REVIEW 4 major objections 6 minor 21 references
Stable FP4 LLM training works once forward and backward passes use the same block scales after transposition.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.5
2026-07-31 05:00 UTC pith:YXMOX7HI
load-bearing objection Useful 30B-scale FP4 training result with a clean diagnosis, but the 2D-geometry causal claim is not isolated from known stabilizers. the 4 major comments →
Stable FP4 Training via Transposition-Invariant Block Quantization
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is that transposition-induced scale mismatch in 1D microscaling is a primary failure mode of FP4 training, and that enforcing forward–backward scale consistency with 2D square-block FP4 quantization—plus truncation-free scaling, stochastic rounding, and MXFP8 for Q/K—is sufficient for stable end-to-end FP4 training at scales up to 30B parameters and 100B tokens with less than 1.3% degradation versus BF16.
What carries the argument
Transposition-invariant 2D block FP4 quantization: partition tensors into b×b square blocks that share one scale so a block and its transpose keep the same maximum and the same scale (S(B)=S(B⊤)), eliminating the forward/backward scale mismatch of 1D layouts.
Load-bearing premise
That software-emulated FP4 microscaling on hardware without native FP4 tensor cores behaves like real MXFP4/NVFP4 hardware for the stability conclusions that matter.
What would settle it
Train the same OLMo-1B/7B or 30B MoE recipe on native FP4 tensor-core hardware (or a verified bit-accurate emulator) with 1D vs 2D blocks only, and check whether 1D still shows the forward–backward scale mismatch and early divergence while 2D stays within ~1% of BF16 loss and downstream accuracy through 100B tokens.
If this is right
- Stable FP4 training at LLM scale can be reached mainly by making quantization transpose-invariant, not only by shrinking blocks or raising scale precision.
- Most transformer linear layers can stay FP4; only Q/K need higher precision (MXFP8) in this design.
- Weight memory and ideal linear throughput can improve substantially (paper estimates ~65% weight memory cut and up to ~3.6× linear throughput on OLMo-1B) once native kernels exist.
- Larger models can close the FP4–BF16 gap further once systematic scale bias is removed.
- Hardware and formats should expose square-block or otherwise transpose-consistent scaling paths, not only 1D microscaling.
Where Pith is reading between the lines
- Any low-precision path that secretly changes scales under the adjoint (transpose, reshape, gather) may hit the same bias even outside FP4.
- If consistency is the bottleneck, learned or adaptive 2D blocks and fused scale metadata could push past the 32×32 efficiency–accuracy trade-off without new number formats.
- Selective higher precision may generalize: other noise-amplifying ops (softmax, layer-norm, router logits in MoE) could be the next places to keep above FP4 rather than whole layers.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper identifies transposition-induced scale inconsistency in 1D microscaling formats (MXFP4/NVFP4) as a source of FP4 training instability: after transposition in backpropagation, values are regrouped into different scaling blocks, so forward and backward passes quantize the same tensor with different scales, biasing gradients under the STE. The proposed fix is 2D square-block FP4 quantization (32×32) for weights and gradients, which preserves block membership under transposition, combined with truncation-free scaling (to avoid overflow) and stochastic rounding in the backward pass (for unbiased gradients), plus MXFP8 for query/key projections. Experiments train OLMo-1B, OLMo-7B, and a Qwen 30B MoE on 100B tokens under software emulation, reporting stable convergence within ~0.8–1.6% of BF16 perplexity and ~1 point on downstream benchmarks.
Significance. If the results hold, this is a useful contribution to low-precision LLM training: stable end-to-end FP4 training up to 30B parameters / 100B tokens with ~1% of BF16 would be a practical step beyond FP8, and the transposition-invariance argument for 2D blocks is simple and mechanistically plausible. Notable strengths: a fully specified, reproducible training configuration (Appendix B: optimizer, schedule, batch size, quantization placement per tensor type); an explicit transposition-invariance proof for square blocks (Appendix D); matched hyperparameters across BF16 and FP4 variants so differences are attributable to the format; and evaluation against external BF16 baselines on public benchmarks rather than internal proxies. The paper is also appropriately candid about the software-emulation limitation (§4.1, §5), and its central design prediction — 2D block geometry should matter independently of scaling/rounding choices — is directly falsifiable with one additional experiment.
