Fourier Magnitude KV Cache Quantization

A technical analysis of Fourier Magnitude KV Cache Quantization concludes that while the original claim of Fourier/phase being uniquely special is not supported, a substantive signal remains: the exact K representation being quantized matters, and low-bit perturbation of the K stored after normalization and RoPE can be genuinely fragile. The analysis, based on the Gemma 3 implementation in Hugging Face Transformers v5.15.0, suggests that the effect survives a post-forward storage-only control, indicating a real cache-reuse phenomenon, and recommends focusing on K quantizer geometry and joint K/V rate accounting for practical low-bit cache improvements.

It looks like you’ve pushed the exploration quite a bit further: My short version is: I think the correction is substantive, but I do not think it removes the interesting signal. It narrows it into something more testable. What I would carry forward from the current state is less: Fourier / phase is uniquely special selective layers are redundant early decode anchors give a sub-int8 recipe and more: the exact K representation being quantized matters a lot, and low-bit perturbation of the K that is actually stored/reused after normalization + RoPE can be genuinely fragile. I would probably make that tensor/locus distinction the organizing boundary from here. For current Transformers Gemma 3, the K path is essentially: hidden state ↓ k proj ↓ k norm ↓ RoPE ↓ past key values.update K, V, ... ↓ attention You can see that ordering directly in the Gemma 3 implementation in Transformers https://github.com/huggingface/transformers/blob/v5.15.0/src/transformers/models/gemma3/modular gemma3.py . That means a hook on k proj is testing a different object from the post-KNorm/post-RoPE K entering the cache. Once I separated those, the picture became much easier to interpret. My default route from here would be: intervention/storage locus ↓ storage-only mechanism ↓ teacher-forced distributional validation ↓ better K quantizer geometry ↓ joint K/V rate accounting ↓ only then packed bytes / kernels / serving claims The part I found most convincing is that the effect still survives a post-forward storage-only control : keeping the forward that creates a K state completely full precision, then demoting only the persistent cached copy, can still alter later logits and later generation. So there does seem to be a real cache-reuse phenomenon left after the retractions. What seems to survive the selective-layer correctionIf I were choosing the next path by goal, I would use something like this: If the goal is to understand the mechanism: corrected storage-only teacher-forced KL/NLL ↓ locate which older prompt regions / K states matter ↓ test whether sensitivity tracks age, content, attention, or quantizer error If the goal is a practical low-bit cache: stop increasing the FP window for now ↓ improve K quantizer geometry ↓ compare against a strong int8 / KIVI-like baseline ↓ reintroduce a residual only if it buys a real rate-quality improvement If the goal is a deployable codec: first get a fidelity-successful K/V policy ↓ define an actual packed representation ↓ count metadata and real bytes ↓ then measure latency / throughput / context scaling So, from my side, I would not read the corrections as the exploration collapsing. The interesting part seems to have narrowed from a fairly broad Fourier/anchor story into a cleaner question about K representation, cache locus, and future error propagation . That is a smaller claim, but it also looks much easier to falsify, compare against existing KV methods, and turn into a reproducible next experiment.