[Bugfix][Spec Decode] Reach TP consensus on the drafter-pass gate before launching drafter collectives (fixes rank-divergent wedge at max_model_len boundary) - #49027
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…nk-divergent input_fits_in_drafter at the max_model_len boundary)
With MTP speculative decoding under TP>1, each rank independently
evaluates the drafter gate (input_fits_in_drafter-style conditions) in
sample_tokens. Near the served context ceiling (the gate can only flip
at max_seq_len >= max_model_len - spec_tokens + 1) a bit-divergent
rejection count is enough for one rank to run the drafter's collectives
while another skips them; NCCL pairs by issue order, so the group either
blocks forever or runs corrupt-and-continue until it wedges at a shape
boundary. A dummy-run guard exists for the DP case; the TP case is
unguarded. On sm_121 there is no post-launch recovery (ncclCommAbort
does not unwedge under graph replay), so prevention must be pre-launch.
Fix, as deployed and validated on 4x DGX Spark TP=4 (GLM-5.2 + MTP
spec=4): in sample_tokens, BEFORE any drafter collective is launched,
all-gather a small consensus tuple (proposed_pass_count, sched_count,
fits, md_none) over the TP gloo cpu_group (CPU transport: works under
graph replay, never touches NCCL pre-consensus) and adopt MAX — {0,N}
means all ranks run, safe because the drafter clamps out-of-window
positions and masks their KV slots to PADDING_SLOT_ID
(spec_decode/utils.py compute_new_slot_mapping). Degenerate states
(any rank md_none, sched-count split) converge to all-skip. Env-gated
rollout: GLM52_DRAFTER_GATE=off|observe|enforce (default off); upstream
rename to e.g. VLLM_SPEC_GATE_CONSENSUS suggested in the PR description.
This carries the deployed gpu_model_runner.py verbatim, including the
per-step cross-rank fingerprint instrument whose gloo cpu_group the
gate shares, and the RAS evidence-before-exit dump the gate calls on
divergence.
Validation: live capture of the divergence in observe mode (would-pass
totals 123/122/122/122 at seq 199,997-200,000, followed by the wedge);
30 + 500 consecutive enforce ceiling sessions with zero incidents;
90-session mode sweep (observe/enforce/off) clean. Efficacy honestly
marked NOT EXERCISED after the companion sm12x attention-route change
removed the platform's nondeterminism seed — this guard is
defense-in-depth for any future source of per-rank nondeterminism.
Ref: RFC vllm-project#48720 §4/§5. Companion PR: sm12x Triton sparse-MLA route.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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vllm-project#49027 documents a wedge / corrupt-and-continue class where TP ranks disagree on the drafter-pass gate near the max_model_len ceiling and launch mismatched drafter collectives. Traced our V1 data flow: the gate cannot diverge here by construction -- its inputs are scheduler-broadcast CPU state (optimistic all-accepted seq lens), and the rank-local acceptance correction is applied only to the GPU num_computed_tokens buffer, never written back to the CPU tensor. A consensus collective would therefore add cost and, if entered conditionally on rank-local values, introduce the very split it guards against. What CAN rot is the invariant itself, silently, in a future merge. Pin it three ways: an invariant comment at the gate, a rate-limited per-rank WARNING when the gate turns off (only reachable within spec_tokens of the ceiling, so quiet in normal serving; per-rank lines make offline cross-rank comparison possible), and AST-level tests asserting the gate reads no rank-local state and _prepare_inputs never writes the GPU correction back into num_computed_tokens_cpu_tensor. Co-authored-by: Claude <noreply@anthropic.com>
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Summary
With MTP speculative decoding under TP>1, each rank independently evaluates the drafter gate (
input_fits_in_drafter-style conditions) insample_tokens. The decision depends on per-rank state that is only guaranteed identical if per-rank compute is bit-identical. Near the served context ceiling (the gate can only flip atmax_seq_len ≥ max_model_len − spec_tokens + 1; with spec=4 that is ceiling−3), a bit-divergent rejection count is enough to make rank A run the drafter's collectives while rank B skips them. The TP group's collective sequences misalign; NCCL pairs by issue order, so either the next collective blocks forever (shape-unequal pairing) or the group keeps running misaligned until it wedges at a shape boundary. The second regime is not hypothetical — incident A's per-rank launched-op counts (27911/27890/27872/27860, spread 51; Evidence below) are a direct receipt that roughly 50 collectives launched past the divergence point before the wedge, i.e. an observed corrupt-and-continue window. That the shape-equal all-reduces inside that window completed with mixed activations (silent output corruption) is the inference NCCL's issue-order pairing forces; direct activation-level proof was not captured. On sm_121 there is no post-launch recovery (ncclCommAbortdoes not unwedge under graph replay); prevention must be pre-launch.There is an existing dummy-run guard for the DP case; the TP case is unguarded (the divergence site we captured: the MTP MoE final all-reduce, intra-pass op #2).
