REVIEW 4 major objections 4 minor 1 cited by
Is your batch size the problem? Revisiting the Adam-SGD gap in language modeling
T0 review · 4 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Tuned SGD with momentum matches Adam on language models when the batch is small, and the paper argues batch size rather than architecture or data explains the optimizer gap.
desk verdict Batch size is a real and informative axis of the Adam-SGD gap in LMs, but the headline parity claim is built on best-run selection and no seed variance, so treat the quantitative claim as provisional. 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 the stochastic differential equation (SDE) approximation of SGD and of sign-based updates. For signSGD the paper cites a theorem giving the drift term as $-\operatorname{erf}\big(\sqrt{B/2}\,\Sigma^{-1/2}\nabla f(x)\big)$, which is approximately linear in $\sqrt{B}$ near zero and saturates beyond a critical batch size, while SGD's drift is just $-\nabla f(x)$ with noise $\sqrt{\eta\Sigma/B}\,dW_t$. This drift-versus-batch-size separation explains both why Adam-type methods improve with larger batches and why SGD's small-batch success is really an iteration-count effect. A secondary instrument is the heterogeneous quadratic testbed with block-diagonal Hessians, used to show the batch-size effect appears even without Transformer-specific structure, and grafting experiments that isolate the update direction as the failing component of large-batch SGD.
What would settle it
Run the small-batch comparison (e.g., batch size 64, 160M model on SlimPajama) with at least five seeds per configuration, tuning the learning rate grid separately at that batch size, and compare the median final validation perplexity of SGD with momentum against Adam; if the median SGD perplexity is clearly worse than Adam's, then the parity claim holds only for best-luck runs, not for typical training.
Extended reading notes
Core claim
The central claim is that SGD with momentum can be made competitive with Adam in Transformer language modeling, provided the batch size is small and both momentum and learning rate are properly tuned: at batch size 64 and a 1.3B-token budget, a 160M model reaches final validation perplexity close to Adam's (30.76 vs 28.77), and at 410M and 1B parameters SGD can actually outperform Adam when trained with very small batches for many steps. The paper further argues that the reason is not any of the previously proposed mechanisms, all of which predict a gap that should persist at small batch sizes; instead, the gap is governed by how each optimizer responds to gradient noise. For signed and adaptive updates, the drift in the SDE limit scales as an error function of the square root of the batch size times the preconditioned gradient, giving an acceleration with batch size up to a critical point, whereas SGD's drift is batch-size independent, so its progress is set by iteration count.
Load-bearing premise
The claim that SGD matches Adam rests on reporting each optimizer's best stable run rather than its typical run, with learning-rate grids tuned at one batch size and only roughly rescaled for others; under a stricter median-seed, per-batch-size protocol, the small-batch parity may vanish.
Editorial extensions
If this is right
- In small-batch regimes, SGD with momentum is a memory-cheaper alternative to Adam for training language models, since it does not store second-order moment estimates.
- The Adam-SGD gap is not intrinsic to Transformers or text data; batch size and gradient noise scale are first-order moderators of the gap.
- Increasing batch size accelerates Adam-type methods through a $\sqrt{B}$ drift effect up to a critical batch size, while SGD gains nothing from larger batches in early training.
- Prior explanations of the gap, including heavy-tailed class imbalance, directional sharpness, and Hessian heterogeneity, are incomplete because they predict a gap that should remain even where SGD is observed to work well.
- At scale, SGD can outperform Adam under small-batch, long-step training, suggesting that optimizer choice interacts with compute budget and memory constraints rather than being fixed by architecture alone.
Reading between the lines
- Extension beyond the paper: if batch size controls the gap, then compute-optimal training recipes that rely on very large batches may systematically favor Adam, while low-resource regimes that use tiny batches are precisely where simpler optimizers could win.
- Extension beyond the paper: the SDE analysis implies that other sign-based or compressed-gradient methods should inherit Adam's batch-size acceleration; a direct test would compare SignSGD and Adam across batch sizes and check whether their gap follows the predicted $\sqrt{B}$ drift scaling.
