nano-vllm/CLAUDE.md

CLAUDE.md

This file provides guidance to Claude Code when working with this repository.

Overview

Nano-vLLM is a lightweight vLLM implementation (~1,200 lines) for fast offline LLM inference. Supports Qwen3 models with CPU offload for long-context inference.

Documentation Index

Document	Purpose
docs/architecture_guide.md	Core components, CPU offload system design, ring buffer architecture, stream configuration
docs/sparse_policy_architecture.md	SparsePolicy abstraction: prefill/decode delegation, pipeline modes, policy implementations
docs/sparse_policy_implementation_guide.md	How to implement custom SparsePolicy: required methods, hooks, ring buffer pipeline pattern
docs/sparse_attention_guide.md	Block sparse attention methods (XAttention, FlexPrefill, MInference, AvgPool, Quest), computation flow, algorithms
docs/xattention_algorithm_guide.md	XAttention 算法详解: stride reshape、Triton kernels、BSA 依赖、块选择算法
docs/block_sparse_attn_interface.md	BSA (Block Sparse Attention) 接口文档: 函数签名、使用示例、约束条件
docs/debugging_guide.md	PyTorch hooks for debugging, hook positions, tensor comparison, memory profiling
docs/optimization_guide.md	Performance optimizations: sgDMA (15x), Triton merge (4.3x), N-way pipeline (2x)
docs/known_issues.md	Documented bugs and fixes: partial last block bug, block size 4096 race condition
docs/ruler_benchmark_results_32k.md	RULER benchmark results (32K context): 13 tasks, 92.3% accuracy, CPU offload performance
docs/ruler_32k_chunked_offload_issue.md	⚠️ OPEN ISSUE: 32K chunked offload accuracy problem (20% error rate in RULER)
docs/chunked_attention_solutions.md	🔧 SOLUTIONS: Chunked attention 准确性问题的代码分析和解决方案

GPU Mutex for Multi-Instance Debugging

IMPORTANT: When running multiple Claude instances for parallel debugging, different rules apply based on script type:

Benchmarks (bench*.py) - Exclusive GPU Access Required

Before running any bench*.py script, Claude MUST wait for exclusive GPU access:

# Check and wait for GPU to be free
while [ -n "$(nvidia-smi --query-compute-apps=pid --format=csv,noheader)" ]; do
  echo "GPU busy, waiting 10s..."
  sleep 10
done

Other Scripts (tests, examples) - No Special Requirements

For non-benchmark scripts, exclusive GPU access is NOT required. Multiple nanovllm processes can run simultaneously on different GPUs - each process automatically selects a unique port for torch.distributed communication.

Multi-Instance Development with PYTHONPATH

IMPORTANT: When running multiple Claude instances on different worktrees, do NOT use pip install -e . globally as it will affect other instances.

Use PYTHONPATH directly - no pip install needed:

# Set PYTHONPATH to point to the project root directory
PYTHONPATH=/path/to/your/worktree:$PYTHONPATH python <script.py>

# Example: running tests
PYTHONPATH=/home/zijie/Code/nano-vllm:$PYTHONPATH python tests/test_needle.py

Benefits:

• No pip install required
• Code changes take effect immediately (no reinstall needed)
• Each worktree is completely isolated

Configuration

Parameter	Default	Notes
kvcache_block_size	1024	Tokens per block (4096 now works after race condition fix)
max_num_batched_tokens	16384	Set = max_model_len for long context
gpu_memory_utilization	0.9	GPU memory fraction
enable_cpu_offload	False	Enable for long context
enforce_eager	False	Set True to disable CUDA graphs

Benchmarking

Files: bench.py (GPU), bench_offload.py (CPU offload), bench_vllm.py (comparison)

Common Issues:

1. max_num_batched_tokens < max_model_len: Set equal for long context
2. CUDA graph dimension mismatch: Ensure input_len + output_len <= max_model_len
3. RoPE out of bounds: Check model's max_position_embeddings in config.json

Model Limits:

• Qwen3-0.6B/4B: 40960 tokens
• Qwen2.5-7B-Instruct-1M: 1048576 tokens

Performance (Qwen3-0.6B):

• GPU: ~18k tok/s (prefill), ~100 tok/s (decode)
• CPU Offload (16K): ~14k tok/s (prefill)
• CPU Offload (32K): ~13k tok/s (prefill)

---

Author: Zijie Tian