Qwen3.8-27B-W4A16-AutoRound

An int4 weight-only (W4A16) quantization of Qwen/Qwen3.8-27B, made with Intel AutoRound and packaged in compressed-tensors for vLLM. The weights are int4 and the activations stay BF16, so it runs on any Ampere-or-newer NVIDIA GPU through the Marlin int4 kernel — no Blackwell needed. It's about 19.5 GB on disk, down from roughly 52 GB at BF16.

This build is aimed at non-Blackwell GPUs. If you have a Blackwell card (RTX 50-series, B200, DGX Spark), the NVFP4 (W4A4) sibling uses the FP4 tensor cores and will be faster. Accuracy notes are in the Evaluation section below.

Model overview

• Base model: Qwen/Qwen3.8-27B — a hybrid GatedDeltaNet (linear-attention) + full-attention multimodal decoder (64 layers: 48 linear-attn + 16 full-attn) with a vision tower; 262K context.
• Quantization: int4 W4A16 (weight-only, group size 128) on the language-decoder Linear layers, except the two tiny per-layer recurrence-control projections (in_proj_a, in_proj_b) which stay BF16.
• Kept BF16: linear_attn.in_proj_a / in_proj_b, the entire visual.* vision tower, the mtp head, and lm_head.
• Format: compressed-tensors (pack-quantized), auto-detected by vLLM (Marlin int4).
• Quantizer: Intel AutoRound (arXiv:2309.05516), SignRound block reconstruction.

Quantization recipe

• Scheme: W4A16 — 4-bit weights, group size 128, symmetric; activations stay BF16.
• Quantized: the 48 GatedDeltaNet layers' in_proj_qkv / in_proj_z / out_proj + all MLP gate/up/down + the 16 full-attention layers' q/k/v/o projections.
• Kept BF16: in_proj_a, in_proj_b, visual.*, mtp, lm_head.
• Calibration: NeelNanda/pile-10k, 128 samples, seqlen 2048, 200 tuning iters (screen recipe).
• Cost: about 35 minutes on a single 96 GB Blackwell GPU; peak 24 GB VRAM, 26 GB host RAM.

Deployment (vLLM)

vLLM auto-detects the int4 scheme from config.json — no quantization flag needed. Serves as Qwen3_5ForConditionalGeneration.

vllm serve dbirks/Qwen3.8-27B-W4A16-AutoRound --max-model-len 8192 --trust-remote-code

Example compose.yaml

services:
  qwen38-w4a16:
    image: vllm/vllm-openai:latest   # any recent vLLM with Qwen3.8 (qwen3_5) support; Marlin int4 runs on any Ampere+ GPU
    ports:
      - "8000:8000"
    ipc: host
    volumes:
      - ~/.cache/huggingface:/root/.cache/huggingface
    command:
      - "--model=dbirks/Qwen3.8-27B-W4A16-AutoRound"
      - "--served-model-name=qwen3.8-27b"
      - "--max-model-len=8192"
      - "--trust-remote-code"
    deploy:
      resources:
        reservations:
          devices:
            - driver: nvidia
              count: all
              capabilities: [gpu]

Evaluation

Measured in the model's default thinking mode with its recommended sampling (temperature 1.0, top-p 0.95, top-k 20), using EleutherAI lm-evaluation-harness.

Across all three tasks the quant matches the BF16 base within the confidence interval, so there's no measurable accuracy loss. (MMLU-Pro here is 100 questions per subject, so it's an internal reference against our own BF16 baseline, not directly comparable to a full-split MMLU-Pro number elsewhere.)

Hardware & format notes

• Runs on any Ampere-or-newer NVIDIA GPU via the Marlin int4 kernel — no Blackwell required.
• Weight-only (W4A16): activations stay BF16 → maximal accuracy + compatibility, but no activation-quant speedup. For Blackwell FP4 (W4A4) throughput, use the NVFP4 sibling.
• Only the language decoder is quantized; the vision tower stays BF16 (intentional; vLLM requires it).

FAQ

Why is this W4A16 build (19.5 GB) smaller than the NVFP4 (W4A4) sibling (20.6 GB), even though it keeps activations at 16-bit?

Activations are never stored in the file — they're intermediate values computed at runtime and then thrown away, so "A16 vs A4" has no effect on file size at all. Both files store only 4-bit weights. The size difference comes from scale metadata (group size):

NVFP4 stores a scale for every 16 weights (finer, which helps FP4 accuracy but adds metadata); int4 stores one for every 128 (coarser, less metadata). That difference of roughly 0.4 bits per weight, across about 24 billion quantized weights, works out to the 1 GB gap. So NVFP4 is a little bigger because of accuracy metadata, not because of anything to do with activations.

Hugging Face shows around 6 B parameters for this model — is it really a 6 B model?

No, it's the full 27.78 B model, same as the base. Quantization changes precision, never the number of parameters. Hugging Face's params widget sums the stored tensor elements, and int4 weights are packed 8 to an int32, so about 24 billion logical weights are stored as roughly 3 billion int32 elements, and it undercounts. The "by dtype" breakdown shows the real figure: 24.33 B in the 4-bit tensors plus 3.45 B in BF16, which is 27.78 B. This quirk shows up on every quantized model.

Which variant should I use — this W4A16 or the NVFP4 sibling?

• Blackwell GPU (RTX 50-series, B200/B300, DGX Spark): use the NVFP4 (W4A4) sibling, which runs on the FP4 tensor cores for faster compute.
• Anything older (Ampere/Ada/Hopper — A100, L40S, RTX 40-series): use this W4A16. Int4 weights run through the Marlin kernel on any modern GPU, and those cards don't have FP4 cores. The 1 GB size difference is negligible; pick based on whether your GPU has FP4 tensor cores.

Reproducibility

from auto_round import AutoRound
# keep in_proj_a/b + visual + mtp in BF16 via a bits=16 layer_config (built from module names)
ar = AutoRound("Qwen/Qwen3.8-27B", scheme="W4A16", dataset="NeelNanda/pile-10k",
               nsamples=128, seqlen=2048, batch_size=4, iters=200,
               device_map=0, trust_remote_code=True, quant_nontext_module=False, seed=42,
               layer_config=BF16_FOR_IN_PROJ_AB_VISUAL_MTP)
ar.quantize_and_save(output_dir="Qwen3.8-27B-W4A16-AutoRound", format="llm_compressor")

Toolchain: auto-round 0.15.0, transformers-from-source (qwen3_5 arch), compressed-tensors, torch 2.13+cu130.

Citation

@article{cheng2023optimize, title={Optimize Weight Rounding via Signed Gradient Descent for the Quantization of LLMs}, author={Cheng, Wenhua and others}, journal={arXiv:2309.05516}, year={2023}}