- Extension beyond the paper: the observation that clipping occurs most often in normalization layers at large batch sizes suggests that per-coordinate scaling of a few outlier coordinates is the active ingredient, pointing to a possible middle-ground optimizer that adaptively clips only rare large coordinates instead of maintaining full Adam states.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper revisits the commonly observed performance gap between Adam and SGD in Transformer language modeling. Through controlled sweeps on nanoGPT-style models (160M up to 1B parameters) trained on SlimPajama and FineWeb, the authors report that SGD with momentum, gradient clipping, and carefully tuned hyperparameters can match or even outperform Adam when the batch size is small, while the gap reappears at larger batch sizes. They then test several existing explanations (heavy-tailed class imbalance, directional sharpness, Hessian heterogeneity), find them insufficient to explain the small-batch result, and propose an SDE-based account in which signed-gradient methods (and by extension Adam) benefit from larger batches through an erf-shaped drift term, whereas plain SGD does not. The central empirical claim is that batch size, not just optimizer choice, is a decisive factor in the Adam-SGD gap.
Significance. If the central claim is robust, the paper provides a valuable and somewhat counterintuitive result: at small batch sizes, SGD with momentum can be competitive with Adam in Transformer language modeling at the 410M and 1B parameter scale. This would reframe the optimizer gap as a batch-size-dependent phenomenon, with practical implications for memory-constrained training and for theories of adaptive methods. The paper is also useful as a systematic empirical probe of prior explanations, and it makes an explicit, falsifiable prediction: that SGD's performance degrades with batch size while Adam's improves, up to a critical batch size. The authors are transparent about the heuristic bridge from SignSGD to Adam in the theory, which is a strength in presentation, though it leaves the theoretical contribution incomplete.
major comments (4)
- [Section 2.2, Table 1, Figure 5] The evaluation protocol does not support the headline parity claim as stated. The text says: "Some settings become unstable at very large learning rates, where one run may succeed, even if the median run diverges. In those settings, we report runs at the largest stable learning rate as optimal." This is best-run selection under instability, and no seed variance is reported anywhere in Section 2 or for Figure 5. Table 1's SGD result at batch size 64 (PPL 30.76, lr=0.5) and the 410M/1B trajectories in Figure 5 could therefore be draws from a distribution whose median is much worse. Please rerun the key comparisons with at least 3-5 seeds, report median and interquartile ranges or full seed curves, and define the stability criterion explicitly (e.g., the fraction of runs that diverge). Without this, the central claim that SGD can match or outperform Adam in small-batch settings is not established.
- [Section 2.2 and Section 2.3] The learning-rate grids are not independently tuned per batch size, despite the abstract's claim of "comprehensively tuned" baseline runs. In Section 2.2, the grid is tuned at batch size 256 and then scaled to batch sizes 64 and 1024 with only three values per batch size. In Section 2.3, fixed grids [0.25, 0.5, 1.0] for SGD and [0.001, 0.002, 0.004] for Adam are used across batch sizes. The reported Adam-SGD gap may therefore be an artifact of insufficient Adam tuning at small batch sizes or insufficient SGD tuning at large batch sizes. Please perform per-batch-size sweeps over wider grids, or at least show a learning-rate response curve for each optimizer and batch size, and justify that the scaled grids cover each optimizer's optimal region.
- [Section 4.3, Theorem 1 and Figure 9] The theoretical claim that large batch sizes accelerate Adam is not directly supported by the stated theorem. Theorem 1 is a result for SignSGD, and the paper says the conclusion holds "in signSGD (and likely also in closely-related algorithms, like Adam)". The quadratic experiments in Figure 9 compare SGD and SignSGD+m, not Adam, so the bridge from the theorem to the actual Adam behavior is a heuristic analogy. This is acknowledged in the text, but it is load-bearing for the paper's proposed explanation of the empirical results. Please state this limitation more prominently, or provide a formal or at least direct empirical argument that the erf drift acceleration carries over to Adam's coordinate-wise normalization rather than only to sign-based updates.
- [Section 2.3, Figure 5, Figure 14] The scaling experiments that support "SGD can outperform Adam even at a 410M and 1B scale" are presented using training perplexity trajectories (Figure 5), not held-out validation perplexity. The learning-rate sweeps in Figure 14 do report validation perplexity, but those appear to be single-run evaluations. Since the headline claim is about final model quality, please report final validation perplexities for the chosen runs, together with seed information, and clarify whether the outperformance holds on held-out data or only on the training set.
minor comments (4)
- [Appendix D] There is a typo: "Leraning rates" should be "Learning rates".
