Qwen3.8-27B-INT5-Flat-5.7bpw-AutoRound

Qwen3.8-27B INT5-Flat 5.7bpw AutoRound

A 5.73 bpw quantization of Qwen3.8-27B, built with AutoRound and stored in the compressed-tensors pack-quantized format.

This checkpoint does not load on stock vLLM. It needs a two-line patch to qwen3_5.py / qwen3_5_mtp.py, supplied in vllm-patch/. See How to use before downloading this.

Measurements

KL divergence against the BF16 base model, WikiText-2 test, 240 sequences x 512 tokens, scored on the first 511 position of each sequence for 240 × 511 = 122,640 scored next-token positions, KL(P_base || Q_quant) in nats. Same reference distribution and the same token sequence as every other row, and the tokenization was verified byte-identical against the previous build's stored token array.

KL divergence cannot be compared across models, datasets, or evaluation methods, and is only comparable if all of them are the same. You CANNOT compare these KLD values to others' KLD reports!

If other quantized models report a very low / high KLD compared to mine in their READMEs, it is typically due to differences in the evaluation methodology.

Layout

64 language-model layers, of which 48 use gated-delta linear attention and 16 use full attention.

This time the layout looks lazy. I actually used dynamic programming to test around 18000 different configs for bit allocation. At a 18.5 gb budget the best setup is actually just flat. The script just put every body projection on INT5 gs64 by itself. I didnt force it to be flat it just came out that way.

Going from 5.73 down to 5.16 bpw roughly doubles the KL and drops the model into the middle of the 4-bit field, where fifteen other checkpoints already sit and the only remaining advantage is a gigabyte of download. The KL/bit slope here is about 3.3x per bit, which makes 5.5 bpw the practical floor for this architecture. Below that, plain flat INT4 build would be the best.

The MTP block is INT5 at group_size 128. Acceptance-rate testing showed INT4 draft heads are measurably worse while INT5 through INT8 are indistinguishable, I chose INT5. group_size is 128 because there's no reason to lower the group size. I actually thought about quantizing MTP to INT5 per-channel, but the gain is small so it's 128.

How to use

I recommend using the vLLM docker image as it's the easiest way to use this model.

1. A vLLM with the humming kernel

The INT5 tier dispatches to HummingLinearKernel through CompressedTensorsWNA16; the INT8 embedding/head tier uses Marlin. You need a build where:

• WNA16_SUPPORTED_TYPES_MAP covers 5/6/7 bits (vLLM PR #46389, merged 2026-06-24),
• and the humming-kernels package is installed. It is in requirements/cuda.txt, so a stock CUDA wheel or image has it.

Check both in one line:

python -c "import importlib.metadata as m; print(m.version('humming-kernels'));
from vllm.model_executor.layers.quantization.compressed_tensors.schemes.compressed_tensors_wNa16 \
import WNA16_SUPPORTED_TYPES_MAP as M; print(sorted(M))"
# humming-kernels 0.1.12
# [2, 3, 4, 5, 6, 7, 8]

HummingLinearKernel.get_min_capability() is 75, so sm86 (RTX 3090) is fine.

2. Patch vLLM for the quantized embedding

vLLM has a working quantized-embedding implementation (CompressedTensorsEmbeddingWNA16Int) that Qwen3.5's model definition never reaches, because models/qwen3_5.py builds

self.embed_tokens = VocabParallelEmbedding(self.vocab_size, config.hidden_size)

with neither quant_config nor prefix. vllm-patch/apply.sh pulls the two files out of your image, applies the diffs, and writes the bind-mount flags:

cd vllm-patch && ./apply.sh <your-vllm-image>
podman run ... $(cat mounts.txt) <your-vllm-image> --model /model ...

On a checkpoint with unquantized embeddings the layer falls back exactly as before, so it is safe to leave mounted for other models.

If your vLLM image is newer than the patch, apply.sh may fail to apply cleanly — upstream has been moving this file. Diff against the copy inside your image before assuming the patch is current.

3. Serve

vllm serve /path/to/model \
  --tensor-parallel-size 2 \
  --gpu-memory-utilization 0.97 \
  --max-model-len 262144 \
  --speculative-config '{"method":"mtp","num_speculative_tokens":3}'

Measured on 2x RTX 3090 (sm86), TP2, vision and MTP both enabled:

Using HummingLinearKernel for CompressedTensorsWNA16
Model loading took 9.62 GiB memory and 47.76 seconds
Available KV cache memory: 11.55 GiB
GPU KV cache size: 340,099 tokens, Maximum concurrency for 262,144 tokens per request: 1.30x
Auto-fit max_model_len: full model context length 262144 fits in available GPU memory
Actual usage is 9.79 GiB for consumed memory (weights + non-torch),
1.51 GiB for peak activation, and 0.03 GiB for CUDAGraph memory.

As shown, it has 77,955 tokens of headroom over the full 262,144 context. My INT6-Mixed build fits the same context with only 1,528 to spare, but this one sure can load the Dflash-2 drafter with full context.

Reproducing this model

AutoRound refuses 5/6/7-bit export to the llm_compressor format out of the box. Two bit checks in auto_round/export/formats/backends/llm_compressor.py reject them before any work happens; the packing itself is delegated to compressed-tensors and has handled 1-8 bits for a while. The patch is in auto-round-patch/:

-        if scheme.bits not in [4, 8, 16]:
+        if scheme.bits not in [4, 5, 6, 7, 8, 16]:
-        if scheme.data_type == "int" and scheme.bits not in [4, 8]:
+        if scheme.data_type == "int" and scheme.bits not in [4, 5, 6, 7, 8]:

plus W5A16 / W6A16 in support_schemes. With that applied, pack_layer produces exactly the shapes vLLM expects. Verified at 4, 5, 6, 7 and 8 bits against ceil(in_features * bits / 32), and end-to-end through a vLLM load.

Calibration: iters=500, nsamples=768, seqlen=2048, batch_size=2, gradient_accumulate_steps=4, dataset NeelNanda/pile-10k (256) plus codeparrot/github-code-clean (768). 5 h 54 m on 2x RTX 3090 with data-parallel calibration across both cards.

If you are building something similar, note why the embedding and head targets are regexes. vLLM rewrites quantization targets through the model's WeightsMapper, but apply_vllm_mapper treats anything without a dot as a class name and passes it through untouched. A target spelled lm_head never reaches language_model.lm_head. And model.language_model.embed_tokens reaches the main model but not the MTP draft, whose module sits at mtp.embed_tokens. re: targets skip the rewrite and are matched as regular expressions, so they hit every copy.

Files

model-0000{1..8}-of-00008.safetensors   weights
model-mtp.safetensors                   MTP block
config.json                             includes the compressed-tensors config
layer_config.json                       the allocation, as fed to AutoRound
vllm-patch/                             the two-line vLLM embedding patch
auto-round-patch/                       the 5/6/7-bit export gate patch

Acknowledgements

• Alibaba / Qwen team for the base Qwen3.8-27B model
• Intel AutoRound team for the quantization framework
• vLLM project for the inference engine and Qwen3_5 MTP support