REVIEW 2 major objections 5 minor 1 cited by
Improving Block-Wise LLM Quantization by 4-bit Block-Wise Optimal Float (BOF4): Analysis and Variations
T0 review · 2 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read The paper shows NF4 and AF4 optimize the wrong objective and derives BOF4, 4-bit block-wise codebooks that minimize the true weight quantization error; signed and outlier-preserving variants reach the best data-free 4-bit perplexity.
desk verdict Useful and mostly sound incremental improvement for block-wise 4-bit LLM quantization; the MSE side is solid, but the MAE-optimality derivation in Appendix B.2.2 has a real missing term and the 'information-theoretically optimal' label oversells it. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is a modified Lloyd's algorithm for block-wise absmax quantization. In each maximization step, the reconstruction level of a Voronoi region is updated not to the plain mean or median of its normalized samples but to a weighted version: for MSE, $\hat{x}(\ell)=\sum_k w_k^2 x_k \big/ \sum_k w_k^2$, where $x_k$ are the normalized weights falling in the region and $w_k$ is the block maximum of the block containing each sample; for MAE, the weighted median with weights $w_k$. The paper proves these centroid conditions by deriving them from the end-to-end quantization error, and shows that the theoretical integral solution and the Monte-Carlo solution agree to about $-56\,\text{dB}$ of relative squared error. BOF4-S rests on the companion mechanism of signed absmax normalization: choosing as the block constant the sign-carrying maximum weight means only the $+1$ endpoint is a mandated reconstruction level, so the level that would otherwise be spent on $-1$ can be re-placed to lower the error.
What would settle it
Run the paper's Monte-Carlo centroid rule on the actual per-layer weights of a large pretrained LLM, quantize with the resulting per-layer codebooks, and compare MAE, MSE, and perplexity against the fixed Gaussian-derived BOF4-S codebook at the same block size. If the data-driven codebooks do not measurably beat the Gaussian-derived ones, the distributional assumption is adequate and the claimed optimality stands; if they do beat it, the claim of optimality is limited by that assumption.
Extended reading notes
Core claim
The paper claims that the optimality story behind NF4 is wrong on two counts: equal utilization of the 16 reconstruction levels is not a valid criterion for rate-distortion optimality, and the AF4 correction still minimizes the error of the normalized weights, not the error of the weights that are actually quantized. In block-wise absmax quantization a weight is reconstructed as $m\hat{x}$ from its normalized value $x$ and the block maximum $m$, so the end-to-end squared error is $m^2(x-\hat{x})^2$; the correct MSE centroid of a Voronoi region is therefore the mean of its normalized samples weighted by squared block maxima, and the correct MAE centroid is the median weighted by block maxima. Feeding these modified centroid updates into Lloyd's algorithm yields the BOF4 codebooks, and changing the normalization—using the signed absolute maximum so that only the $+1$ endpoint is a fixed reconstruction level—yields BOF4-S, which reduces the error further. The paper further claims that outlier weights violate the Gaussian assumption on which the analysis rests, and its outlier-preserving quantization (OPQ) responds by storing those outliers in bfloat16, replacing them with zero before normalization and thereby improving perplexity, especially at large block sizes.
Load-bearing premise
The codebooks are derived under the assumption that model weights are independent draws from a single zero-mean symmetric distribution, taken as Gaussian; real LLM weights have layer-dependent and outlier structure that violates this, and the paper's own OPQ mechanism exists precisely because of that failure.
Editorial extensions
If this is right
- Data-free 4-bit block-wise quantization of LLM weights can be made lower in MAE and MSE than NF4 and AF4 at zero extra memory cost, since only the codebook values change.
- Signed absmax normalization (BOF4-S) is a stand-alone accuracy gain that transfers to any block-wise quantizer, not just the codebooks derived here.
- MSE-optimized codebooks generally give lower perplexity than MAE-optimized ones, making the error metric of codebook optimization itself a design choice with measurable language-modeling consequences.
- Outlier-preserving quantization keeps perplexity low at larger block sizes, reducing the memory used by quantization constants.
- QLoRA-style fine-tuning inherits the ranking: instruction-following and code-generation models fine-tuned on BOF4-based weights match or exceed those fine-tuned on NF4 or AF4 weights.
Reading between the lines
- The $m^2$- and $m$-weighted centroid rules are not specific to LLM weights: any block-scaled scalar quantizer, including block-wise optimizer-state quantization, suffers the same objective mismatch, so re-deriving its codebooks with the same argument is a direct testable transfer.
