vllm-xpu-bench

name

vllm-xpu-bench

description

Benchmark a **running vLLM-XPU OpenAI-compatible server** on an Intel GPU using `vllm bench`. Measures TTFT (time-to-first-token), TPOT (time-per-output-token), ITL (inter-token latency), end-to-end latency, and throughput under concurrency. Covers online (`vllm bench serve`) and offline (`vllm bench throughput`) modes; concurrency sweeps and quant comparison live in `references/sweep-and-compare.md`. Use after **vllm-xpu-run** when the user asks "how fast is this?".

vllm-xpu-bench

vllm bench is the same CLI on Intel as on CUDA. The XPU-specific levers are the serve-side flags from vllm-xpu-run (--enforce-eager, --max-model-len, --gpu-memory-utilization, --block-size=64). Pure measurement; for fixes see profiling skills.

Preflight — find the container and verify the server

Before benchmarking, identify the running vLLM container. The bench client must always run inside the container via docker exec — never on the host. This shares the engine's network namespace and reuses the already-loaded tokenizer cache.

1. Find the vLLM container name

docker ps --format 'table {{.Names}}\t{{.Image}}\t{{.Ports}}\t{{.Status}}' | grep -iE 'vllm|8000'

This gives you <container-name>. If multiple containers appear, ask the user which one to bench. Do not proceed without a confirmed container name.

2. Verify the API is reachable and confirm the server is vLLM

docker exec <container-name> curl -s http://127.0.0.1:8000/v1/models

Check the owned_by field in the response:

docker exec <container-name> curl -s http://127.0.0.1:8000/v1/models | \
    python3 -c "import sys,json; d=json.load(sys.stdin); print(d['data'][0]['owned_by'])"

• owned_by: "vllm" → correct server, proceed.
• owned_by: "sglang" → stop. This is a SGLang server; use the sglang-xpu-bench skill instead.
• Any other value → ask the user to confirm the server type before proceeding.

Note the id field — you need it for --model and --served-model-name in the bench command.

If the API is unreachable, check container logs:

docker logs <container-name> 2>&1 | tail -30

3. No server running — start one

If no vLLM container is running, launch one per vllm-xpu-run. Confirm the model and image tag with the user before starting. Use --no-enable-prefix-caching and --disable-log-stats for fair benchmarks.

docker run -d --name <container-name> \
    --device /dev/dri \
    -v /dev/dri/by-path:/dev/dri/by-path:ro \
    --group-add "$(getent group render | cut -d: -f3)" \
    --ipc=host \
    -e ZE_AFFINITY_MASK=0 \
    -e VLLM_WORKER_MULTIPROC_METHOD=spawn \
    -e HTTP_PROXY -e HTTPS_PROXY -e NO_PROXY \
    -e http_proxy -e https_proxy -e no_proxy \
    -e HF_TOKEN="$HF_TOKEN" \
    -v "$HOME/.cache/huggingface:/root/.cache/huggingface" \
    -p 8000:8000 \
    intel/vllm:<version>-xpu \
    vllm serve <model-id> \
        --dtype bfloat16 \
        --enforce-eager \
        --block-size=64 \
        --max-model-len 4096 \
        --gpu-memory-utilization 0.85 \
        --no-enable-prefix-caching \
        --disable-log-stats

Wait for Application startup complete in docker logs, then confirm:

curl -s http://localhost:8000/v1/models | python3 -m json.tool

4. Verify XPU is in use (optional)

Only needed when starting a new server (step 3). If the server was already running and /v1/models returned a model list, XPU is confirmed — skip this step.

xpu-smi dump -d 0 -m 5,18 -i 1 -n 3

VRAM should be non-zero. If zero, the model loaded on CPU — check --device /dev/dri and ZE_AFFINITY_MASK.

