GPTQ 8-bit quantization of Qwen/Qwen3.8-27B, produced and validated on 4x AMD Instinct MI100 (gfx908 / CDNA1) over XGMI. This is the reference checkpoint of an int8-native serving stack tuned specifically for gfx908, where int8 matrix rate is 2x every dtype except fp16 (equal) at half the memory bandwidth.

Quantization details

Parameter	Value
Format	GPTQ
Bits	8
Group size	128
Symmetric	true
desc_act	false
true_sequential	true
lm_head	false (kept fp16 — tied weights feed the logits GEMM)
Vision encoder / MTP module	untouched BF16 (carried in checkpoint)
Quantizer	GPTQModel 7.3.4, RTN-free (Hessian-compensated)
Calibration	512 samples, mixed evol-codealpaca-v1 (code) + C4 (general text), binned uniformly 256–2048 tokens

Why group_size 128: the accompanying runtime dispatches AITER true int8×int8 compute (W8A8) for every decode and prefill GEMM shape, which requires one weight scale per 128-wide K block. Quality was verified by a teacher-forced KL-divergence sweep over 6 precision configurations before baking — this config measured KLD 0.011 vs the gs32 reference (gate 0.02) and showed activation quantization adds only +0.0032 KLD on top.

Serving (vLLM, gfx908 fork)

Use with the companion fork stack:

vLLM: curvedinf/int8-vllm — branch main (AITER W8A8 INT8 GEMMs everywhere, int8 embedding gather, int8 per-token-head KV, vLLM custom all-reduce)
AITER: curvedinf/int8-aiter — branch main (int8 unified-attention kernels + gfx908 tuning)

bash

VLLM_ROCM_USE_AITER=1 \
VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION=1 \
VLLM_GFX908_INT8_LM_HEAD=1 \
VLLM_GFX908_ACT_QUANT=round \
VLLM_DISABLED_KERNELS=TritonW8A16LinearKernel \
vllm serve <this-model-dir> \
  --tensor-parallel-size 4 \
  --max-num-seqs 8 \
  --dtype half \
  --max-model-len 65536 \
  --kv-cache-dtype int8_per_token_head \
  --mamba-ssm-cache-dtype float32 \
  --speculative-config '{
    "method": "dflash",
    "model": "curvedinf/Qwen3.8-27B-DFlash2-GPTQ-INT8-W8A8-GS128",
    "num_speculative_tokens": 15,
    "kv_cache_dtype": "int8_per_token_head"
  }'

This is one fixed TP4/C8 contract: both GS128 models, AITER W8A8 INT8 GEMMs at every M, AITER unified attention, vLLM custom all-reduce, fp32 Mamba/GDN state, and INT8 target/draft KV. --dtype half is the model's residual/native dtype; GEMM inputs are dynamically quantized to INT8 by the AITER W8A8 path.

The top-level KV flag configures this target; the nested field configures the DFlash2 draft. Both use INT8 per-token-head KV. Do not substitute TRITON_ATTN, W8A16, RCCL or AITER CAR all-reduce, fp16 KV, no speculation, another TP size, or another concurrency in the intended recipe.

Model architecture

Qwen3.8-27B is a hybrid dense multimodal model (model_type qwen3_5): 64 layers — 48 GDN linear-attention + 16 full-attention (repeating 3:1), hidden 5120, 27B parameters, vocab 248,320, context 262,144. This checkpoint serves the language model (--language-model-only); vision weights are carried for completeness.

Reproduction

Quantization script and full experiment ledger (KLD sweep, kernel microbenchmarks, A/B measurements) live in the vLLM fork: ~/models/quantize_qwen38_27b_gptq8.py and logs/c8_optimization/experiments.md.

curvedinf/Qwen3.8-27B-GPTQ-INT8-W8A8-GS128

Qwen3.8-27B GPTQ INT8 W8A8 GS128

Quantization details

Serving (vLLM, gfx908 fork)

Model architecture

Reproduction

curvedinf/
Qwen3.8-27B-GPTQ-INT8-W8A8-GS128