- Because the Monte-Carlo and theoretical codebooks coincide, the natural next experiment is to run the centroid rule on real per-layer weight samples; if per-layer codebooks outperform the fixed Gaussian-derived one, the 'optimal' label moves from one global codebook to a data-dependent family.
- The OPQ threshold $q$ is fixed against a theoretical distribution of block maxima; treating it as a per-layer or per-tensor knob rather than a global hyperparameter is a testable way to trade memory against perplexity, especially for small models where OPQ showed the largest gains.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies 4-bit block-wise absmax quantization of LLM weights, as used in QLoRA-style fine-tuning. It proposes BOF4 and BOF4-S, codebooks obtained by a Lloyd-style EM algorithm with centroid rules that minimize the end-to-end MSE or MAE of the unnormalized weights, rather than the error of the normalized weights. It also proposes signed absmax normalization (BOF4-S), an outlier-preserving mixed-precision scheme (OPQ), and reports experiments on Llama, Qwen, and Mistral models showing lower quantization error and perplexity than NF4 and AF4. A theoretical solution and a Monte-Carlo empirical solution are both presented, with a numerical equivalence check for the MSE case.
Significance. The MSE-based BOF4-S results, if correct, would be a useful, reproducible contribution: the modified centroid derivation is nontrivial, the codebooks are published, the MSE theoretical/empirical agreement at -56 dB is a strong sanity check, and the perplexity improvements over NF4/AF4, while small, are consistent across several models. The OPQ mechanism is a practical addition for handling outliers without abandoning the Gaussian weight assumption. However, the paper's central theoretical novelty is weakened by an algebraic error in the MAE centroid derivation, which is one of the two optimality results claimed in the abstract. Since the error is local and the practical results are dominated by MSE optimization, the contribution is defensible after a substantial revision.
major comments (2)
- [Appendix B.2.2, Eq. (59) and Section 3.2, Eq. (7)] The derivation of the MAE centroid condition is algebraically incorrect. Starting from Eq. (57) and substituting pM(m|X∈R_l) = pM(m) A / P[X∈R_l] with A = FX(ξ(l)|M=m) - FX(ξ(l-1)|M=m), and the truncated CDF FX(x̂|M=m,X∈R_l) = (FX(x̂|M=m) - FX(ξ(l-1)|M=m))/A, one obtains ∫ m pM(m)[FX(x̂|M=m) - FX(ξ(l-1)|M=m) - A/2] dm = 0. Equation (59) omits the term -FX(ξ(l-1)|M=m). This is not a notational variant: for a uniform source on [-1,1] and region [0,1], Eq. (59) gives x̂ = -0.5, whereas the correct MAE-optimal level is the median 0.5. Consequently, the MAE-optimized codebooks in Table 6 are not proven to minimize MAE even under the paper's Gaussian model, and the claimed equivalence between the theoretical and data-driven MAE solutions (Eq. (8) vs. Eq. (59)) is unverified; Table 8 covers MSE only. The theoretical MAE derivation must be corrected or the MAE-optimality claims must be withdrawn.
- [Abstract, Section 1, and Section 3.2] The codebooks are described as 'information-theoretically optimal' and 'the correct, information-theoretically optimal codebook'. Appendix B.2 itself states that the modified centroid criterion establishes only local optimality of a Lloyd-algorithm fixed point. Lloyd's algorithm is not guaranteed to find a global optimum, so the unqualified 'information-theoretically optimal' overstates the result. Please replace with 'locally optimal under the stated necessary conditions' or equivalent wording.
minor comments (5)
- [Section 3.2, Eq. (7)] The bracket notation in Eq. (7) contains a typographical artifact ('FX(ˆx(ℓ)|M =m)]− 1/2'); the intended expression appears to be FX(ˆx(ℓ)|M =m) − 1/2 [FX(x|M =m)]ξ(ℓ)ξ(ℓ−1), matching Eq. (59). Please correct the typesetting.
- [Tables 1, 3, 4 and Appendix G] The reported perplexity differences between methods are small (e.g., 8.43 vs. 8.53 for Llama-3.1 8B in Table 1), and the fine-tuning experiments in Tables 3 and 4 involve stochastic training runs without variance or multiple seeds. Please report standard deviations or multiple seeds for the fine-tuning evaluations to support the ranking claims.
- [Appendix C, Table 8] The numerical equivalence between the theoretical and empirical centroid solutions is demonstrated only for MSE. Given the MAE derivation issue, please provide an analogous comparison for MAE or explicitly state that equivalence was verified only for MSE.