Evidence
max_position_embeddings≫ servedmax_model_len,effective_drafter_max_model_len= served ceiling, so the gate is only exercisable within spec_tokens of the ceiling — quiet mid-context traffic proves nothing about this bug.Fix (deployed and validated on 4× DGX Spark, TP=4, GLM-5.2 + MTP spec=4)
In
sample_tokens, before any drafter collective is launched, all-gather a small consensus tuple over the TP gloo cpu_group (the same group used by our fingerprint guard; CPU transport so it works under graph replay and never touches NCCL pre-consensus):and adopt MAX across ranks:
vllm/v1/spec_decode/utils.py, functioncompute_new_slot_mapping()(defined at line 241):clamped_positions = torch.clamp(new_positions, max=max_model_len - 1)at line 258 prevents OOB block-table indexing, and positions>= max_model_lenare masked toPADDING_SLOT_ID = -1at lines 266–267 (constant at line 11), so no KV-cache slot is ever written for them. Verified twice: read in the container-extracted deployed tree when the fix was built (2026-07-15), and re-confirmed by a read-only grep inside the running deployment's container (vLLM 0.23.1rc1.dev893+gd3a66aa7e, 2026-07-18 — the line numbers above are from that tree). A rank forced to run a pass it would have skipped therefore produces clamped, discarded proposals — never OOB.md_none(no metadata) or the scheduled-count split disagrees, all ranks skip — degenerate states converge to the safe side.GLM52_DRAFTER_GATE=off|observe|enforce(defaultoff).observejournals per-rank decisions + would-enforce totals without intervening (this mode produced the live capture above);enforceadopts the consensus.Validation (receipts available)
enforceat seq ceiling−3..ceiling: no crash, no OOB, no quality collapse, no false enforcement, engine steps lockstep across ranks. Extended overnight to 500 consecutive enforce ceiling sessions, zero incidents. Gate jurisdiction demonstrably exercised — scoped to where the numbers come from: in each of the 120K-ceiling 30-session rounds (observe, enforce, and off), 20GLM52_DRAFTER skipevents per rank, lockstep-identical across all four ranks; the single 200K-ceiling probe session (2026-07-18) showed 3 skips per rank, again lockstep 4/4, consistent with its session count.Design notes / known caveats
GLM52_*env vars are our deployment's; upstream should rename to a vLLM-appropriate flag (e.g.VLLM_SPEC_GATE_CONSENSUS=off|observe|enforce) — semantics as above.Validation context and base
All validation above was performed on our production deployment, which is built from the jasl fork lineage (vLLM 0.23.1rc1.dev893+gd3a66aa7e). This branch is the port of that change onto upstream main at 3775d5f.
The port is compile-checked but has not been re-executed against the upstream base; happy to rebase onto current main on maintainer request.
AI-assistance disclosure (per AGENTS.md §1 Accountability)
AI assistance was used throughout this contribution. The investigation, evidence collection (live R1 boundary capture, RAS monitoring, 500-session overnight soak), consensus-guard design, and drafting of this PR involved AI agents (Claude, operating as orchestrator and technical writer) under continuous human direction and review by @marksunner. All claims are backed by the linked evidence pack; every receipt cited in this PR description is a real artifact from the investigation.
Not duplicating an existing PR: No open PR addresses the TP-consensus gap in the MTP drafter gate. The existing dummy-run guard covers the DP case; the TP rank-divergence site documented here is unguarded in upstream.
Testing: Validation was performed on a 4× DGX Spark production deployment (GLM-5.2 int4-int8mix, TP=4, MTP spec=4). The branch is ported onto upstream main at 3775d5f and compile-checked; full re-execution results are from the jasl fork lineage deployment as disclosed in "Validation context and base" above. 500 consecutive enforce-mode ceiling sessions with zero incidents, 90-session mode sweep (observe/enforce/off), and a 200K-boundary probe — all clean. No model evaluation regression observed; byte-identical smoke completions across boots.
Refs
evidence-pack/r1-capture/(R1 boundary-capture summary + per-rank gate-line counts),evidence-pack/ras-dumps/(NCCL RAS per-rank op-count excerpts for R1 and incident A),evidence-pack/soak-logs/(R-series 30+30+30 rounds, the 500-session statistics run, and the 200K probe driver logs). Full raw bundles retained privately.