Qwen3.5-2B ยท FP8 (dynamic)

8-bit quantization of Qwen/Qwen3.5-2B, produced for efficient vLLM serving. Quantized by axteam.

Base model Qwen/Qwen3.5-2B
Method FP8 (dynamic)
Format compressed-tensors float-quantized (scheme FP8_DYNAMIC)
Weights / activations W8A8 โ€” FP8 E4M3 weights (per-output-channel scale), dynamic per-token FP8 activations
Architecture Hybrid (linear-attention + full-attention) ยท Dense ยท multimodal (vision) ยท MTP head
Size on disk ~3.2 GB
Calibration None (data-free static weight scales)
License Apache-2.0

Quantization details

  • Scheme โ€” W8A8 โ€” FP8 E4M3 weights (per-output-channel scale), dynamic per-token FP8 activations, stored as compressed-tensors float-quantized (scheme FP8_DYNAMIC).
  • Kept in BF16 โ€” the vision tower & projector (model.visual.*), the multi-token-prediction head (mtp.*), all RMSNorms, token embeddings, the LM head, the SSM non-linear parts (linear_attn.conv1d/A_log/dt_bias). Left unquantized to preserve quality (about 40% of all parameters in this build).
  • Tokenizer / chat_template / generation_config are identical to the original. config.json is the original plus a quantization_config block.

This is post-training quantization (PTQ) โ€” no retraining. FP8 dynamic is calibration-free.

Benchmarks (measured)

Every column was measured with the same harness and settings โ€” lm-evaluation-harness on vLLM, greedy, few-shot completion (no chat template). Percentages are recovery vs. the BF16 original.

Benchmark BF16 (original) FP8-dynamic (this repo) AWQ-8bit NVFP4A16 NVFP4 AWQ-4bit
MMLU (5-shot) 53.33 48.18 (90.3%) 53.54 (100.4%) 54.27 (101.8%) 55.22 (103.5%) 60.38 (113.2%)
GSM8K (5-shot, strict) 58.23 56.63 (97.3%) 56.56 (97.1%) 54.51 (93.6%) 51.71 (88.8%) 54.06 (92.8%)
ARC-Challenge (0-shot) 41.47 41.98 (101.2%) 41.30 (99.6%) 41.04 (99.0%) 39.51 (95.3%) 39.59 (95.5%)
Winogrande (5-shot) 64.56 63.22 (97.9%) 64.48 (99.9%) 64.48 (99.9%) 61.09 (94.6%) 62.51 (96.8%)
KMMLU (5-shot, direct) 43.29 42.58 (98.4%) 43.20 (99.8%) 42.01 (97.1%) 41.14 (95.0%) 42.10 (97.3%)
HAE-RAE (0-shot) 45.65 45.00 (98.6%) 45.10 (98.8%) 44.91 (98.4%) 42.07 (92.2%) 41.06 (90.0%)
Average 51.09 49.60 (97.1%) 50.70 (99.2%) 50.21 (98.3%) 48.46 (94.8%) 49.95 (97.8%)

Evaluation (measured)

Fidelity to the BF16 original on an internal mixed English/Korean set (100 prompts, BF16 greedy answers scored under each build โ€” same harness for every row).

Metric This model BF16 original
Perplexity (lower = closer) 1.8710 1.8515 (+1.1%)
Top-1 next-token agreement 94.4% 98.9%

These measure fidelity to the base model, not absolute task scores.

Requirements

Runtime vLLM โ€” the architecture is in vLLM โ‰ฅ 0.17.0; tested here with vLLM 0.32.0.
vLLM install vLLM 0.32 itself needs Python 3.11โ€“3.14, torch 2.13 (CUDA 13.0 build, so an NVIDIA driver with CUDA 13 support), transformers โ‰ฅ5.16.1,<5.20 and compressed-tensors 0.17.0 (both pinned by vLLM; 0.17.0 reads this checkpoint, which was saved with 0.19.0 โ€” do not upgrade it separately).
GPU NVIDIA Turing (sm75, e.g. T4) or newer. FP8 math runs on Ada / Hopper / Blackwell (sm89+); on Turing and Ampere vLLM switches to weight-only FP8 automatically. V100 (sm70) is not supported. (Kernel choice per GPU from vLLM 0.32's selection code; only B200 was run here.)
Memory Weights: 3.2 GB. Add KV cache (grows with context length ร— concurrent requests) and a few GB for activations / CUDA graphs; use --tensor-parallel-size N when the weights do not fit one GPU.
Transformers (without vLLM) transformers>=5.2.0 + compressed-tensors>=0.15.0 (checked on CPU with Qwen3.5-0.8B FP8 / AWQ-4bit / NVFP4A16 / NVFP4).
trust_remote_code not needed

Usage (vLLM)

Start an OpenAI-compatible server:

vllm serve axteam/Qwen3.5-2B-FP8-dynamic --max-model-len 32768

Query it:

curl http://localhost:8000/v1/chat/completions -H "Content-Type: application/json" -d '{
  "model": "axteam/Qwen3.5-2B-FP8-dynamic",
  "messages": [{"role": "user", "content": "๋Œ€ํ•œ๋ฏผ๊ตญ์˜ ์ˆ˜๋„๋Š” ์–ด๋””์ธ๊ฐ€์š”?"}],
  "max_tokens": 512
}'

Offline inference in Python:

from vllm import LLM, SamplingParams

llm = LLM(model="axteam/Qwen3.5-2B-FP8-dynamic", max_model_len=32768)
params = SamplingParams(max_tokens=1024)
out = llm.chat([{"role": "user", "content": "Explain model quantization in two sentences."}], params)
print(out[0].outputs[0].text)

# image input (URL or data: URI)
messages = [{"role": "user", "content": [
    {"type": "image_url", "image_url": {"url": "https://example.com/your-image.png"}},
    {"type": "text", "text": "Describe this image."},
]}]
out = llm.chat(messages, params)
print(out[0].outputs[0].text)

Thinking mode is off by default in this model's chat template; to turn it on pass chat_template_kwargs={"enable_thinking": True} (Python llm.chat(..., chat_template_kwargs=...), or "chat_template_kwargs": {"enable_thinking": true} in the request body). Text-only serving: add --limit-mm-per-prompt '{"image": 0, "video": 0}' to skip loading the vision encoder's memory budget.

