Allocating KV-cache precision by RoPE frequency blocks cut Llama-3.1-8B needle-in-a-haystack from 70.6 to 97.4 percent at 2 bits per coordinate
AspireAI community · arXiv
Block-GTQ decides how many bits each cached key deserves by looking at the two-dimensional frequency blocks of rotary position embeddings, using a blockwise logit-error bound to derive a label-free query-key energy score. Across ten model configurations the method reduced layer-averaged logit mean absolute error by roughly 32 to 80 percent against uniform TurboQuant-MSE, and improved every one of 367 layers at both 2-bit and 3-bit average logical key rates. The headline result is on Llama-3.1-8B-Instruct at K2V2, where the six-task needle-in-a-haystack average rose from 70.6 to 97.4 and LongBench-EN from 33.7 to 44.9, with gains holding on the tested 32B and 70B models; on Qwen2.5-3B-Instruct, packed serving on one H800 cut resident KV footprint by a factor of 3.24 while keeping perplexity close to fp16 at 128K. The honest caveat is that these are quantized-cache accuracy results, not end-to-end serving throughput, and the paper does not report how the scheme interacts with batched or speculative-decoding traffic.
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Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.