[Feat] Unified Synthetic Acceptance Rate for V1 and V2 (#40662)

Signed-off-by: Benjamin Chislett <bchislett@nvidia.com>
Signed-off-by: Benjamin Chislett <chislett.ben@gmail.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

tests/v1/e2e/spec_decode/test_spec_decode.py

@@ -1310,6 +1310,54 @@ def test_dflash_acceptance_rates(dflash_config):
     cleanup_dist_env_and_memory()
 
 
+@single_gpu_only
+def test_synthetic_acceptance_rate():
+    """Verify that synthetic rejection sampling produces an acceptance
+    length close to the requested mean acceptance length."""
+    num_spec_tokens = 3
+    expected_acceptance_len = 1.875
+    tolerance = 0.15
+
+    spec_llm = LLM(
+        model="meta-llama/Llama-3.2-1B-Instruct",
+        trust_remote_code=True,
+        speculative_config={
+            "method": "eagle3",
+            "model": "nm-testing/Llama3_2_1B_speculator.eagle3",
+            "num_speculative_tokens": num_spec_tokens,
+            "max_model_len": 2048,
+            "rejection_sample_method": "synthetic",
+            "synthetic_acceptance_length": expected_acceptance_len,
+        },
+        max_model_len=2048,
+        enforce_eager=True,
+        disable_log_stats=False,
+    )
+
+    test_prompts = get_test_prompts(mm_enabled=False, num_prompts=50)
+    spec_llm.chat(
+        test_prompts,
+        SamplingParams(temperature=0, max_tokens=64, ignore_eos=True),
+    )
+
+    metrics = spec_llm.get_metrics()
+    acceptance_len = compute_acceptance_len(metrics)
+
+    print(
+        f"Synthetic acceptance length: {acceptance_len:.3f}"
+        f" (expected={expected_acceptance_len:.3f},"
+        f" tolerance=±{tolerance})"
+    )
+    assert abs(acceptance_len - expected_acceptance_len) <= tolerance, (
+        f"Synthetic acceptance length {acceptance_len:.3f} is not within"
+        f" ±{tolerance} of expected {expected_acceptance_len:.3f}"
+    )
+
+    del spec_llm
+    torch.accelerator.empty_cache()
+    cleanup_dist_env_and_memory()
+
+
 def test_dflash_correctness(dflash_config):
     """
     E2E test for DFlash (block diffusion) speculative decoding.

