arXiv:2608.29702cs.CL2026-08

嵌入表中的短行簇会夸大内在维度估计,移除后结果更真实。

A Hub of Short Rows Inflates Intrinsic Dimension Estimation of Token Embeddings

  • 发现嵌入表近原点存在短行簇,干扰邻近估计结果。
  • 移除短行簇后,11个模型的维度从波动变为稳定在10-17之间。
  • 归一化处理可替代移除,同样降低错误维度估计。

词嵌入表在原点附近存在一个由短行构成的簇,该簇会偏差最近邻内在维度(ID)估计结果。由于测度集中效应,任一词向量到中心簇的距离均小于到其他词向量的距离,导致其前两个邻居均为簇内行且距离相近,致使如TwoNN等估计器报告的维度远高于真实值。单个词测量时维度呈重尾分布;全词汇测量时,维度随模型参数量增长。但移除该簇后,重尾消失,11个模型(从GPT-2到K3、GLM-4.7)的维度收缩至窄区间。仅数百行短行即足以完全夸大估计值。我们复现了Pythia模型中嵌入表内在维度从160M到12B参数时由27增至122的结果,但移除簇后,所有规模下维度稳定在10至17之间。该簇包含被欠训练检测器标记的子集,但在Pythia上,整个簇在训练中被更新,表明其特征仅为长度而非未更新。最后证明,对行进行归一化可获得与移除相同的效果。

原文摘要 · Abstract (English)

A token-embedding table holds a hub of short rows near its origin, and we show that this cluster biases what nearest-neighbor intrinsic-dimension (ID) estimators report. Because of the concentration of measure, a token is closer to the central cluster than to any other token, so its first two neighbors are both hub rows at nearly the same distance. As a result, the ID estimators such as TwoNN return a dimension far above the real ID. Measured one token at a time, dimension is a heavy-tailed distribution. Measured on the full vocabulary, it grows with the model's parameter count. However, when we remove the hub, the heavy tail disappears and the measured dimension collapses to a narrow range for eleven models, from GPT-2 to models such as K3 and GLM-4.7. The hub acts as a switch: a few hundred rows are enough to fully inflate the estimate. We reproduced an experiment stating that the intrinsic dimension (ID) of Pythia's token-embedding table grows with the parameter count, from $27$ to $122$ between 160M and 12B parameters. We show that this result disappears when the hub is removed: the table then reads $10$ to $17$ at every size. The hub contains a subset of the population that under-trained-token detectors flag, but on Pythia the hub that we detected and removed as a whole was updated during training: what seem to characterize these rows is simply their length, not an absence of updates. Finally, we show that normalizing the rows instead of removing them gives the same lower reading.

嵌入分析内在维度模型机制归一化

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