Qwen3.8-27B-DFlash2-W4A16

incoai/Qwen3.8-27B-DFlash2 — the DFlash2 block drafter for Qwen3.8-27B (5 Qwen3-style layers, 1.92B parameters, 3.85 GB in bf16) — requantized to W4A16 compressed-tensors (pack-quantized, group 128, symmetric, Marlin) with GPTQ, so it fits next to a 27B target on a 24 GB card: 1.19 GB.

Built by and for syv-ai/qwen38-27b-rtx3090 (Qwen3.8-27B on one RTX 3090 with vLLM): single-user/start_qwen.sh with SPEC=dflash2.

What is quantized

tensors	precision	note
layers.N.self_attn.{q,k,v,o}_proj, layers.N.mlp.{gate,up,down}_proj (1.61B params)	int4 GPTQ, g128	Hessians from the drafter's own inputs on 400 real prompts (~290k rows per layer)
fc (5120 × 25600, projects the target's layer 5/19/33/47/61 hidden states)	int4 GPTQ, g128	keeping it bf16 measured no acceptance difference
grouped-conv kernel_projections, candidate_selector, norms	bf16	tiny; vLLM builds them unquantized

Measured on the W4A16 target (RTX 3090, 8 realistic chat prompts × 1,024 tokens, vLLM 0.27.1 with the repo's patches): same acceptance as the bf16 drafter at greedy (3.34-3.65 vs 3.54 tokens per step), about 5% lower at the model's default sampling; per decode step it reads 2.7 GB less, which is what makes it a net win on this card. Details and the pipeline (drafter/capture_dflash2.py, drafter/quant_dflash2.py) in the repo's drafter/README.md.

A variant whose k/v Hessians also blended the context-KV precompute's input distribution — which is the theoretically tidier calibration — measured 7% worse greedy acceptance (3.12 vs 3.34 tokens per step, 118 vs 126 tok/s end to end) and is not what ships here.

Use

With vLLM 0.27.1 + the repo's patches/dflash2-backport.patch (backport of vLLM PR #52816), or vLLM main with PR #52816:

vllm serve <Qwen3.8-27B target> \
  --speculative-config '{"method":"dflash","model":"syvai/Qwen3.8-27B-DFlash2-W4A16","num_speculative_tokens":7}' \
  --attention-backend FLASH_ATTN --kv-cache-dtype bfloat16

The drafter shares the target's embeddings and lm_head (it ships neither); with a quantized target lm_head the repo's patch is needed (upstream refuses a non-bf16 lm_head for the candidate top-k).

Apache-2.0, like the original drafter by Inco.