major comments (4)
- [§4.2, Fig. 4, Fig. 5, Table 4] The paper's central causal claim — that transposition-induced scale inconsistency is the dominant source of FP4 training instability, and that 2D block geometry is what fixes it — is never isolated experimentally. The only controlled ablations (Fig. 5) vary truncation-free scaling and stochastic rounding, both of which are established stabilizers in the cited prior work (e.g., refs [5, 6, 17]). Table 4 varies 2D block *size* (8×8 to 64×64) but never compares 1D vs 2D geometry at a matched recipe. Figure 4 compares against unnamed 'alternative microscaling schemes' whose configurations are unspecified. The load-bearing experiment is missing: 1D MXFP4 (1×32) with identical truncation-free scaling, stochastic rounding, and MXFP8 Q/K vs the proposed 2D-FP4 with the same components. This experiment would be especially informative because a 32×32 block shares one scale across 1024 values versu
- [§3.2, Table 2, Table 3, Appendix G] The MXFP8 Q/K variant consistently performs *worse* than the base 2D-FP4 variant across all reported metrics: Table 2 shows larger perplexity degradation for +MXFP8 at every scale (e.g., 1.6% vs 1.1% avg on OLMo-1B; 1.2% vs 0.9% on OLMo-7B), Table 3 shows lower average accuracy at every scale, and §4.2 acknowledges the loss gap 'increases to 1.6%.' Yet §3.2 motivates MXFP8 Q/K as necessary for stability ('directly quantizing Q and K to FP4 introduces errors that... distort the attention distribution'), and Appendix G reframes it as 'a stability-oriented mixed-precision option.' If the FP4-only configuration is uniformly closer to BF16 and equally stable in all reported runs, the case for the MXFP8 component is unsupported by the paper's own evidence. Either (a) show the FP4-Q/K configuration that fails or destabilizes (currently absent — the only divergence shown is 'naive FP4 without mi
- [Abstract, Table 2, Appendix B] The abstract states 'less than 1.3% degradation in perplexity and downstream accuracy,' but Table 2 reports 1.6% average perplexity degradation for the 2D-FP4+MXFP8 configuration on OLMo-1B, and Fig. 4/§4.2 repeat the 1.6% figure. The claim is only true for the base 2D-FP4 variant. Since the +MXFP8 configuration is presented as the proposed practical design (§3.2, Table 1, Appendix B where 'Query/Key Precision: MXFP8' appears in the default training configuration), the headline number is inconsistent with the paper's own tables. Please reconcile the abstract claim with Table 2, and clarify which configuration the claim refers to. Separately, Appendix B lists 'Query/Key Precision MXFP8' as the default for all models while the main text treats 2D-FP4 (without MXFP8) as 'Ours' — the default configuration should be stated unambiguously.
- [Fig. 4, §4.2] Figure 4 compares the method against 'alternative microscaling schemes' and 'competing methods' that are never named or cited in the caption or text, and no direct numerical comparison against the most relevant prior FP4 training work — MXFP4 training [17], NVFP4 pretraining [1], or Quartet [4] — appears anywhere in Tables 2–3. Given that [1] reports NVFP4 pretraining results and [4] reports native FP4 training, the absence of any controlled comparison to these baselines makes it impossible to assess the claimed improvement ('final loss gap approximately 1.1%, compared to around 2% for competing methods'). At minimum, the baselines in Fig. 4 must be identified, configured, and cited; ideally one named prior recipe should be run under the same token budget and hyperparameters.
minor comments (6)
- [Fig. 1] Fig. 1(d) reports 'effective weight deviation' but the quantity is never defined in the text or appendix. Please give a formula.
- [Table 1] Table 1's 'ideal linear throughput ~3.56×' and 'activation bandwidth ~0.30×' are described as 'idealized'; given that all experiments are software emulation (§4.1) and no kernel measurements exist, consider moving Table 1 to the appendix or clearly labeling it as a projection, since it does not reflect any measured system.
