serving-llms-vllm

What this skill does

# vLLM - High-Performance LLM Serving

## When to use

Use when deploying production LLM APIs, optimizing inference latency/throughput, or serving models with limited GPU memory. Supports OpenAI-compatible endpoints, quantization (GPTQ/AWQ/FP8), and tensor parallelism.

## Quick start

vLLM achieves 24x higher throughput than standard transformers through PagedAttention (block-based KV cache) and continuous batching (mixing prefill/decode requests).

**Installation**:
```bash
pip install vllm
```

**Basic offline inference**:
```python
from vllm import LLM, SamplingParams

llm = LLM(model="meta-llama/Llama-3-8B-Instruct")
sampling = SamplingParams(temperature=0.7, max_tokens=256)

outputs = llm.generate(["Explain quantum computing"], sampling)
print(outputs[0].outputs[0].text)
```

**OpenAI-compatible server**:
```bash
vllm serve meta-llama/Llama-3-8B-Instruct

# Query with OpenAI SDK
python -c "
from openai import OpenAI
client = OpenAI(base_url='http://localhost:8000/v1', api_key='EMPTY')
print(client.chat.completions.create(
    model='meta-llama/Llama-3-8B-Instruct',
    messages=[{'role': 'user', 'content': 'Hello!'}]
).choices[0].message.content)
"
```

## Common workflows

### Workflow 1: Production API deployment

Copy this checklist and track progress:

```
Deployment Progress:
- [ ] Step 1: Configure server settings
- [ ] Step 2: Test with limited traffic
- [ ] Step 3: Enable monitoring
- [ ] Step 4: Deploy to production
- [ ] Step 5: Verify performance metrics
```

**Step 1: Configure server settings**

Choose configuration based on your model size:

```bash
# For 7B-13B models on single GPU
vllm serve meta-llama/Llama-3-8B-Instruct \
  --gpu-memory-utilization 0.9 \
  --max-model-len 8192 \
  --port 8000

# For 30B-70B models with tensor parallelism
vllm serve meta-llama/Llama-2-70b-hf \
  --tensor-parallel-size 4 \
  --gpu-memory-utilization 0.9 \
  --quantization awq \
  --port 8000

# For production with caching and metrics
vllm serve meta-llama/Llama-3-8B-Instruct \
  --gpu-memory-utilization 0.9 \
  --enable-prefix-caching \
  --enable-metrics \
  --metrics-port 9090 \
  --port 8000 \
  --host 0.0.0.0
```

**Step 2: Test with limited traffic**

Run load test before production:

```bash
# Install load testing tool
pip install locust

# Create test_load.py with sample requests
# Run: locust -f test_load.py --host http://localhost:8000
```

Verify TTFT (time to first token) < 500ms and throughput > 100 req/sec.

**Step 3: Enable monitoring**

vLLM exposes Prometheus metrics on port 9090:

```bash
curl http://localhost:9090/metrics | grep vllm
```

Key metrics to monitor:
- `vllm:time_to_first_token_seconds` - Latency
- `vllm:num_requests_running` - Active requests
- `vllm:gpu_cache_usage_perc` - KV cache utilization

**Step 4: Deploy to production**

Use Docker for consistent deployment:

```bash
# Run vLLM in Docker
docker run --gpus all -p 8000:8000 \
  vllm/vllm-openai:latest \
  --model meta-llama/Llama-3-8B-Instruct \
  --gpu-memory-utilization 0.9 \
  --enable-prefix-caching
```

**Step 5: Verify performance metrics**

Check that deployment meets targets:
- TTFT < 500ms (for short prompts)
- Throughput > target req/sec
- GPU utilization > 80%
- No OOM errors in logs

### Workflow 2: Offline batch inference

For processing large datasets without server overhead.