Reducing KV-cache memory (optional, serve-time)

The KV cache is kept in BF16 by default. KV-cache precision is a runtime choice, independent of the weight quantization above; for long contexts or large batches FP8 roughly halves KV-cache memory:

vllm serve axteam/Qwen3.5-2B-FP8-dynamic --kv-cache-dtype fp8

License

Quantized derivative of Qwen/Qwen3.5-2B (Apache-2.0). All original weights, tokenizer and chat template belong to the Qwen team.


ํ•œ๊ตญ์–ด ์š”์•ฝ

Qwen/Qwen3.5-2B ๋ฅผ FP8 (dynamic) ๋กœ ์–‘์žํ™”ํ•œ ์„œ๋น™์šฉ(8-bit) ๋ชจ๋ธ์ž…๋‹ˆ๋‹ค. ์ž‘์„ฑ: axteam.

  • ๋ฐฉ์‹ โ€” FP8 E4M3. ๊ฐ€์ค‘์น˜๋Š” ์ถœ๋ ฅ ์ฑ„๋„๋งˆ๋‹ค ์Šค์ผ€์ผ ํ•˜๋‚˜(์ •์ ยท๋ณด์ • ๋ถˆํ•„์š”), ํ™œ์„ฑํ™”๋Š” ์ถ”๋ก  ์‹œ ํ† ํฐ๋งˆ๋‹ค ๋™์  FP8. ํ˜•์‹ compressed-tensors float-quantized (scheme FP8_DYNAMIC).
  • BF16 ์œ ์ง€ โ€” ๋น„์ „ ํƒ€์›ŒยทMTP ํ—ค๋“œยท๋ชจ๋“  normยท์ž„๋ฒ ๋”ฉ/LM ํ—ค๋“œยทSSM ๋น„์„ ํ˜•๋ถ€(conv1d/A_log/dt_bias).
  • PTQ โ€” ์žฌํ•™์Šต ์—†์Œ.
  • ํ’ˆ์งˆ(์‹ค์ธก) โ€” BF16 ์›๋ณธ ๋Œ€๋น„ perplexity 1.8710(์›๋ณธ 1.8515, +1.1%), top-1 ์ผ์น˜๋„ 94.4%(์›๋ณธ 98.9%). ๋‚ด๋ถ€ ์˜/ํ•œ 100๋ฌธํ•ญ, ๊ฐ™์€ ํ•˜๋„ค์Šค.
  • ๋ฒค์น˜๋งˆํฌ(์‹ค์ธก) โ€” MMLU ยท GSM8K ยท ARC-Challenge ยท Winogrande ยท KMMLU ยท HAE-RAE ํ‰๊ท  BF16 51.09 โ†’ ์ด ๋ชจ๋ธ 49.60 (ํšŒ๋ณต๋ฅ  97.1%). lm-evaluation-harness ยท vLLM, ๋ชจ๋“  ํ˜•์‹ ๊ฐ™์€ ์„ค์ •.
  • ๋ผ์ด์„ ์Šค โ€” ์›๋ณธ Apache-2.0 ํŒŒ์ƒ๋ฌผ. ๊ฐ€์ค‘์น˜ยทํ† ํฌ๋‚˜์ด์ €ยทchat_template ๊ถŒ๋ฆฌ๋Š” Qwen ํŒ€.
  • ์š”๊ตฌ์‚ฌํ•ญ โ€” vLLM โ‰ฅ0.17.0 (0.32.0 ์—์„œ ์‹ค์ธก); GPU Turing(sm75) ์ด์ƒ. FP8 ์—ฐ์‚ฐ์€ AdaยทHopperยทBlackwell(sm89+), ๊ทธ ์•„๋ž˜๋Š” vLLM ์ด ๊ฐ€์ค‘์น˜ ์ „์šฉ FP8 ๋กœ ์ž๋™ ์ „ํ™˜; ๊ฐ€์ค‘์น˜ 3.2 GB + KV ์บ์‹œ; Transformers ์ง์ ‘ ์‚ฌ์šฉ: transformers โ‰ฅ5.2.0 + compressed-tensors โ‰ฅ0.15.0.
  • ์‹คํ–‰ โ€” vllm serve axteam/Qwen3.5-2B-FP8-dynamic --max-model-len 32768 ๋กœ OpenAI ํ˜ธํ™˜ ์„œ๋ฒ„(ํฌํŠธ 8000), ๋˜๋Š” Python LLM(model="axteam/Qwen3.5-2B-FP8-dynamic").chat(...) (์œ„ ์˜ˆ์‹œ). thinking ์€ ๊ธฐ๋ณธ์œผ๋กœ ๊บผ์ ธ ์žˆ์Œ โ€” ์ผœ๋ ค๋ฉด chat_template_kwargs={"enable_thinking": True}. ์ด๋ฏธ์ง€ ์ž…๋ ฅ์€ image_url ๋ฉ”์‹œ์ง€๋กœ.
  • KV ์บ์‹œ ์ ˆ๊ฐ(๋Ÿฐํƒ€์ž„) โ€” --kv-cache-dtype fp8.
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