Title: HAPPI: Efficient KV cache compression with Hadamard PCA-based Power iteration
Primary area: other topics in machine learning (i.e., none of the above)
Keywords: Low-rank approximation, KV Cache Compression, LLM Inference Optimization

Abstract: Truncated Singular Value Decomposition (SVD) has recently attracted renewed attention for its effectiveness in model optimizations, such as LoRA initialization and KV-cache compression. However, exact SVD remains computationally expensive, while approximate methods like power iteration often introduce non-negligible errors. In this paper, we present Hadamard PCA-based Power Iteration (HaPPI), a new algorithm that significantly improves the accuracy of low-rank approximation while retaining efficiency. Compared to prior methods, HaPPI achieves the lowest approximation error at a practical computational cost. Building on this foundation, we further propose HaPPI-KV, which combines HaPPI with key whitening and residual quantization to deliver high compression ratios for key–value caches. By leveraging both the efficiency and precision of HaPPI, HaPPI-KV achieves state-of-the-art trade-offs between memory efficiency and model quality, highlighting the superiority of our approach.