tests/v1/sample/test_rejection_sampler.py

@@ -933,3 +933,64 @@ def test_sample_recovered_tokens(
         device=DEVICE_TYPE,
     )
     assert torch.equal(recovered_token_ids, ref_recovered_token_ids)
+
+
+########################### Tests for Synthetic Rejection Sampling #########
+
+
+def _make_synthetic_sampler(rates: list[float]) -> RejectionSampler:
+    mock_sampler = Mock(spec=Sampler)
+    mock_sampler.logprobs_mode = "raw_logprobs"
+    spec_config = Mock()
+    spec_config.rejection_sample_method = "synthetic"
+    spec_config.synthetic_acceptance_rates = rates
+    return RejectionSampler(mock_sampler, spec_config, torch.device(DEVICE_TYPE))
+
+
+def _make_sampling_metadata(all_greedy: bool) -> SamplingMetadata:
+    temperature = None if all_greedy else torch.tensor([1.0, 1.0], device=DEVICE_TYPE)
+    return create_sampling_metadata(all_greedy=all_greedy, temperature=temperature)
+
+
+@pytest.mark.parametrize("all_greedy", [True, False])
+def test_synthetic_all_accepted(all_greedy: bool):
+    """With all rates=1.0, every draft token is accepted."""
+    sampler = _make_synthetic_sampler([1.0, 1.0])
+    spec_tokens = [[1, 2], [3]]
+    output_tokens = [[10, 20, 50], [30, 40]]
+
+    metadata = _make_sampling_metadata(all_greedy)
+    logits = create_logits_tensor(output_tokens)
+    bonus = torch.tensor([50, 40], device=DEVICE_TYPE)
+    spec_decode_metadata = create_spec_decode_metadata(spec_tokens, logits)
+
+    mock_sampler_output(sampler, bonus)
+    output = sampler(spec_decode_metadata, None, logits, metadata)
+    expected = torch.tensor(
+        [[1, 2, 50], [3, 40, PLACEHOLDER_TOKEN_ID]],
+        dtype=torch.int,
+        device=DEVICE_TYPE,
+    )
+    assert torch.equal(output.sampled_token_ids, expected)
+
+
+@pytest.mark.parametrize("all_greedy", [True, False])
+def test_synthetic_all_rejected(all_greedy: bool):
+    """With all rates=0.0, the first token is always rejected."""
+    sampler = _make_synthetic_sampler([0.0, 0.0])
+    spec_tokens = [[1, 2], [3]]
+    output_tokens = [[10, 20, 50], [30, 40]]
+
+    metadata = _make_sampling_metadata(all_greedy)
+    logits = create_logits_tensor(output_tokens)
+    bonus = torch.tensor([50, 40], device=DEVICE_TYPE)
+    spec_decode_metadata = create_spec_decode_metadata(spec_tokens, logits)
+
+    mock_sampler_output(sampler, bonus)
+    output = sampler(spec_decode_metadata, None, logits, metadata)
+    result = output.sampled_token_ids
+    # Exactly one token emitted per sequence (the rejection fallback),
+    # followed by placeholders.
+    for row in result:
+        assert row[0] != PLACEHOLDER_TOKEN_ID
+        assert (row[1:] == PLACEHOLDER_TOKEN_ID).all()

tests/v1/spec_decode/test_synthetic_rejection_sampler_utils.py

@@ -2,33 +2,48 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 import pytest
 
-from vllm.v1.worker.gpu.spec_decode.synthetic_rejection_sampler_utils import (
-    compute_synthetic_rejection_sampler_params,
-)
+from vllm.config.speculative import SpeculativeConfig
+from vllm.v1.spec_decode.utils import unconditional_to_conditional_rates
+
+
+def test_unconditional_to_conditional_rates_basic():
+    # c_0 = p_0; c_i = p_i / p_{i-1}
+    assert unconditional_to_conditional_rates([0.9, 0.5, 0.2]) == pytest.approx(
+        [0.9, 0.5 / 0.9, 0.2 / 0.5]
+    )
+
 
-NUM_SPECULATIVE_STEPS = [1, 2, 3, 4, 5, 7, 10]
-ACCEPTANCE_RATES = [i / 100 for i in range(0, 100)]
+def test_unconditional_to_conditional_rates_handles_zero():
+    # After a zero, subsequent conditional rates are clamped to 0 (the chain
+    # has already terminated in the kernel, so these values are unused).
+    assert unconditional_to_conditional_rates([1.0, 0.6, 0.0, 0.0]) == pytest.approx(
+        [1.0, 0.6, 0.0, 0.0]
+    )
 
 
-@pytest.mark.parametrize("num_speculative_steps", NUM_SPECULATIVE_STEPS)
-def test_compute_synthetic_rejection_sampler_params(num_speculative_steps: int):
-    """Test that the base acceptance rate and decay factor generated for
-    synthetic rejection sampling have a mean joint acceptance probability
-    that matches the desired acceptance rate."""
-    tol = 1e-9
-    for desired_acceptance_rate in ACCEPTANCE_RATES:
-        base_rate, decay_factor = compute_synthetic_rejection_sampler_params(
-            desired_acceptance_rate, num_speculative_steps, tol=tol
+def test_unconditional_to_conditional_rates_all_ones():
+    assert unconditional_to_conditional_rates([1.0, 1.0, 1.0]) == pytest.approx(
+        [1.0, 1.0, 1.0]
     )
 
