Why We Built Bitweave: Sub-Millisecond Hybrid Retrieval in <1.1 MB RSS Memory

When building local RAG (Retrieval-Augmented Generation) applications, edge agents, or serverless AI pipelines, developers usually hit a wall with standard vector stores: memory overhead. Running a dedicated vector database locally often demands hundreds of megabytes—or gigabytes—of RAM just to keep indices warm. On the flip side, lightweight local options like scanning raw JSON files or querying SQLite don't scale well when vector dimensions climb into the thousands (1536d+). We built Bitweave to solve this exact trade-off: a zero-copy, SIMD-accelerated hybrid retrieval engine in Rust (with Python bindings) that handles categorical filtering and vector search while locking its active heap footprint under 1.1 MB RSS. The Architecture: How Bitweave Achieves Sub-Millisecond Speed at <1.1 MB RAM Bitweave relies on a 3-part design to maximize search speed while keeping memory consumption negligible: [ Categorical Filters ] ---> Bit-Sliced Bitmaps │ ▼ [ Query Vector (1536d) ] --> 1-Bit SIMD Pre-Filtering (Hamming Distance) │ (Top K Candidates) ▼ [ Raw Embeddings Buffer ] -> Zero-Copy Float32 Rescoring (exact_rescore=True) │ ▼ Top-K Results Array (NumPy) Zero-Copy Memory Mapping (memmap2) Instead of deserializing index files into Python RAM or Rust heap space, Bitweave uses memory-mapped files (.bweave). The operating system's page cache handles lazy loading of index segments directly from disk into virtual address space. As a result, the active RSS memory footprint remains static around 1.1 MB, whether your index holds 5,000 or 200,000 records. 1-Bit Vector Quantization & SIMD Hamming Distance High-dimensional float32 vectors (1536d) are quantized down to 1-bit sign masks (where values > 0 map to 1 and <= 0 map to 0). During pre-ranking, Bitweave uses SIMD bitwise XOR and POPCNT operations to compute Hamming distances across candidate vectors in microseconds. Zero-Copy 2-Pass Float32 Rescoring (exact_rescore=True) Quantization speeds up initial candidate selection, but f