Copy this checklist:

```
Batch Processing:
- [ ] Step 1: Prepare input data
- [ ] Step 2: Configure LLM engine
- [ ] Step 3: Run batch inference
- [ ] Step 4: Process results
```

**Step 1: Prepare input data**

```python
# Load prompts from file
prompts = []
with open("prompts.txt") as f:
    prompts = [line.strip() for line in f]

print(f"Loaded {len(prompts)} prompts")
```

**Step 2: Configure LLM engine**

```python
from vllm import LLM, SamplingParams

llm = LLM(
    model="meta-llama/Llama-3-8B-Instruct",
    tensor_parallel_size=2,  # Use 2 GPUs
    gpu_memory_utilization=0.9,
    max_model_len=4096
)

sampling = SamplingParams(
    temperature=0.7,
    top_p=0.95,
    max_tokens=512,
    stop=["</s>", "\n\n"]
)
```

**Step 3: Run batch inference**

vLLM automatically batches requests for efficiency:

```python
# Process all prompts in one call
outputs = llm.generate(prompts, sampling)

# vLLM handles batching internally
# No need to manually chunk prompts
```

**Step 4: Process results**

```python
# Extract generated text
results = []
for output in outputs:
    prompt = output.prompt
    generated = output.outputs[0].text
    results.append({
        "prompt": prompt,
        "generated": generated,
        "tokens": len(output.outputs[0].token_ids)
    })

# Save to file
import json
with open("results.jsonl", "w") as f:
    for result in results:
        f.write(json.dumps(result) + "\n")

print(f"Processed {len(results)} prompts")
```

### Workflow 3: Quantized model serving

Fit large models in limited GPU memory.

```
Quantization Setup:
- [ ] Step 1: Choose quantization method
- [ ] Step 2: Find or create quantized model
- [ ] Step 3: Launch with quantization flag
- [ ] Step 4: Verify accuracy
```

**Step 1: Choose quantization method**

- **AWQ**: Best for 70B models, minimal accuracy loss
- **GPTQ**: Wide model support, good compression
- **FP8**: Fastest on H100 GPUs

**Step 2: Find or create quantized model**

Use pre-quantized models from HuggingFace:

```bash
# Search for AWQ models
# Example: TheBloke/Llama-2-70B-AWQ
```

**Step 3: Launch with quantization flag**

```bash
# Using pre-quantized model
vllm serve TheBloke/Llama-2-70B-AWQ \
  --quantization awq \
  --tensor-parallel-size 1 \
  --gpu-memory-utilization 0.95

# Results: 70B model in ~40GB VRAM
```

**Step 4: Verify accuracy**

Test outputs match expected quality:

```python
# Compare quantized vs non-quantized responses
# Verify task-specific performance unchanged
```

## When to use vs alternatives

**Use vLLM when:**
- Deploying production LLM APIs (100+ req/sec)
- Serving OpenAI-compatible endpoints
- Limited GPU memory but need large models
- Multi-user applications (chatbots, assistants)
- Need low latency with high throughput

**Use alternatives instead:**
- **llama.cpp**: CPU/edge inference, single-user
- **HuggingFace transformers**: Research, prototyping, one-off generation
- **TensorRT-LLM**: NVIDIA-only, need absolute maximum performance
- **Text-Generation-Inference**: Already in HuggingFace ecosystem

## Common issues

**Issue: Out of memory during model loading**

Reduce memory usage:
```bash
vllm serve MODEL \
  --gpu-memory-utilization 0.7 \
  --max-model-len 4096
```

Or use quantization:
```bash
vllm serve MODEL --quantization awq
```

**Issue: Slow first token (TTFT > 1 second)**

Enable prefix caching for repeated prompts:
```bash
vllm serve MODEL --enable-prefix-caching
```

For long prompts, enable chunked prefill:
```bash
vllm serve MODEL --enable-chunked-prefill
```

**Issue: Model not found error**

Use `--trust-remote-code` for custom models:
```bash
vllm serve MODEL --trust-remote-code
```

**Issue: Low throughput (<50 req/sec)**

Increase concurrent sequences:
```bash
vllm serve MODEL --max-num-seqs 512
```

Check GPU utilization with `nvidia-smi` - should be >80%.

**Issue: Inference slower than expected**

Verify tensor parallelism uses power of 2 GPUs:
```bash
vllm serve MODEL --tensor-parallel-size 4  # Not 3
```

Enable speculative decoding for faster generation:
```bash
vllm serve MODEL --speculative-model DRAFT_MODEL
```

## Advanced topics

**Server deployment patterns**: See [references/server-deployment.md](references/server-deployment.md) for Docker, Kubernetes, and load balancing configurations.

**Performance optimization**: See [references/optimization.md](references/optimization.md) for PagedAttention tuning, continuous batching details, and benchmark results.

**Quantization guide**: See [references/quantization.md](references/quantization.md) for AWQ/GPTQ/FP8 setup, model preparation, and accuracy comparisons.

**Troubleshooting**: See [refere