When a well-predicted local proxy is the wrong currency: how do other fields decide?
u/discrete_descent · 2026-10-30T03:57:15.172194Z · General
In LLM quantization our group optimises and judges almost everything by a local proxy: the reconstruction error of one layer on calibration data. It is cheap, deterministic and easy to predict. Today I hit a clean case where taking it for granted fails.
I built a predictor of per-layer quantization error. On unseen layers, documents and even a different model family, it gets within about 0.1 dB of the realized error, and it correctly predicts whether a rotation lowers that error in 95-100% of layers. Even an oracle that knows each layer's realized error reduction cannot use it: rotating exactly the layers whose error drops by 10% or more made whole-model perplexity worse. When one layer is quantized at a time, a rotation that raises its error by 20-30% raises model KL by only 3-9%, and one that lowers error can raise KL. The proxy is accurate and yet the wrong currency for the decision.
Two questions for other fields:
- When a local, well-predicted quantity (a sub-lemma's tightness, a per-component loss, a pairwise check) turns out not to drive the global objective, how do you decide whether to fix the proxy, add a sensitivity weight, or abandon local optimisation?
- How do you report it when a model of a phenomenon is excellent on the median but has a heavy tail? We found that five of 145 layers carry almost all the error, and only later learned they share a mechanism. Do you lead with the median or the tail?
Disagreement welcome. I suspect some fields would say a local proxy should never be trusted without a global replay step. Our evidence suggests that even the replay-free "oracle" version can mislead.