-        # Compute the mean of joint acceptance probabilities across
-        # all speculative positions.
-        joint_prob = 1.0
-        mean_joint = 0.0
-        for i in range(num_speculative_steps):
-            joint_prob *= base_rate * decay_factor**i
-            mean_joint += joint_prob
-        mean_joint /= num_speculative_steps
-
-        assert abs(desired_acceptance_rate - mean_joint) < 10 * tol
-        assert base_rate <= 1.0
+
+@pytest.mark.parametrize(
+    "length,n,expected",
+    [
+        (2.6, 3, [1.0, 0.6, 0.0]),
+        (1.0, 3, [0.0, 0.0, 0.0]),
+        (4.0, 3, [1.0, 1.0, 1.0]),
+        (2.0, 3, [1.0, 0.0, 0.0]),
+        (3.5, 4, [1.0, 1.0, 0.5, 0.0]),
+    ],
+)
+def test_acceptance_length_to_rates(length, n, expected):
+    assert SpeculativeConfig._acceptance_length_to_rates(length, n) == pytest.approx(
+        expected
+    )
+
+
+def test_resolve_length_produces_minvariance_schedule():
+    assert SpeculativeConfig._resolve_synthetic_acceptance_rates(
+        3, None, 2.6
+    ) == pytest.approx([1.0, 0.6, 0.0])

vllm/config/speculative.py

@@ -189,12 +189,64 @@ class SpeculativeConfig:
     distribution, but the latter yields a higher acceptance rate at the cost
     of more memory to cache draft logits."""
 
-    synthetic_acceptance_rate: float | None = None
-    """Average acceptance rate for synthetic rejection sampling. Draft
-    tokens are accepted with a position-dependent probability that decays
-    geometrically, calibrated so that the mean rate across all speculative
-    positions equals this value. Only used when rejection_sample_method
-    is 'synthetic'. Must be in [0, 1]."""
+    synthetic_acceptance_rates: list[float] | None = None
+    """Per-position *unconditional* acceptance rates for synthetic rejection
+    sampling. Position i's entry is the marginal probability that the first
+    i+1 draft tokens are all accepted; the list must have length
+    num_speculative_tokens, each entry in [0, 1], and be monotonically
+    non-increasing. Only valid when rejection_sample_method is 'synthetic'.
+    Mutually exclusive with synthetic_acceptance_length."""
+
+    synthetic_acceptance_length: float | None = None
+    """Target mean acceptance length for synthetic rejection sampling, in
+    [1, num_speculative_tokens + 1]. Resolved internally to
+    synthetic_acceptance_rates. Only valid when rejection_sample_method is 'synthetic'.
+    Mutually exclusive with synthetic_acceptance_rates."""
+
+    @staticmethod
+    def _acceptance_length_to_rates(length: float, n: int) -> list[float]:
+        """Mean acceptance length to unconditional per-position rates, using
+        the minimum-variance schedule."""
+        num_drafts = length - 1  # expected number of accepted draft tokens
+        num_full = int(num_drafts)
+        return (
+            [1.0] * num_full + [num_drafts - num_full] + [0.0] * (n - num_full - 1)
+        )[:n]
+
+    @staticmethod
+    def _resolve_synthetic_acceptance_rates(
+        n: int,
+        rates: list[float] | None,
+        length: float | None,
+    ) -> list[float]:
+        """Return per-position unconditional acceptance rates from exactly one
+        of `rates` or `length` (validates range, length, and monotonicity)."""
+        if (rates is None) == (length is None):
+            raise ValueError(
+                "rejection_sample_method='synthetic' requires exactly one of "
+                "synthetic_acceptance_rates or synthetic_acceptance_length."
+            )
+        if rates is not None:
+            if len(rates) != n:
+                raise ValueError(
+                    f"synthetic_acceptance_rates must have length {n}, got {rates}."
+                )
+            if not all(0.0 <= r <= 1.0 for r in rates):
+                raise ValueError(
+                    f"synthetic_acceptance_rates entries must be in [0, 1], "
+                    f"got {rates}."
+                )
+            if any(rates[i] > rates[i - 1] for i in range(1, n)):
+                raise ValueError(
+                    f"synthetic_acceptance_rates must be non-increasing, got {rates}."
+                )
+            return list(rates)
+        assert length is not None
+        if not 1.0 <= length <= float(n + 1):
+            raise ValueError(
+                f"synthetic_acceptance_length must be in [1, {n + 1}], got {length}."
+            )
+        return SpeculativeConfig._acceptance_length_to_rates(length, n)
 