- [Table 4] Table 4 reports ranges ('~0.8–1.0', '~0.9–1.1') rather than single measurements, unlike Tables 2–3. Please report actual measured values with the model/token budget at which they were obtained.
- [Table 3, §4.2] The claim that FP4 exceeding BF16 on SciQ/COPA (Table 3) 'may be attributed to a regularization effect induced by quantization noise' is speculative; either provide supporting evidence (e.g., variance over seeds) or remove. Relatedly, no seed variance or error bars are reported anywhere — for gaps as small as 0.8–1.6%, run-to-run variance matters.
- [§3.1, Table 2] Notation: Qp and Qn are used in the scale formula (§3.1) before being defined; the E2M1 bounds should be stated explicitly. Also '∆' column header in Table 2 is undefined at first use (defined only in Appendix F).
- [§3, Appendix C] Activations use 1D-FP4 (Appendix B/C) while weights/gradients use 2D — but per the paper's own thesis, activations also participate in transposed backward GEMMs (∇X = ∇Y W). A brief discussion of why the 1D activation layout does not reintroduce the inconsistency the paper identifies would close an apparent gap in the argument.
Circularity Check
No circularity: method is a constructive design validated against external BF16 baselines, not a prediction forced by its inputs.
full rationale
The paper’s chain is: (i) 1D microscaling reassigns scales under transposition, inducing forward–backward mismatch; (ii) square 2D blocks preserve block maxima under transposition by construction, so S_fwd = S_bwd; (iii) add truncation-free scaling and stochastic rounding; (iv) evaluate end-to-end training against BF16 on held-out perplexity and reasoning benchmarks. Step (ii) is a design identity (square blocks are transposition-invariant because they contain the same values after transpose), not a fitted quantity renamed as a prediction, nor a uniqueness theorem imported from the authors. There is no parameter fit that is then reported as an independent forecast; block size 32×32 and learning rates are ordinary hyperparameters. Citations for STE, stochastic rounding, and microscaling formats are external prior art, not load-bearing self-citations that force the result. Empirical claims are checked against BF16 and public-style datasets (Wikitext, Pile, C4, SciQ, COPA, ARC-E, HellaSwag). Missing 1D-vs-2D ablations affect causal isolation of the mechanism, not circularity of the derivation. Score 0.
Axiom & Free-Parameter Ledger
free parameters (3)
- 2D block size (default 32×32) =
32×32
- Peak learning rates per model =
4e-4 (1B), 3e-4 (7B), 1e-4 (30B MoE)
- FP4 format bounds (Qp, Qn) inside scale formula =
E2M1 representable bounds (format-defined)
axioms (5)
- domain assumption Straight-through estimator: ∂L/∂X ≈ ∂L/∂X̂ through the non-differentiable quantizer
- standard math Stochastic rounding yields unbiased gradients in expectation: E[round_stoch(x)] = x
- standard math Square b×b blocks preserve max-abs and therefore scale under transposition
- ad hoc to paper Software emulation of MX/NV-style FP4 matches the stability behavior of future native tensor-core FP4
- domain assumption Activations may remain 1D-quantized while weights/gradients are 2D without reintroducing fatal inconsistency
invented entities (1)
-
Transposition-invariant 2D block FP4 training recipe (2D blocks + truncation-free scale + stochastic rounding + MXFP8 Q/K)
no independent evidence
read the original abstract
Reducing training precision is a key lever for improving the e ciency of large language model (LLM) training, but pushing beyond FP8 to 4-bit oating point (FP4) remains challenging due to instability during optimization. We identify a fundamental source of this instability in existing microscaling approaches: scale inconsistency induced by tensor transposition. In conventional 1D block quantization, forward and backward passes assign di erent scaling factors to the same values after transposition, leading to biased and unstable gradient updates. To address this issue, we propose a low-precision training framework based on 2D block FP4 quantization, which enforces transposition-invariant scaling and preserves consistency between forward and backward computations. We further combine this with truncation-free scaling and stochastic rounding to control quantization error and maintain unbiased gradients. To handle the sensitivity of attention mechanisms, we adopt MXFP8 quantization for query and key projections, yielding a practical mixed-precision design. We evaluate our method on dense LLMs up to 7B parameters and a 30B Mixture-of-Experts model, trained on up to 100B tokens. Across all settings, our approach achieves stable end-to-end FP4 training and closely matches BF16 performance, with less than 1.3% degradation in perplexity and downstream accuracy. These results demonstrate that enforcing forwardbackward scaling consistency is su cient to enable practical FP4 training at scale, providing a simple and e ective pathway toward more e cient LLM training.
