[Bugfix][Spec Decode] DFlash2: accept unquantized linear LM heads in the candidate selector - #52883
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Two additions to the DFlash drafter, carried by a separate architecture: a checkpoint declaring DFlash2DraftModel gets them, and every existing DFlashDraftModel checkpoint resolves to the class it resolves to today. Grouped dynamic depthwise convolution inside each block, so a proposal position can see the ones before it without another backbone pass: out[i,c] = sum_t (base[t,c] + delta[i,t,g(c)]) * x[i-t,c], taps zero across the block boundary. Each sublayer is wrapped in and out from one projection of its input. Sized by conv_kernel_size and conv_group_size in the draft config. Candidate selector. Instead of an independent argmax per slot, keep the target head's top-K per slot, score adjacent transitions edge(p->c) = <A[p] * project(h), B[c]> + unary[c], and walk the best path from the verified anchor. At T>0 it walks by inverse CDF and returns q over the K candidates for the lossless verify. Sized by selector_rank and selector_top_k. The selector runs after the draft model's forward and carries its own @support_torch_compile. The path walk is one Triton program per request: a slot's K scores stay in registers, and the slot-to-slot dependency is a loop inside the program rather than a kernel per slot. The vocabulary top-k, the selector's largest single cost, uses FlashInfer's radix kernel where FlashInfer is available and torch.topk otherwise. DFlash2 runs on the V2 model runner, which is where its speculator lives. use_v2_model_runner selects V2 for a DFlash2 draft, as it already does for dspark: the V1 DFlashProposer has no candidate selector, so a DFlash2 checkpoint reaching it would draft as DFlash1 without raising. Co-authored-by: SubSir <tiancaizhangdaxian@sjtu.edu.cn>
…the candidate selector compute_candidates guards the LM head with isinstance(quant_method, UnquantizedEmbeddingMethod), but quant configs (INC, ModelOpt, fp8 excluded layers, ...) return UnquantizedLinearMethod for an unquantized ParallelLMHead; only a build with no quant_config at all yields UnquantizedEmbeddingMethod. A quantized-body target with an unquantized head therefore failed to start: ValueError: DFlash2 requires an unquantized target LM head for candidate TopK. Both methods are unquantized and their apply() dispatches the same unquantized GEMM, so accept either; the error now names the offending method. Depends on vllm-project#52816 (stacked; retarget to main once it merges). Co-authored-by: pi coding agent <agent@fieldio.com> Signed-off-by: Andreas Echavez <oceanplexian@gmail.com>
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…issue (#2) The try/except fallback in _topk was added because FlashInfer's radix top-k failed to JIT-compile on CUDA 13. Root cause turned out to be the CPATH-based include setup pulling in a mismatched host_runtime.h; with curand*.h symlinked into the system include path instead, FlashInfer compiles and works fine on CUDA 13. The fallback is unnecessary and masks real errors, so drop it and use FlashInfer directly. Also types the config parameter as Qwen3Config to match the base class signature, aligning this file with vllm-project#52883. Signed-off-by: Andreas Echavez <oceanplexian@gmail.com> Co-authored-by: pi coding agent <agent@fieldio.com>
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One additional LM-head compatibility data point that goes beyond the I tested: The target's I relaxed the unquantized-only guard but left the existing candidate-logit dispatch unchanged: logits = self.lm_head.quant_method.apply(
self.lm_head,
hidden_states,
bias=None,
)The quantized head successfully executes the DFlash2 candidate Top-K path. After fixing a separate I ran So at least for compressed-tensors FP8, the existing |
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This pull request has merge conflicts that must be resolved before it can be |
Three changes make this recipe serve the checkpoint people actually have, at the window they want. The target LM-head guard is patched out. DFlash2 rejects a quantized lm_head, which excluded unsloth (FP8 head) and RadixArk (NVFP4) and forced a third-party checkpoint. The candidate Top-K dispatches through quant_method.apply() either way and a compressed-tensors FP8 head runs it (reported in vllm-project/vllm#52883, reproduced here). Acceptance on unsloth reaches 4.7, the highest of the three NVFP4 checkpoints tried, and the accuracy smoke is unchanged: ppl 19.59, gsm8k_cot_lite 0.9600. The drafter gets a bigger position table. It ships max_position_embeddings=262144, so a prompt past that indexes its cos/sin cache out of bounds and dies on a device-side assert. make_drafter_overlay.sh raises it to 524288 and touches nothing else: the drafter attends in a 2048-token sliding window, so acceptance is unchanged (4.29 measured on a 267,946-token prompt, needle retrieved). The image now applies PR #52816's head, which carries the candidate-selector compile-cache namespace fix, instead of #52883's conflicting branch. Serving shape: 512k YaRN, util 0.92, max-num-seqs 4, 648,991-token pool (1.24x at 512k), 132 tok/s solo against 96 without the drafter. 0.94 serves fine and then OOMs on a prompt_logprobs request, which is why it is not the default. start.sh now builds both overlays, defaults to no restart policy, and refuses to start while another process holds the GPUs. Two dead ends are documented: the W4A16 drafter cannot load (fused-KV precompute reads qkv_proj.weight), and LMCache cannot run at all (every KV connector disables the hybrid KV cache manager, which this model's GDN state cannot survive).
