Aligning `top_p` and `top_k` Sampling (#1885)

* Align top_p and top_k with huggingface

* remove _get_prompt_and_output_tokens

* rename _apply_top_p_top_k

* compare top_p top_k with hf

* fix test errors

tests/samplers/test_sampler.py

@@ -4,6 +4,7 @@ from unittest.mock import patch
 
 import pytest
 import torch
+from transformers import GenerationConfig, GenerationMixin
 
 from vllm.model_executor.layers.sampler import Sampler
 from vllm.model_executor.utils import set_random_seed
@@ -233,3 +234,65 @@ def test_sampler_logits_processors(seed: int):
     for _, sequence_output in enumerate(sampler_output):
         for idx, nth_output in enumerate(sequence_output.samples):
             assert nth_output.output_token == idx
+
+
+@pytest.mark.parametrize("seed", RANDOM_SEEDS)
+def test_sampler_top_k_top_p(seed: int):
+    set_random_seed(seed)
+    batch_size = random.randint(1, 256)
+    top_k = random.randint(100, 500)
+    top_p = random.random() * 0.1
+    vocab_size = 32000
+    input_tensor = torch.rand((batch_size, 1024),
+                              device="cuda",
+                              dtype=torch.float16)
+    fake_logits = torch.normal(0,
+                               5,
+                               size=(batch_size, vocab_size),
+                               device=input_tensor.device,
+                               dtype=input_tensor.dtype)
+    sampler = MockLogitsSampler(32000, fake_logits)
+    model_runner = ModelRunner(None, None, None)
+
+    generation_model = GenerationMixin()
+    generation_config = GenerationConfig(top_k=top_k,
+                                         top_p=top_p,
+                                         do_sample=True)
+    warpers = generation_model._get_logits_warper(generation_config)
+    assert len(warpers) == 2  # top_p and top_k
+
+    seq_group_metadata_list = []
+    prompt_lens = []
+    for i in range(batch_size):
+        seq_group_metadata_list.append(
+            SequenceGroupMetadata(
+                request_id=f"test_{i}",
+                is_prompt=True,
+                seq_data={0: SequenceData([1, 2, 3])},
+                sampling_params=SamplingParams(
+                    temperature=1,
+                    top_k=top_k,
+                    top_p=top_p,
+                ),
+                block_tables={0: [1]},
+            ))
+        prompt_lens.append(seq_group_metadata_list[-1].seq_data[0].get_len())
+
+    sampling_metadata = model_runner._prepare_sample(seq_group_metadata_list,
+                                                     prompt_lens)
+
+    sample_probs = None
+
+    def mock_sample(probs, logprobs, sampling_metadata):
+        nonlocal sample_probs
+        sample_probs = probs
+        return [[prob.topk(1, dim=-1).indices.tolist(), [0]] for prob in probs]
+
+    with patch("vllm.model_executor.layers.sampler._sample", mock_sample):
+        sampler(embedding=None,
+                hidden_states=input_tensor,
+                sampling_metadata=sampling_metadata)
+    hf_probs = warpers(torch.zeros_like(fake_logits), fake_logits.clone())
+    hf_probs = torch.softmax(hf_probs, dim=-1, dtype=torch.float)
+    assert torch.allclose(hf_probs, sample_probs, atol=1e-5)
+    assert torch.equal(hf_probs.eq(0), sample_probs.eq(0))

vllm/model_executor/layers/sampler.py

@@ -76,7 +76,7 @@ class Sampler(nn.Module):
         logits.div_(sampling_tensors.temperatures.unsqueeze_(dim=1))
 
         if do_top_p_top_k:
-            logits = _apply_top_p_top_k(logits, sampling_tensors.top_ps,
+            logits = _apply_top_k_top_p(logits, sampling_tensors.top_ps,
                                         sampling_tensors.top_ks)
 
         if do_min_p:
@@ -185,27 +185,27 @@ def _apply_penalties(logits: torch.Tensor, prompt_tokens_tensor: torch.Tensor,
     return logits
 
 
-def _apply_top_p_top_k(
+def _apply_top_k_top_p(
     logits: torch.Tensor,
     p: torch.Tensor,
     k: torch.Tensor,
 ) -> torch.Tensor:
-    logits_sort, logits_idx = logits.sort(dim=-1, descending=True)
+    logits_sort, logits_idx = logits.sort(dim=-1, descending=False)
+
+    # Apply top-k.
+    top_k_mask = logits_sort.size(1) - k.to(torch.long)
+    # Get all the top_k values.
+    top_k_mask = logits_sort.gather(1, top_k_mask.unsqueeze(dim=1))
+    top_k_mask = logits_sort < top_k_mask
+    logits_sort.masked_fill_(top_k_mask, -float("inf"))
 
     # Apply top-p.
     probs_sort = logits_sort.softmax(dim=-1)
-    probs_sum = probs_sort.cumsum(dim=-1).sub_(probs_sort)
-    top_p_mask = probs_sum > p.unsqueeze_(dim=1)
-
-    # Apply top-k.
-    # Create a mask for the top-k elements.
-    top_k_mask = torch.arange(logits_idx.shape[-1], device=logits_idx.device)
-    top_k_mask = top_k_mask.expand(logits_idx.shape[0], -1)
-    top_k_mask = top_k_mask >= k.unsqueeze_(dim=1)
-
-    # Final mask.
-    mask = (top_p_mask | top_k_mask)
-    logits_sort.masked_fill_(mask, -float("inf"))
+    probs_sum = probs_sort.cumsum(dim=-1)
+    top_p_mask = probs_sum <= 1 - p.unsqueeze(dim=1)
+    # at least one
+    top_p_mask[:, -1] = False
+    logits_sort.masked_fill_(top_p_mask, -float("inf"))
 
     # Re-sort the probabilities.
     src = torch.arange(logits_idx.shape[-1],