Figures
Reference graph
Works this paper leans on
-
[1]
Pretraining large language models with nvfp4.arXiv preprint arXiv:2509.25149, 2025
Felix Abecassis, Anjulie Agrusa, Dong Ahn, Jonah Alben, Stefania Alborghetti, Michael An- dersch, Sivakumar Arayandi, Alexis Bjorlin, Aaron Blakeman, Evan Briones, et al. Pretraining large language models with nvfp4.arXiv preprint arXiv:2509.25149, 2025
arXiv 2025
-
[2]
gpt-oss-120b & gpt-oss-20b model card.arXiv preprint arXiv:2508.10925, 2025
Sandhini Agarwal, Lama Ahmad, Jason Ai, Sam Altman, Andy Applebaum, Edwin Arbus, Rahul K Arora, Yu Bai, Bowen Baker, Haiming Bao, et al. gpt-oss-120b & gpt-oss-20b model card.arXiv preprint arXiv:2508.10925, 2025
Pith/arXiv arXiv 2025
-
[3]
Yoshua Bengio, Nicholas Léonard, and Aaron Courville. Estimating or propagating gradients through stochastic neurons for conditional computation.arXiv preprint arXiv:1308.3432, 2013
Pith/arXiv arXiv 2013
-
[4]
Roberto L Castro, Andrei Panferov, Soroush Tabesh, Oliver Sieberling, Jiale Chen, Mahdi Nikdan, Saleh Ashkboos, and Dan Alistarh. Quartet: Native fp4 training can be optimal for large language models.arXiv preprint arXiv:2505.14669, 2025
arXiv 2025
-
[5]
Oscillation-reduced mxfp4 training for vision transformers.arXiv preprint arXiv:2502.20853, 2025
Yuxiang Chen, Haocheng Xi, Jun Zhu, and Jianfei Chen. Oscillation-reduced mxfp4 training for vision transformers.arXiv preprint arXiv:2502.20853, 2025
Pith/arXiv arXiv 2025
-
[6]
Fp4 all the way: Fully quantized training of llms.arXiv preprint arXiv:2505.19115, 2025
Brian Chmiel, Maxim Fishman, Ron Banner, and Daniel Soudry. Fp4 all the way: Fully quantized training of llms.arXiv preprint arXiv:2505.19115, 2025. 11
Pith/arXiv arXiv 2025
-
[7]
A survey of low-bit large language models: Basics, systems, and algorithms.Neural Networks, page 107856, 2025
Ruihao Gong, Yifu Ding, Zining Wang, Chengtao Lv, Xingyu Zheng, Jinyang Du, Yang Yong, Shiqiao Gu, Haotong Qin, Jinyang Guo, et al. A survey of low-bit large language models: Basics, systems, and algorithms.Neural Networks, page 107856, 2025
2025
-
[8]
Zhiwei Hao, Jianyuan Guo, Li Shen, Yong Luo, Han Hu, Guoxia Wang, Dianhai Yu, Yonggang Wen, and Dacheng Tao. Low-precision training of large language models: Methods, challenges, and opportunities.arXiv preprint arXiv:2505.01043, 2025
Pith/arXiv arXiv 2025
-
[9]
Towards fully fp8 gemm llm training at scale.arXiv preprint arXiv:2505.20524, 2025
Alejandro Hernández-Cano, Dhia Garbaya, Imanol Schlag, and Martin Jaggi. Towards fully fp8 gemm llm training at scale.arXiv preprint arXiv:2505.20524, 2025
arXiv 2025
-
[10]
How hungry is ai? benchmarking energy, water, and carbon footprint of llm inference, 2025
Nidhal Jegham, Marwan Abdelatti, Chan Young Koh, Lassad Elmoubarki, and Abdeltawab Hendawi. How hungry is ai? benchmarking energy, water, and carbon footprint of llm inference, 2025
2025
-
[11]
A comprehensive study on quantization tech- niquesforlargelanguagemodels