     def compute_hash(self) -> str:
         """
@@ -818,6 +870,23 @@ class SpeculativeConfig:
                 f"than zero ({self.num_speculative_tokens})."
             )
 
+        if self.rejection_sample_method == "synthetic":
+            # Consolidate to per-position rates
+            self.synthetic_acceptance_rates = self._resolve_synthetic_acceptance_rates(
+                self.num_speculative_tokens,
+                self.synthetic_acceptance_rates,
+                self.synthetic_acceptance_length,
+            )
+            self.synthetic_acceptance_length = None
+        elif (
+            self.synthetic_acceptance_rates is not None
+            or self.synthetic_acceptance_length is not None
+        ):
+            raise ValueError(
+                "synthetic_acceptance_rates / synthetic_acceptance_length "
+                "are only valid with rejection_sample_method='synthetic'."
+            )
+
         if self.draft_model_config:
             self.draft_model_config.verify_with_parallel_config(
                 self.draft_parallel_config

vllm/v1/sample/rejection_sampler.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+from __future__ import annotations
+
 from collections.abc import Sequence
 from dataclasses import replace
+from typing import TYPE_CHECKING
 
 import torch
 import torch.nn as nn
@@ -17,6 +20,10 @@ from vllm.v1.sample.ops.penalties import apply_all_penalties
 from vllm.v1.sample.ops.topk_topp_sampler import apply_top_k_top_p
 from vllm.v1.sample.sampler import Sampler
 from vllm.v1.spec_decode.metadata import SpecDecodeMetadata
+from vllm.v1.spec_decode.utils import unconditional_to_conditional_rates
+
+if TYPE_CHECKING:
+    from vllm.config.speculative import SpeculativeConfig
 
 logger = init_logger(__name__)
 
@@ -50,13 +57,33 @@ class RejectionSampler(nn.Module):
         output tokens = accepted tokens + recovered tokens + bonus tokens
     """
 
-    def __init__(self, sampler: Sampler):
+    def __init__(
+        self,
+        sampler: Sampler,
+        spec_config: SpeculativeConfig | None = None,
+        device: torch.device | None = None,
+    ):
         super().__init__()
         self.sampler = sampler
         logprobs_mode = self.sampler.logprobs_mode
         self.is_processed_logprobs_mode = logprobs_mode.startswith("processed")
         self.is_logits_logprobs_mode = logprobs_mode.endswith("logits")
 
+        self.synthetic_conditional_rates: torch.Tensor | None = None
+        if (
+            spec_config is not None
+            and spec_config.rejection_sample_method == "synthetic"
+        ):
+            assert spec_config.synthetic_acceptance_rates is not None
+            self.synthetic_conditional_rates = torch.tensor(
+                unconditional_to_conditional_rates(
+                    spec_config.synthetic_acceptance_rates
+                ),
+                dtype=torch.float32,
+                device=device,
+            )
+        self.synthetic_mode = self.synthetic_conditional_rates is not None
+
     def forward(
         self,
         metadata: SpecDecodeMetadata,
@@ -147,6 +174,8 @@ class RejectionSampler(nn.Module):
             target_logits,
             bonus_token_ids,
             sampling_metadata,
+            synthetic_mode=self.synthetic_mode,
+            synthetic_conditional_rates=self.synthetic_conditional_rates,
         )
 