Metrics

• TTFT — wall time to first generated token. Dominated by prefill; sensitive to prefix caching.
• TPOT — mean wall time per generated token after the first. Dominated by decode kernel + KV bandwidth.
• ITL — per-token inter-arrival; same source as TPOT but reported per-token so percentiles are meaningful.
• E2EL — wall time of one request, issue to last token.
• Throughput — output_tokens / wall_seconds.

Modes

Mode	Subcommand	When
Online (server up)	vllm bench serve	Default — client-observed metrics.
Offline (no server)	vllm bench throughput	Quick batch throughput; loads the model in the bench process.
Other	vllm bench latency / sweep / startup / mm-processor	vllm bench --help.

Canonical client flag set

For apples-to-apples decode/throughput numbers, pin these flags:

vllm bench serve \
    --backend openai \
    --endpoint /v1/completions \
    --model <ID> --served-model-name <ID> \
    --dataset-name random \
    --random-input-len 1024 --random-output-len 1024 \
    --num-prompts 10 --max-concurrency <C> \
    --request-rate inf \
    --percentile-metrics ttft,tpot,itl,e2el \
    --metric-percentiles 90,99 \
    --ignore-eos

Why each flag:

• random + fixed in/out lengths → removes prompt-distribution variance.
• --num-prompts 10 → enough for median and p90/p99 at typical XPU throughput. Raise to 200+ only when specifically asked for tight p99 repeatability.
• --request-rate inf → saturate the server (throughput-at- saturation, not rate-limited TPOT).
• --ignore-eos → short completions otherwise terminate early and inflate per-token rate.
• --percentile-metrics ttft,tpot,itl,e2el -> the four numbers that describe a serving stack.
• --backend openai + --endpoint /v1/completions -> bypass the chat-template and any structured-output parser. See next note.

Use /v1/completions for random-token datasets. For gpt-oss or any Harmony-based model, always use /v1/completions regardless of dataset: the Harmony streaming parser can trip on control-token-like substrings in any response. For non-Harmony models with random tokens, the chat endpoint wraps each sequence in system/role tokens, inflating input length and skewing throughput. Use --backend openai-chat only for non-Harmony models and a real-prompt dataset (sharegpt, sonnet) where you specifically want to measure chat-template overhead.

Add --trust-remote-code only when the model declares repo-local code and you trust the publisher. It executes Python from the model repo in the bench client.

Server-side prerequisites for fair benchmarks (add to vllm serve for the bench window only):

• --no-enable-prefix-caching — prefix cache hits report TTFT as scheduler-only (~25 ms) and contaminate the metric.
• --disable-log-stats — avoid per-step log IO.

Online bench (server up)

Always run the bench client inside the container via docker exec. Never search for or invoke vllm on the host — the binary lives inside the container image.

docker exec <vllm-container-name> bash -c '
    mkdir -p /root/bench-out
    vllm bench serve \
        --backend openai \
        --endpoint /v1/completions \
        --host 127.0.0.1 --port 8000 \
        --model Qwen/Qwen2.5-1.5B-Instruct \
        --served-model-name Qwen/Qwen2.5-1.5B-Instruct \
        --dataset-name random \
        --random-input-len 1024 --random-output-len 1024 \
        --num-prompts 10 --max-concurrency 4 \
        --request-rate inf \
        --percentile-metrics ttft,tpot,itl,e2el \
        --metric-percentiles 50,90,99 \
        --ignore-eos \
        --save-result --result-dir /root/bench-out'

Headline rows in the output: Total throughput (tok/s), Mean TTFT, p99 TTFT, Mean TPOT, p99 TPOT. The JSON in /root/bench-out keeps every per-request data point — preserve for regression diffs.

