={self.min_p}, "
                 f"use_beam_search={self.use_beam_search}, "
                 f"length_penalty={self.length_penalty}, "
                 f"early_stopping={self.early_stopping}, "