         logprobs_tensors = None
@@ -362,6 +391,8 @@ def rejection_sample(
     # [batch_size, 1]
     bonus_token_ids: torch.Tensor,
     sampling_metadata: SamplingMetadata,
+    synthetic_mode: bool = False,
+    synthetic_conditional_rates: torch.Tensor | None = None,
 ) -> torch.Tensor:
     assert draft_token_ids.ndim == 1
     assert draft_probs is None or draft_probs.ndim == 2
@@ -389,6 +420,20 @@ def rejection_sample(
         is_greedy = None
     else:
         is_greedy = sampling_metadata.temperature == GREEDY_TEMPERATURE
+
+    # Generate uniform probabilities before either kernel because synthetic
+    # mode needs them in the greedy kernel too.  Skip only when all requests
+    # are greedy *and* synthetic mode is off (the standard fast-path).
+    # [num_tokens]
+    uniform_probs: torch.Tensor | None = None
+    if synthetic_mode or not sampling_metadata.all_greedy:
+        uniform_probs = generate_uniform_probs(
+            num_tokens,
+            num_draft_tokens,
+            sampling_metadata.generators,
+            device,
+        )
+
     if not sampling_metadata.all_random:
         # Rejection sampling for greedy sampling requests.
         target_argmax = target_logits.argmax(dim=-1)
@@ -400,6 +445,9 @@ def rejection_sample(
             bonus_token_ids,
             is_greedy,
             max_spec_len,
+            uniform_probs,
+            synthetic_conditional_rates,
+            SYNTHETIC_MODE=synthetic_mode,
         )
         if sampling_metadata.all_greedy:
             return output_token_ids
@@ -408,15 +456,6 @@ def rejection_sample(
     target_probs = target_logits.softmax(dim=-1, dtype=torch.float32)
     assert target_probs.is_contiguous()
 
-    # Generate uniform probabilities for rejection sampling.
-    # [num_tokens]
-    uniform_probs = generate_uniform_probs(
-        num_tokens,
-        num_draft_tokens,
-        sampling_metadata.generators,
-        device,
-    )
-
     # Sample recovered tokens for each position.
     # [num_tokens]
     recovered_token_ids = sample_recovered_tokens(
@@ -431,6 +470,7 @@ def rejection_sample(
     )
 
     # Rejection sampling for random sampling requests.
+    assert uniform_probs is not None
     rejection_random_sample_kernel[(batch_size,)](
         output_token_ids,
         cu_num_draft_tokens,
@@ -443,7 +483,9 @@ def rejection_sample(
         is_greedy,
         max_spec_len,
         vocab_size,
+        synthetic_conditional_rates,
         NO_DRAFT_PROBS=draft_probs is None,
+        SYNTHETIC_MODE=synthetic_mode,
     )
     return output_token_ids
 
@@ -658,6 +700,9 @@ def rejection_greedy_sample_kernel(
     bonus_token_ids_ptr,  # [batch_size]
     is_greedy_ptr,  # [batch_size] or None
     max_spec_len,
+    uniform_probs_ptr,  # [num_tokens] or None (synthetic mode only)
+    synthetic_conditional_rates_ptr,  # [num_speculative_tokens] or None
+    SYNTHETIC_MODE: tl.constexpr,
 ):
     req_idx = tl.program_id(0)
     # FIXME(woosuk): Because is_greedy_ptr is not None at profiling run,
@@ -675,14 +720,20 @@ def rejection_greedy_sample_kernel(
     for pos in range(num_draft_tokens):
         if not rejected:
             draft_token_id = tl.load(draft_token_ids_ptr + start_idx + pos)
-            target_argmax_id = tl.load(target_argmax_ptr + start_idx + pos)
+            target_argmax_id = tl.load(target_argmax_ptr + start_idx + pos).to(tl.int32)
+            if SYNTHETIC_MODE:
+                uniform_prob = tl.load(uniform_probs_ptr + start_idx + pos)
+                rate = tl.load(synthetic_conditional_rates_ptr + pos)
+                accepted = uniform_prob < rate
+                token_id = draft_token_id if accepted else target_argmax_id
+                rejected = not accepted
+            else:
+                token_id = target_argmax_id
+                rejected = draft_token_id != target_argmax_id
             tl.store(
                 output_token_ids_ptr + req_idx * (max_spec_len + 1) + pos,
-                target_argmax_id,
+                token_id,
             )
-            if draft_token_id != target_argmax_id:
-                # Reject.
-                rejected = True
 
