LEANN/docs/configuration-guide.md

LEANN Configuration Guide

This guide helps you optimize LEANN for different use cases and understand the trade-offs between various configuration options.

Getting Started: Simple is Better

When first trying LEANN, start with a small dataset to quickly validate your approach:

For document RAG: The default data/ directory works perfectly - just a few PDFs let you test in minutes

python -m apps.document_rag --query "What techniques does LEANN use?"

For other data sources: Limit the dataset size for quick testing

# WeChat: Test with recent messages only
python -m apps.wechat_rag --max-items 100 --query "昨天聊了什么"

# Browser history: Last few days
python -m apps.browser_rag --max-items 500 --query "AI papers I read"

# Email: Recent inbox
python -m apps.email_rag --max-items 200 --query "meeting schedules"

Once validated, scale up gradually:

• 100 documents → 1,000 → 10,000 → full dataset
• This helps identify issues early before committing to long processing times

Embedding Model Selection: Understanding the Trade-offs

Based on our experience developing LEANN, embedding models fall into three categories:

Small Models (< 100M parameters)

Example: sentence-transformers/all-MiniLM-L6-v2 (22M params)

• Pros: Lightweight, fast for both indexing and inference
• Cons: Lower semantic understanding, may miss nuanced relationships
• Use when: Speed is critical, handling simple queries, on interactive mode or just experimenting with LEANN

Medium Models (100M-500M parameters)

Example: facebook/contriever (110M params), BAAI/bge-base-en-v1.5 (110M params)

• Pros: Balanced performance, good multilingual support, reasonable speed
• Cons: Requires more compute than small models
• Use when: Need quality results without extreme compute requirements, general-purpose RAG applications

Large Models (500M+ parameters)

Example: Qwen/Qwen3-Embedding-0.6B (600M params), intfloat/multilingual-e5-large (560M params)

• Pros: Best semantic understanding, captures complex relationships, excellent multilingual support
• Cons: Slower inference, longer index build times
• Use when: Quality is paramount and you have sufficient compute resources

Cloud vs Local Trade-offs

OpenAI Embeddings (text-embedding-3-small/large)

• Pros: No local compute needed, consistently fast, high quality
• Cons: Requires API key, costs money, data leaves your system, known limitations with certain languages
• When to use: Prototyping, non-sensitive data, need immediate results

Local Embeddings

• Pros: Complete privacy, no ongoing costs, full control, can sometimes outperform OpenAI embeddings
• Cons: Slower than cloud APIs, requires local compute resources
• When to use: Production systems, sensitive data, cost-sensitive applications

Index Selection: Matching Your Scale

HNSW (Hierarchical Navigable Small World)

Best for: Small to medium datasets (< 10M vectors)

• Full recomputation required
• High memory usage during build phase
• Excellent recall (95%+)

# Optimal for most use cases
--backend-name hnsw --graph-degree 32 --build-complexity 64

DiskANN

Best for: Large datasets (> 10M vectors, 10GB+ index size)

• Uses Product Quantization (PQ) for coarse filtering during graph traversal
• Recomputes only top candidates for exact distance calculation

# For billion-scale deployments
--backend-name diskann --graph-degree 64 --build-complexity 128

LLM Selection: Engine and Model Comparison

LLM Engines

OpenAI (--llm openai)

• Pros: Best quality, consistent performance, no local resources needed
• Cons: Costs money ($0.15-2.5 per million tokens), requires internet, data privacy concerns
• Models: gpt-4o-mini (fast, cheap), gpt-4o (best quality), o3-mini (reasoning, not so expensive)

Ollama (--llm ollama)

• Pros: Fully local, free, privacy-preserving, good model variety
• Cons: Requires local GPU/CPU resources, slower than cloud APIs, need to pre-download models by ollama pull
• Models: qwen3:1.7b (best general quality), deepseek-r1:1.5b (reasoning)

HuggingFace (--llm hf)

• Pros: Free tier available, huge model selection, direct model loading (vs Ollama's server-based approach)
• Cons: More complex initial setup
• Models: Qwen/Qwen3-1.7B-FP8

Parameter Tuning Guide

Search Complexity Parameters

--build-complexity (index building)

• Controls thoroughness during index construction
• Higher = better recall but slower build
• Recommendations:
  • 32: Quick prototyping
  • 64: Balanced (default)
  • 128: Production systems
  • 256: Maximum quality

--search-complexity (query time)

• Controls search thoroughness
• Higher = better results but slower
• Recommendations:
  • 16: Fast/Interactive search (500-1000ms on consumer hardware)
  • 32: High quality with diversity (1000-2000ms)
  • 64+: Maximum accuracy (2000ms+)

Top-K Selection

--top-k (number of retrieved chunks)

• More chunks = better context but slower LLM processing
• Should be always smaller than --search-complexity
• Guidelines:
  • 3-5: Simple factual queries
  • 5-10: General questions (default)
  • 10+: Complex multi-hop reasoning

Trade-off formula:

• Retrieval time ∝ log(n) × search_complexity
• LLM processing time ∝ top_k × chunk_size
• Total context = top_k × chunk_size tokens

Graph Degree (HNSW/DiskANN)

--graph-degree

• Number of connections per node in the graph
• Higher = better recall but more memory
• HNSW: 16-32 (default: 32)
• DiskANN: 32-128 (default: 64)

Performance Optimization Checklist

If Embedding is Too Slow

1. Switch to smaller model:

   # From large model
   --embedding-model Qwen/Qwen3-Embedding
   # To small model
   --embedding-model sentence-transformers/all-MiniLM-L6-v2
   

2. Use MLX on Apple Silicon:

   --embedding-mode mlx --embedding-model mlx-community/multilingual-e5-base-mlx
   

3. Limit dataset size for testing:

   --max-items 1000  # Process first 1k items only
   

If Search Quality is Poor

1. Increase retrieval count:

   --top-k 30  # Retrieve more candidates
   

2. Tune chunk size for your content:

   • Technical docs: --chunk-size 512
   • Chat messages: --chunk-size 128
   • Mixed content: --chunk-size 256

3. Upgrade embedding model:

   # For English
   --embedding-model BAAI/bge-base-en-v1.5
   # For multilingual
   --embedding-model intfloat/multilingual-e5-large
   

Understanding the Trade-offs

Every configuration choice involves trade-offs:

Factor	Small/Fast	Large/Quality
Embedding Model	all-MiniLM-L6-v2	Qwen/Qwen3-Embedding-0.6B
Chunk Size	512 tokens	128 tokens
Index Type	HNSW	DiskANN
LLM	qwen3:1.7b	gpt-4o

The key is finding the right balance for your specific use case. Start small and simple, measure performance, then scale up only where needed.

Deep Dive: Critical Configuration Decisions

When to Disable Recomputation

LEANN's recomputation feature provides exact distance calculations but can be disabled for extreme QPS requirements:

--no-recompute  # Disable selective recomputation

Trade-offs:

• With recomputation (default): Exact distances, best quality, higher latency, minimal storage (only stores metadata, recomputes embeddings on-demand)
• Without recomputation: Must store full embeddings, significantly higher memory and storage usage (10-100x more), but faster search

Disable when:

• You have abundant storage and memory
• Need extremely low latency (< 100ms)
• Running a read-heavy workload where storage cost is acceptable

Further Reading

• Lessons Learned Developing LEANN
• LEANN Technical Paper
• DiskANN Original Paper