perplexity-ai/pplx-qwen-3-8-27b-dflash2-20260819

pplx-computer-qwen-3-8-27b-dspark2-20260819

Qwen3.8-27B (arch class Qwen3_5ForConditionalGeneration) quantized to NVFP4 (ModelOpt mixed-precision mirroring RadixArk/Qwen3.8-27B-NVFP4's recipe, FP8 KV cache) with one deliberate change: an unquantized bf16 lm_head (DFlash2 refuses a quantized one — every public NVFP4 quant packs it to FP4), plus baked-in dspark2 MTP draft tensors (mtp.* in model-mtp.safetensors, referenced by model.safetensors.index.json).

Snapshot of the checkpoint serving on the pplx-computer DGX Spark fleet as of 2026-08-19. Verified perf on one GB10: ~30–35 tok/s single-stream decode, 84 tok/s aggregate @ 4 streams (2.9× over no-spec NVFP4).

How to run (DGX Spark / GB10)

Everything needed is in serving/. Stock vLLM can not serve this with DFlash2 — it needs the pure-Python vLLM PR #52816 overlay plus a one-line lm_head fix (see notes below), packaged here as two Dockerfiles.

# 0) Auth (private repo) — needs a perplexity-ai read token
hf auth login

# 1) Download this checkpoint + the DFlash2 draft (draft/) + the serving kit
hf download perplexity-ai/pplx-computer-qwen-3-8-27b-dspark2-20260819 \
  --local-dir ~/models/qwen38-dspark2-20260819

# 2) Build the patched vLLM image (pure-Python patches, no kernel rebuild)
cd ~/models/qwen38-dspark2-20260819/serving
docker build -f Dockerfile          -t vllm-dflash2:local     .
docker build -f Dockerfile.lmheadfix -t vllm-dflash2:lmheadfix .

# 3) Serve
MODEL_DIR=~/models/qwen38-dspark2-20260819 docker compose up -d

# 4) Verify (first start compiles graphs — allow up to ~10 min)
curl -sf localhost:8000/health && \
curl -s localhost:8000/v1/chat/completions -H 'Content-Type: application/json' -d '{
  "model": "pplx-computer-qwen-3-8-27b-dspark2-20260819",
  "messages": [{"role": "user", "content": "Say hi in five words."}],
  "max_tokens": 64
}' | python3 -m json.tool

Tunables (env vars on docker compose up): GPU_UTIL (default 0.75 — raise to ~0.9 on a dedicated box), MAX_LEN (262144), MAX_SEQS (4), SERVED_NAME (the OpenAI-API model id; defaults to this repo's name — override if your client expects a different id).

Serve flags

vllm serve /models/qwen38 \
  --served-model-name pplx-computer-qwen-3-8-27b-dspark2-20260819 \
  --host 0.0.0.0 --port 8000 \
  --gpu-memory-utilization 0.75 \
  --max-model-len 262144 \
  --max-num-seqs 4 \
  --max-num-batched-tokens 8192 \
  --async-scheduling \
  --speculative-config '{"method": "dflash", "model": "<model-dir>/draft", "num_speculative_tokens": 7}' \
  --reasoning-parser qwen3 \
  --tool-call-parser qwen3_coder \
  --enable-auto-tool-choice

Notes / gotchas

• The DFlash2 draft is vendored at draft/ (a mirror of incoai/Qwen3.8-27B-DFlash2, 2026-08-19) and referenced by local path in the compose — the repo is fully self-contained, no external downloads at serve time. The draft is separate from the target weights: the in-checkpoint mtp.* tensors are the dspark2 MTP module — MTP k=7 reaches similar acceptance but decodes ~2× slower (sequential draft passes), so DFlash2 is the recommended path.
• bf16 lm_head is load-bearing. DFlash2 requires an unquantized target lm_head; it is excluded via exclude_modules in both hf_quant_config.json and config.json (vLLM's modelopt loader keys on the exclude wildcards, not on absence from quantized_layers).
• Why Dockerfile.lmheadfix: vLLM gives an excluded lm_head an UnquantizedLinearMethod, which PR #52816's isinstance check wrongly rejects (it only accepts UnquantizedEmbeddingMethod). The second image layer relaxes that check.
• --max-num-batched-tokens 8192 is required: with spec decode enabled, vLLM otherwise clamps scheduling to 2048 and prefill collapses.