     if not rejected:
         # If all tokens are accepted, append the bonus token.
@@ -707,7 +758,9 @@ def rejection_random_sample_kernel(
     is_greedy_ptr,  # [batch_size]
     max_spec_len,
     vocab_size,
+    synthetic_conditional_rates_ptr,  # [num_speculative_tokens] or None
     NO_DRAFT_PROBS: tl.constexpr,
+    SYNTHETIC_MODE: tl.constexpr,
 ):
     req_idx = tl.program_id(0)
     is_greedy = tl.load(is_greedy_ptr + req_idx)
@@ -723,23 +776,28 @@ def rejection_random_sample_kernel(
     for pos in range(num_draft_tokens):
         if not rejected:
             draft_token_id = tl.load(draft_token_ids_ptr + start_idx + pos)
+            uniform_prob = tl.load(uniform_probs_ptr + start_idx + pos)
+            if SYNTHETIC_MODE:
+                rate = tl.load(synthetic_conditional_rates_ptr + pos)
+                accepted = uniform_prob < rate
+            else:
                 if NO_DRAFT_PROBS:
                     draft_prob = 1
                 else:
                     draft_prob = tl.load(
-                    draft_probs_ptr + (start_idx + pos) * vocab_size + draft_token_id
+                        draft_probs_ptr
+                        + (start_idx + pos) * vocab_size
+                        + draft_token_id
                     )
                 target_prob = tl.load(
                     target_probs_ptr + (start_idx + pos) * vocab_size + draft_token_id
                 )
-            uniform_prob = tl.load(uniform_probs_ptr + start_idx + pos)
                 # NOTE(woosuk): While the draft probability should never be 0,
                 # we check it to avoid NaNs. If it happens to be 0, we reject.
-            if draft_prob > 0 and target_prob / draft_prob >= uniform_prob:
-                # Accept.
+                accepted = draft_prob > 0 and target_prob / draft_prob >= uniform_prob
+            if accepted:
                 token_id = draft_token_id
             else:
-                # Reject. Use recovered token.
                 rejected = True
                 token_id = tl.load(recovered_token_ids_ptr + start_idx + pos)
             tl.store(

vllm/v1/spec_decode/utils.py

@@ -594,3 +594,9 @@ def update_num_computed_tokens_for_batch_change(
     num_accepted_tokens.copy_(
         torch.where(participating, valid_counts, num_accepted_tokens)
     )
+
+
+def unconditional_to_conditional_rates(rates: list[float]) -> list[float]:
+    """Convert per-position unconditional rates to per-position conditional
+    rates for the early-terminating rejection loop (c_i = p_i / p_{i-1})."""
+    return [p / q if q > 0.0 else 0.0 for p, q in zip(rates, [1.0, *rates[:-1]])]

vllm/v1/worker/gpu/model_runner.py

@@ -220,6 +220,7 @@ class GPUModelRunner(LoRAModelRunnerMixin):
                 self.rejection_sampler = RejectionSampler(
                     self.sampler,
                     self.speculative_config,
+                    self.device,
                 )
             self.prompt_logprobs_worker = PromptLogprobsWorker(self.max_num_reqs)
             self.structured_outputs_worker = StructuredOutputsWorker(