Offline throughput (no server)

docker run --rm \
    --device /dev/dri \
    -v /dev/dri/by-path:/dev/dri/by-path:ro \
    --group-add "$(getent group render | cut -d: -f3)" \
    --ipc=host \
    -e ZE_AFFINITY_MASK=0 \
    -e HF_TOKEN="$HF_TOKEN" \
    -v "$HOME/.cache/huggingface:/root/.cache/huggingface" \
    intel/vllm:<version>-xpu \
    bench throughput \
        --model Qwen/Qwen2.5-1.5B-Instruct \
        --dtype bfloat16 \
        --enforce-eager \
        --max-model-len 4096 \
        --gpu-memory-utilization 0.85 \
        --dataset-name random \
        --input-len 512 --output-len 128 \
        --num-prompts 200

Concurrency sweep (find the knee)

docker exec <vllm-container-name> bash -c '
for c in 1 2; do
    mkdir -p /root/bench-out/c$c
    vllm bench serve \
        --backend openai --endpoint /v1/completions \
        --host 127.0.0.1 --port 8000 \
        --model "$MODEL" \
        --dataset-name random \
        --random-input-len 512 --random-output-len 128 \
        --num-prompts 10 \
        --max-concurrency "$c" \
        --ignore-eos \
        --metric-percentiles 50,90,99 \
        --save-result --result-dir "/root/bench-out/c$c"
done'

The "knee" is the concurrency where throughput plateaus while p99 TPOT starts climbing. On Arc Pro B70 with a 1–3B model that's usually around 8–16. Beyond it you trade latency for nothing.

Benchmarking quantised serving

Bench numbers are meaningful only when the quant kernel is actually engaged. Most common silent failure: W4A8 falling through to W4A16 (vLLM #38064) — int4 weights load, requests succeed, activations are still FP16.

1. Confirm the kernel:

   docker logs <name> 2>&1 | grep -E "Selected.*Kernel|XPUFP8|gemm"
   docker logs <name> 2>&1 | grep -i "Unknown vLLM environment"
   

   If you asked for AWQ/GPTQ but see int4_gemm_w4a16, you're hitting the fall-through.
2. Read 2–3 sample completions from the saved JSON before trusting throughput. Some quant kernels return non-language output without raising; HTTP 200 doesn't prove correctness.

For apples-to-apples between quant kinds, keep --no-enable-prefix-caching on the server — random prompts can get artificial cache hits otherwise.

What "good" looks like on Arc Pro B70 (BF16, single GPU, prompt 512 / gen 128, concurrency 1)

Model size	TTFT (ms)	TPOT (ms/tok)	Single-stream tok/s
0.5 B	tens	low single digits	low hundreds
1.5 B	tens	~10	~100
7–8 B	hundreds	tens	~30
14 B BF16	OOM at default	—	needs FP8 or -tp

Order-of-magnitude only; real numbers move with each vLLM release. Save a baseline JSON, diff against it on upgrades.

Common XPU surprises

• First run slow, second fast → XPU Triton kernel compile. Mount TRITON_CACHE_DIR so the next start is hot.
• TTFT fine, TPOT awful → an op fell back to CPU. Set PYTORCH_ENABLE_XPU_FALLBACK=0 for the bench; the engine errors and names the op.
• Throughput collapses past concurrency 8 → KV cache exhausted. Lower --max-model-len or raise --gpu-memory-utilization.
• Bench TPOT 30–60% worse than a single curl → bench is at --max-concurrency, curl is concurrency 1.

Env vars

Variable	Purpose
ZE_AFFINITY_MASK	Pin offline-mode bench client to one XPU.
TRITON_CACHE_DIR	Persist XPU Triton kernels across runs.
PYTORCH_ENABLE_XPU_FALLBACK=0	Make CPU-fallback errors loud.
HF_TOKEN	Gated checkpoints.

What this skill does NOT cover

• Profiling → vllm-xpu-profile, xpu-profile-unitrace.
• Fixing unsupported ops — out of scope.
• SGLang benches → sglang-xpu-bench.
• Pure PyTorch benches → torch-xpu-bench.

References

• references/sweep-and-compare.md — concurrency sweep, quant benches, run-vs-run diff
• vLLM bench docs: https://docs.vllm.ai/en/latest/contributing/profiling/profiling_index.html#offline-batched-inference-benchmarks