Jiedong Lang, Zhehao Guo, and Shuyu Huang. A comprehensive study on quantization tech- niquesforlargelanguagemodels. In2024 4th International Conference on Artificial Intelligence, Robotics, and Communication (ICAIRC), pages 224–231. IEEE, 2024
2024
-
[12]
Large language models: A survey, 2025
Shervin Minaee, Tomas Mikolov, Narjes Nikzad, Meysam Chenaghlu, Richard Socher, Xavier Amatriain, and Jianfeng Gao. Large language models: A survey, 2025
2025
-
[13]
Recipes for pre-training llms with mxfp8.arXiv preprint arXiv:2506.08027, 2025
Asit Mishra, Dusan Stosic, Simon Layton, and Paulius Micikevicius. Recipes for pre-training llms with mxfp8.arXiv preprint arXiv:2506.08027, 2025
Pith/arXiv arXiv 2025
-
[14]
Andrei Panferov, Erik Schultheis, Soroush Tabesh, and Dan Alistarh. Quartet ii: Accu- rate llm pre-training in nvfp4 by improved unbiased gradient estimation.arXiv preprint arXiv:2601.22813, 2026
Pith/arXiv arXiv 2026
-
[15]
Microscaling data formats for deep learning.arXiv preprint arXiv:2310.10537, 2023
Bita Darvish Rouhani, Ritchie Zhao, Ankit More, Mathew Hall, Alireza Khodamoradi, Summer Deng, Dhruv Choudhary, Marius Cornea, Eric Dellinger, Kristof Denolf, et al. Microscaling data formats for deep learning.arXiv preprint arXiv:2310.10537, 2023
Pith/arXiv arXiv 2023
-
[16]
Scaling laws for floating point quantization training
Xingwu Sun, Shuaipeng Li, Ruobing Xie, Weidong Han, Kan Wu, Zhen Yang, Yixing Li, An Wang, Shuai Li, Jinbao Xue, et al. Scaling laws for floating point quantization training. arXiv preprint arXiv:2501.02423, 2025
Pith/arXiv arXiv 2025
-
[17]
Training llms with mxfp4.arXiv preprint arXiv:2502.20586, 2025
Albert Tseng, Tao Yu, and Youngsuk Park. Training llms with mxfp4.arXiv preprint arXiv:2502.20586, 2025
Pith/arXiv arXiv 2025
-
[18]
Ruizhe Wang, Yeyun Gong, Xiao Liu, Guoshuai Zhao, Ziyue Yang, Baining Guo, Zhengjun Zha, and Peng Cheng. Optimizing large language model training using fp4 quantization.arXiv preprint arXiv:2501.17116, 2025
Pith/arXiv arXiv 2025
-
[19]
Haocheng Xi, Han Cai, Ligeng Zhu, Yao Lu, Kurt Keutzer, Jianfei Chen, and Song Han. Coat: Compressing optimizer states and activation for memory-efficient fp8 training.arXiv preprint arXiv:2410.19313, 2024
Pith/arXiv arXiv 2024
-
[20]
An empirical study of microscaling formats for low-precision llm training
Hanmei Yang, Summer Deng, Amit Nagpal, Maxim Naumov, Mohammad Janani, Tongping Liu, and Hui Guan. An empirical study of microscaling formats for low-precision llm training. In2025 IEEE 32nd Symposium on Computer Arithmetic (ARITH), pages 1–8. IEEE Computer Society, 2025. 12
2025
-
[21]
Jiecheng Zhou, Ding Tang, Rong Fu, Boni Hu, Haoran Xu, Yi Wang, Zhilin Pei, Zhongling Su, Liang Liu, Xingcheng Zhang, et al. Towards efficient pre-training: Exploring fp4 precision in large language models.arXiv preprint arXiv:2502.11458, 2025. 13 A Supplementary Material Thisappendixprovidesadditionalimplementationandevaluationdetailsfortheproposedtransp...
Pith/arXiv arXiv 2025
discussion (0)
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