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We looked at this PR while coordinating our own DFlash2 work (#53978 touches the same 1. The guard this PR fixes no longer exists on 2. But the same class confusion survives at one other call site. if not isinstance(lm_head.quant_method, UnquantizedEmbeddingMethod):
raise ValueError("A head_dtype different from the model dtype is only "
"supported for an unquantized lm_head.")This branch only runs when So the insight of this PR is still needed — just one file over. Retargeting the accept-both-classes fix (plus the clearer error naming the offending method, which we'd keep) to |
…at would draft the published checkpoint as DFlash1 (mudler#1314) (mudler#1321) DFlash2 is a second DFlash architecture rather than a change to DFlash, and this engine has no route for it. Upstream carries it in two open pull requests ([vllm#52816](vllm-project/vllm#52816) at head `19c9351904df4c63042671bc67a866ca48dc7d6f`, plus the stacked guard fix [vllm#52883](vllm-project/vllm#52883)): DFlash1 gains two subclass seams and keeps every behaviour, so a `DFlashDraftModel` checkpoint resolves exactly as it does today, while `DFlash2DraftModel` adds a grouped dynamic depthwise convolution inside each draft block and a candidate selector that replaces the independent per-slot argmax with a scored path walk over the target head's top-K. This is the spec half of a two-pull-request row, the shape the developer chose at row claim. No production code lands here. `SPEC-DFLASH2` enters the engine matrix as `READY`, which is what a helper dispatch needs before W1 can start. The spec takes its shapes from the published `z-lab/Qwen3.8-27B-DFlash2` checkpoint rather than from the diff. Its safetensors header, range-read on 2026-08-19, is DFlash1's tensor set plus `layers.N.{attention,mlp}_conv.*` and three `candidate_selector` tensors, of which the two codebooks are `(248320, 256)` bf16 each: about 254 MB resident that the DFlash1 lane never allocates. It records one defect that raises nothing. That config declares all five layers `sliding_attention` AND `is_causal false`, while our causality resolution mirrors the OLD upstream rule, so every layer would run causal. The draft would emit plausible tokens, a token gate against our own output would see nothing, and only acceptance would move, which the lossless verify hides. Upstream changes `_dflash_layer_causal` to read `is_causal` first, in the same commit that adds the architecture. Three further decisions are argued rather than assumed. FlashInfer's 3380-line radix top-k is not ported: our shape is K=16 over a 248320 vocabulary for about 224 rows, and `src/vt/cuda/cuda_sample.cu:297-506` already carries the same sort-free pivot-bracket threshold search, so what is owed is emitting the surviving pairs rather than a new kernel. The path walk runs on device from the first landing, because the identical sequential shape in DSpark shipped host-side and measured 28% of the 27B draft step (mudler#436) before it had to be moved. And the GGUF drafter arm lands in the same wave rather than as a follow-on row. BEYOND-PIN by developer decision: the parity pin `555967922` does not carry the architecture at all, anchors cite the pull-request head, and this row does not advance the pin. It is the posture `SPEC-DSPARK-QWEN3-ROUTING` already takes toward vllm#52197. Records: issue mudler#1314 in the index, the `SPEC-DFLASH2` row with its section and total counts, `ENGINE_ROWS` 164 to 165 with its justification paragraph, and the `STATUS.md` projection. The `ENGINE_ROWS` comment block shifted five `ENG-RECORD-ANCHOR-RATCHET` citations into the same file by twelve lines, and those anchors are repaired here rather than left for the ratchet to catch. Gates: `scripts/agent-ready.py` all green at `d72848e06`, including `check-agent-record` at ENGINE=165 with anchor rot unchanged at 38, and `tests/scripts/test_agent_record.py` 97 passed. FOLLOWING_AGENTS_PROTOCOL Following-Agents-Protocol: true AI-Assisted: true Assisted-by: AGENT:deepseek-v4-flash [edit bash]
4 participants
Purpose
Stacked on #52816 — this branch is
19c93519plus one commit, so the diffabove includes that PR's changes. Retarget to
mainonce #52816 merges; thediff then collapses to the single fix commit.
DFlash2Qwen3ForCausalLM.compute_candidatesguards the LM head withisinstance(self.lm_head.quant_method, UnquantizedEmbeddingMethod), but thatclass only appears when the model has no quant config at all. A quantization
config that leaves the head unquantized — INC, ModelOpt, fp8 with excluded
layers, ... — returns
UnquantizedLinearMethodfor aParallelLMHead(thesame class it returns for any unquantized linear). A quantized-body target with
an unquantized head therefore fails to start:
Reproduced with
z-lab/Qwen3.8-27B(INC-int4, head unquantized) as target andz-lab/Qwen3.8-27B-DFlash2as drafter, TP2 on dual RTX 3090.Both classes are unquantized and their
apply()dispatches the sameunquantized GEMM, so the guard now accepts either, and the error names the
offending method. Also types the
configparameter asQwen3Configto matchthe base class signature.
Why this is not a duplicate
#52816 introduces the DFlash2 architecture; this PR only fixes the LM-head
guard in the code that PR adds, on top of its head. No other open PR touches
this path.
Test plan
ruff check/ruff format --checkon the touched file → clean.Test Result
(to be filled)
Note
AI assistance (pi coding agent) was used for the fix and this description;
every changed line was reviewed and tested by the submitter.