- [Title page, Appendix F, References] Please fix the typos: 'Insitute' should be 'Institute' (title page), 'qunaitzation' should be 'quantization' (Appendix F), 'V oronoi' should be 'Voronoi' (Section 3.2), and reference [22] should list 'AAAI' instead of 'AIII'.
- [Section 6, Limitations] The limitation discussion of double quantization is useful. Please also state explicitly that the OPQ threshold q was tuned on a validation split (Appendix E.2), which introduces a small data-dependent element, and briefly discuss how this interacts with the 'data-free' characterization of the method.
Circularity Check
No significant circularity: BOF4(-S) codebooks are derived from an external Gaussian model and validated against external baselines, not from the evaluation data.
full rationale
The central derivation is self-contained: the BOF4(-S) codebooks are obtained by minimizing the end-to-end MSE or MAE of block-wise absmax-quantized weights under an assumed zero-mean symmetric, specifically Gaussian, weight distribution, with closed-form centroid conditions in Eqs. (5) and (7) and matching Monte-Carlo estimators in Eqs. (6) and (8). The comparisons to NF4 and AF4 are made against externally fixed codebooks on fixed LLM weights, so the reported error reductions are not forced by construction. The Gaussian i.i.d. assumption is an external modeling assumption, not an import of the target result, and the empirical/theoretical equivalence check in Table 8 is a consistency test of the same objective, not a disguised fit. The OPQ hyperparameter q is selected on a validation split, but this is ordinary hyperparameter tuning rather than a fitted parameter renamed as a prediction, and it does not enter the derivation of the codebooks. No load-bearing self-citation or imported uniqueness theorem is used; Lloyd's algorithm is standard prior work. The algebraic concern about Eq. (59) raised in the skeptical note concerns the correctness of the MAE centroid derivation, which falls under mathematical validity rather than circularity, and is therefore not scored here.
Assumptions & free parameters
free parameters (2)
- OPQ quantile q =
0.95
- Block size I =
64 (main experiments); codebooks tabulated for 32, 64, 128, 256
assumptions (4)
- domain assumption Network weights are i.i.d. samples from a zero-mean symmetric distribution, specialized to N(0,1).
- standard math The nearest-neighbor assignment is a necessary condition for optimality in the modified Lloyd algorithm.
- domain assumption Within a block, the largest-magnitude weight is unique, so the discrete mass points of normalized weights are at +/-1 (absolute) or +1 (signed).
- standard math Gaussian integral identity (Owen 1980, eq. 101) used to simplify the MSE centroid for Gaussian weights.
Cite this review
Pith. "Pith review of Improving Block-Wise LLM Quantization by 4-bit Block-Wise Optimal Float (BOF4): Analysis and Variations." pith.science (2026). https://pith.science/paper/RVMXILUT
@misc{pith2026250506653,
author = {Pith},
title = {Pith review of: Improving Block-Wise LLM Quantization by 4-bit Block-Wise Optimal Float (BOF4): Analysis and Variations},
year = {2026},
howpublished = {\url{https://pith.science/paper/RVMXILUT}},
note = {Machine review of arXiv:2505.06653}
}
read the original abstract
Large language models (LLMs) demand extensive memory capacity during both fine-tuning and inference. To enable memory-efficient fine-tuning, existing methods apply block-wise quantization techniques, such as NF4 and AF4, to the network weights. We show that these quantization techniques incur suboptimal quantization errors. Therefore, as a first novelty, we propose an optimization approach for block-wise quantization. Using this method, we design a family of quantizers named 4-bit block-wise optimal float (BOF4), which consistently reduces the quantization error compared to both baseline methods. We provide both a theoretical and a data-driven solution for the optimization process and prove their practical equivalence. Secondly, we propose a modification to the employed normalization method based on the signed absolute block maximum (BOF4-S), enabling further reduction of the quantization error and empirically achieving less degradation in language modeling performance. Thirdly, we explore additional variations of block-wise quantization methods applied to LLMs through an experimental study on the importance of accurately representing zero and large-amplitude weights on the one hand, and optimization towards various error metrics on the other hand. Lastly, we introduce a mixed-precision quantization strategy dubbed outlier-preserving quantization (OPQ) to address the distributional mismatch induced by outlier weights in block-wise quantization. By storing outlier weights in 16-bit precision (OPQ) while applying BOF4-S, we achieve top performance among 4-bit block-wise quantization techniques w.r.t. perplexity.
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Forward citations
Cited by 1 Pith paper
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