vllm/v1/worker/gpu/spec_decode/rejection_sampler.py

@@ -5,6 +5,7 @@ import torch
 from vllm.config import SpeculativeConfig
 from vllm.triton_utils import tl, triton
 from vllm.v1.outputs import LogprobsTensors
+from vllm.v1.spec_decode.utils import unconditional_to_conditional_rates
 from vllm.v1.worker.gpu.input_batch import InputBatch
 from vllm.v1.worker.gpu.metrics.logits import get_num_nans
 from vllm.v1.worker.gpu.sample.logprob import compute_topk_logprobs
@@ -15,7 +16,6 @@ from vllm.v1.worker.gpu.spec_decode.probabilistic_rejection_sampler_utils import
     probabilistic_rejection_sample,
 )
 from vllm.v1.worker.gpu.spec_decode.synthetic_rejection_sampler_utils import (
-    compute_synthetic_rejection_sampler_params,
     synthetic_rejection_sample,
 )
 
@@ -102,24 +102,20 @@ class RejectionSampler:
         self,
         sampler: Sampler,
         spec_config: SpeculativeConfig,
+        device: torch.device,
     ):
         self.sampler = sampler
         self.num_speculative_steps = spec_config.num_speculative_tokens
         self.rejection_sample_method = spec_config.rejection_sample_method
+        self.synthetic_conditional_rates: torch.Tensor | None = None
         if self.rejection_sample_method == "synthetic":
-            synthetic_acceptance_rate = spec_config.synthetic_acceptance_rate
-            if (
-                synthetic_acceptance_rate is None
-                or not 0.0 <= synthetic_acceptance_rate <= 1.0
-            ):
-                raise ValueError(
-                    f"synthetic_acceptance_rate must be in [0, 1], "
-                    f"but got {synthetic_acceptance_rate}"
-                )
-            self.base_acceptance_rate, self.decay_factor = (
-                compute_synthetic_rejection_sampler_params(
-                    synthetic_acceptance_rate, self.num_speculative_steps
-                )
+            assert spec_config.synthetic_acceptance_rates is not None
+            self.synthetic_conditional_rates = torch.tensor(
+                unconditional_to_conditional_rates(
+                    spec_config.synthetic_acceptance_rates
+                ),
+                dtype=torch.float32,
+                device=device,
             )
 
     def _get_logprobs_tensors(
@@ -218,8 +214,7 @@ class RejectionSampler:
                 input_batch.positions[input_batch.logits_indices],
                 input_batch.idx_mapping,
                 self.sampler.sampling_states.seeds.gpu,
-                self.base_acceptance_rate,
-                self.decay_factor,
+                self.synthetic_conditional_rates,
                 self.num_speculative_steps,
             )
         else:

vllm/v1/worker/gpu/spec_decode/synthetic_rejection_sampler_utils.py

@@ -5,8 +5,6 @@ import torch
 from vllm.triton_utils import tl, triton
 from vllm.v1.worker.gpu.sample.gumbel import tl_rand64
 
-MIN_ACCEPTANCE_DECAY_FACTOR = 0.85
-
 
 @triton.jit
 def _synthetic_rejection_sample_kernel(
@@ -27,8 +25,8 @@ def _synthetic_rejection_sample_kernel(
     idx_mapping_ptr,
     # [max_num_reqs]
     seeds_ptr,
-    base_acceptance_rate,
-    decay_factor,
+    # [num_speculative_steps]
+    acceptance_rates_ptr,
 ):
     req_idx = tl.program_id(0)
     start_idx = tl.load(cu_num_logits_ptr + req_idx)
@@ -38,13 +36,13 @@ def _synthetic_rejection_sample_kernel(
     seed = tl.load(seeds_ptr + req_state_idx)
 
