PP1 probability-path audit — 2026-10-23

Status: CPU source inspection only; no Spark/x86host access, traffic or service changes. The reference experiment is OFF: nothing in serving imports this directory.

What q the current remote verifier uses

q is the point mass at the proposed token d: q(d)=1, q(v≠d)=0. For target T=1 with ordinary stochastic verification, acceptance is p(d), and a rejection samples the full processed target distribution with d removed and renormalized. A confidence score is not q(d).

Evidence at lane base cc9515fdd4e26197ef13b797c06bcf9cc39a085c:

spark/remote_dspark/speculator.py:52–60 refuses non-greedy proposal mode; its capacity speculator sets draft_logits=None (line 272). HELLO declares SAMPLE_GREEDY. wire.py:379–438 carries IDs, confidence and optional margins, with no categorical proposal distribution.
worker/model.py:402–435 applies sequential Markov bias to each head row, then argmax. Confidence is computed separately. The native/TP2 worker implementations retain the same greedy wire contract.
The installed image's GPU runner vllm/v1/worker/gpu/model_runner.py:1711–1718 passes self.speculator.draft_logits to its rejection sampler. gpu/spec_decode/rejection_sampler.py:244–287 applies target sampling parameters before verification. Grammar masks are applied by the runner before this call (around line 1680).
gpu/spec_decode/rejection_sampler_utils.py:630–665 uses a log probability ratio in standard verification. Without draft logits, the draft logprob is zero. Lines 833–887 implement direct target sampling for bonus/placeholders, the positive residual for general q, or target logits with d masked for point-mass q. Target greedy requests instead compare target argmax.

The separate older v1/worker/gpu_model_runner.py caches take_last_draft_probs() by request ID and calls v1/sample/rejection_sampler.py with dense probabilities. That file's missing-request path warns and returns None. It is not the remote overlay's integration path. Reusing that fallback for sampled proposals would silently substitute point-mass q and be incorrect; a future sampled path must refuse missing q or discard the entire proposal and run target-only.

The installed optional local sampled path

gpu/spec_decode/speculator.py:178–191 allocates full-vocabulary cached pre-temperature logits only for draft_sample_method=probabilistic (default greedy in config/speculative.py:601). DSpark _sample_logits applies Markov bias, scatters a reduced draft vocabulary into target IDs if applicable, then uses gumbel_sample. That routine caches the pre-temperature logits and uses separate draft noise. Verification reloads those logits, upcasts to FP32, divides by the sampling temperature and computes global logsumexp. Thus the local sampled q is softmax of those exact cached, temperature-scaled logits, not a confidence estimate or an independently truncated reconstruction. The default head cache dtype comes from model head_dtype.

Standard ratio verification and block verification are distinct algorithms. The image also supports block verification (prefix joint ratios and a scaled residual); PP1 implements standard tokenwise ratio verification only. No claim of block-verifier parity is made.

HeNryous 0055: an existing negative prose experiment

Pinned donor: HeNryous afe4d5e41ec6fa4db17c5cc88758af439a106913, patch 0055. Historical b1726644b89fbb57b6468d33880cddac55a3c527 does not contain 0055; it must not be used as that patch's citation.

UPSTREAM CLAIM (patch preamble, not our measurements): sampled proposals helped some code/JSON cases, but dense probability handling added about 1 ms per round and prose did not improve. The hybrid mode samples requests already in the full-window regime; other requests use argmax and cache one-hot logits (0 at d, −inf elsewhere). It requires probabilistic drafting and local argmax disabled; their test used block verification. The reported aggregate gain was 0.75%, with single-stream prose −1.6% and four-stream JSON −5.1%; it was not adopted. This supports a bounded new experiment, not an enablement decision.

Provenance and limits

MEASURED (local file inspection): image _version.py identifies 0.31.1.dev0+g6e517b15.dsv41gb10. Read-only image snapshot hashes:

File under `vllm/v1/worker/gpu/`	SHA256
`spec_decode/dspark/speculator.py`	f319e29d7b5492f8504d141474ef0bb58d39971249a24749fa8d464a808e8708
`spec_decode/rejection_sampler.py`	25759f98e8aace9a556bc4252c206f246535f0536ebc919f97e097f92a72066a
`spec_decode/rejection_sampler_utils.py`	ef1d40bf31140eb644a8f3b7a003373209d6a8e297f059e70ea66cf4972d8d90

0055 SHA256: 9affd2a869ef344be901ed5fa9b444544ebff48f714b78c930101392467a05f5. No live revision attestation was attempted: this lane forbids Spark access. Coordinator records identify the production K2 TP2 pair and process proxy; that operational record does not make native source-faithful serving qualified. Local refs at inspection: main 21bed313ac803d38327c4de1ff2e5235f329c716, offload-x86host 26ef0a3ab3df9760ad48e73d16523a2ee1316e5e (moving refs).

Algorithm references: Leviathan et al., ICML 2023, algorithm 1 and appendix A.1, vLLM rejection_sampler.py pinned e4db46c79bca354fef07197465faea14ca563c09. The latter HEAD was resolved via git ls-remote on 2026-10-23; the local image above is authoritative for this deployment, not upstream main.