- [Section 2.3] The phrase "its critical batch size is close to 1" is used without a definition. Please define what is meant by "critical batch size" in this context (e.g., the threshold beyond which larger batches no longer reduce the number of steps to a given loss).
- [Appendix A.2] The sentence "As a sequence length of 160k, given our lack of experience with extremely low batch sizes (shorter sequence length)" is garbled; please rewrite to state the actual configuration used for the Fineweb 160M experiments.
- [Figure 4] The x-axis label "10 160804020" appears malformed; it should read something like "Steps (\times 10^3)" with tick labels 10, 20, 40, 80, 160.
Circularity Check
No significant circularity: the empirical SGD-vs-Adam comparison is self-contained, and the cited SDE theorem for SignSGD is external support that does not encode the paper's target claim.
full rationale
The paper's central claim—that SGD with momentum can match Adam in small-batch Transformer language modeling—is established by direct hyperparameter sweeps (Section 2.2, Figure 1, Table 1) and scaling runs (Section 2.3, Figure 5), not by fitting a parameter and then presenting the same fitted quantity as a prediction. The learning-rate grids at batch sizes 64 and 1024 are scaled from the grid tuned at batch size 256, but the reported perplexities are measurements of separate runs, and Section 2.2's note that unstable settings are reported at the largest stable learning rate is a selection-robustness concern, not a circularity concern. The main theoretical input is Theorem 1, cited from Compagnoni et al. (2025b), which shares co-author Orvieto. This is a self-citation, but it is not load-bearing in a circular way: the theorem is stated with explicit assumptions (i.i.d. Gaussian noise, diagonal covariance) and is about SignSGD; it does not assume or include the paper's empirical result about Adam versus SGD. The paper only heuristically extends the theorem to Adam ('likely also in closely-related algorithms, like Adam'), and that extension is flagged as an analogy rather than derived from the target claim. No equation in the paper reduces to its own input by construction, and no fitted parameter is renamed as a prediction. A stricter evaluation protocol could weaken the empirical conclusion, but that is a correctness/robustness issue, not circularity.
Assumptions & free parameters
assumptions (3)
- domain assumption The SDE approximation of SGD (Eq. 2) with Gaussian noise is a valid proxy for the discrete training dynamics.
- ad hoc to paper The theorem from Compagnoni et al. (2025b) for SignSGD (Theorem 1) is assumed to carry over to Adam.
- domain assumption The heterogeneous quadratic model (Appendix D) is a representative proxy for Transformer optimization.
Cite this review
Pith. "Pith review of Is your batch size the problem? Revisiting the Adam-SGD gap in language modeling." pith.science (2026). https://pith.science/paper/PYZQ7EYQ
@misc{pith2026250612543,
author = {Pith},
title = {Pith review of: Is your batch size the problem? Revisiting the Adam-SGD gap in language modeling},
year = {2026},
howpublished = {\url{https://pith.science/paper/PYZQ7EYQ}},
note = {Machine review of arXiv:2506.12543}
}
read the original abstract
Adam is known to perform significantly better than Stochastic Gradient Descent (SGD) in language models, a phenomenon for which a number of explanations have been proposed. In this work, we revisit this "optimizer gap" through a series of comprehensively tuned baseline training runs for language modeling with Transformers. We exhaustively study how momentum, gradient clipping, and batch size affect the gap between SGD and Adam. Our empirical findings show that SGD with momentum can actually perform similarly to Adam in small-batch settings, if tuned correctly. We revisit existing explanations for Adam's advantage, including heavy-tailed class imbalance, directional sharpness, and Hessian heterogeneity, which struggle to directly explain this phenomenon. Towards bridging this gap in our understanding, by analyzing our Transformer training runs and simple quadratic settings inspired by the literature, we provide new insights, driven by stochastic differential equation models, into the role of batch size on the training dynamics.
Figures
Figures from the paper (15 more)
Forward citations
Cited by 1 Pith paper
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Reference graph
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Reviewed August 7, 2026 · model on record in the stance chip above.
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