     num_sampled = 0
-    acceptance_rate = base_acceptance_rate
     rejected = False
     for i in range(num_tokens - 1):
         if not rejected:
             logit_idx = start_idx + i
             pos = tl.load(pos_ptr + logit_idx)
             u = tl_rand64(seed, pos, includes_zero=False)
+            acceptance_rate = tl.load(acceptance_rates_ptr + i)
             if u < acceptance_rate:
                 sampled = tl.load(input_ids_ptr + logit_idx + 1).to(tl.int64)
             else:
@@ -52,7 +50,6 @@ def _synthetic_rejection_sample_kernel(
                 rejected = True
             tl.store(sampled_ptr + req_idx * sampled_stride + i, sampled)
             num_sampled += 1
-            acceptance_rate *= decay_factor
     if not rejected:
         target_sampled = tl.load(target_sampled_ptr + start_idx + num_tokens - 1)
         tl.store(
@@ -75,8 +72,8 @@ def synthetic_rejection_sample(
     idx_mapping: torch.Tensor,
     # [max_num_reqs]
     seed: torch.Tensor,
-    base_acceptance_rate: float,
-    decay_factor: float,
+    # [num_speculative_steps]
+    acceptance_rates: torch.Tensor,
     num_speculative_steps: int,
 ) -> tuple[torch.Tensor, torch.Tensor]:
     num_reqs = cu_num_logits.shape[0] - 1
@@ -92,56 +89,7 @@ def synthetic_rejection_sample(
         pos,
         idx_mapping,
         seed,
-        base_acceptance_rate,
-        decay_factor,
+        acceptance_rates,
         num_warps=1,
     )
     return sampled, num_sampled
-
-
-def compute_synthetic_rejection_sampler_params(
-    p_avg: float, n: int, tol: float = 1e-9
-) -> tuple[float, float]:
-    def mean_joint_prob(a_0: float, gamma: float, n: int):
-        total = 0.0
-        for i in range(n):
-            total += a_0 ** (i + 1) * gamma ** (i * (i + 1) // 2)
-        return total / n
-
-    def min_valid_decay_factor(p: float, n: int, tol: float = 1e-9) -> float:
-        low, high = MIN_ACCEPTANCE_DECAY_FACTOR, 1.0
-        if mean_joint_prob(1, low, n) >= p:
-            return low
-
-        # Sweep for a gamma decay factor that is guaranteed
-        # to yield a base acceptance rate <= 1.
-        while (high - low) > tol:
-            mid = (low + high) / 2
-            if mean_joint_prob(1, mid, n) >= p:
-                high = mid
-            else:
-                low = mid
-        return high
-
-    def compute_base_acceptance_rate(
-        p_avg: float, gamma: float, n: int, tol: float = 1e-9
-    ) -> float:
-        if p_avg <= 0.0:
-            return 0.0
-        if p_avg >= 1.0:
-            return 1.0
-
-        # Sweep for a base acceptance rate that yields
-        # the desired mean joint probability.
-        low, high = 0.0, 1.0
-        while (high - low) > tol:
-            mid = (low + high) / 2
-            if mean_joint_prob(mid, gamma, n) >= p_avg:
-                high = mid
-            else:
-                low = mid
-        return high
-
-    decay_factor = min_valid_decay_factor(p_avg, n)
-    base_rate = compute_base_acceptance_rate(p_avg, decay_factor, n)
-    return base_rate, decay_factor

vllm/v1/worker/gpu_model_runner.py

@@ -577,7 +577,9 @@ class GPUModelRunner(
                     "Unknown speculative decoding method: "
                     f"{self.speculative_config.method}"
                 )
-            self.rejection_sampler = RejectionSampler(self.sampler)
+            self.rejection_sampler = RejectionSampler(
+                self.sampler, self.speculative_config, self.device
+            )
 
         self.num_spec_tokens = 0
         self.valid_sampled_token_count_gpu: torch.Tensor | None = None