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REVIEW 3 major objections 4 minor 1 cited by

Position: Curvature Matrices Should Be Democratized via Linear Operators

T0 review · 3 major / 4 minor · reviewed 2026-08-09 · deepseek-v4-flash

Pith's one-line read This paper argues that exposing curvature matrices as linear operators—objects that multiply vectors without ever materializing the matrix—unifies exact and approximate curvature computation, as demonstrated by the curvlinops library.

desk verdict A position paper with a real library behind it: the linear-operator claim holds, but the 'without worrying' framing is oversold and the paper's own Section 5 admits it. read the letter →

arxiv 2501.19183 v1 pith:AXJ2DQ24 submitted 2025-01-31 cs.LG

classification cs.LG
keywords linearoperatorscurvaturematricesHessiangeneralizedGauss-NewtonFisherinformationmatrixKFACrandomizedalgebraneuralnetworks
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This position paper argues that curvature matrices—the Hessian of a neural network's empirical risk and its cheaper approximations, such as the generalized Gauss-Newton, Fisher information, and KFAC—should be presented to machine-learning researchers as linear operators, objects that answer matrix-vector products without ever building the full matrix. To back this claim, the paper introduces curvlinops, a library that wraps exact and approximate curvature matrices in one common interface. It argues that this interface delivers three benefits: it hides error-prone implementation details such as automatic differentiation, batch scaling, and stochasticity; it reduces applications like second-order optimization, influence functions, model merging, and pruning to a few lines of code that mirror the underlying mathematics; and it makes existing linear-algebra tooling, including iterative eigensolvers and randomized estimators of trace, diagonal, and spectral density, directly usable. If the position is right, a single abstraction could replace much of the bespoke curvature code in deep learning.

What carries the argument

The load-bearing object is the linear operator: an interface that answers the matrix-vector product $A(v)$ for a matrix $A$ and satisfies the two linearity laws $A(v_1+v_2)=A(v_1)+A(v_2)$ and $A(\alpha v)=\alpha A(v)$. Its work in the argument is to make the representation the same regardless of how the curvature is computed: the Hessian uses nested automatic differentiation, the generalized Gauss-Newton uses vector-Jacobian and Jacobian-vector products, Fisher-vector products are obtained from a Gauss-Newton product on a detached pseudo-loss $f^\top g g^\top f$, and KFAC multiplies vectors through sums of Kronecker products of layer factors. Because every curvature variant lives behind the same multiplication interface, users can compose and invert operators, and swap exact for approximate curvature, without changing their application code.

What would settle it

Run a controlled user study in which practitioners with limited experience in second-order optimization implement a standard task, such as influence-function attribution or Fisher-weighted model merging, given only the linear-operator interface; then compare the curvature matrices they select and the correctness of their results against a reference expert implementation. If a substantial fraction of non-expert users accidentally choose an empirical Fisher where a generalized Gauss-Newton or damped inverse is required, and the library's checks do not catch it, the democratization claim is falsified.

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Extended reading notes

Core claim

The central claim is a design claim, not a new theorem: a single linear-operator interface $v \mapsto A(v)$, where $A$ is a linear map acting like a matrix $A$ but never materializing it, is general enough to cover the curvature matrices that matter in deep learning and convenient enough that application code can treat them like dense matrices. The paper demonstrates this with curvlinops, which provides operators for the Hessian, the generalized Gauss-Newton, the Monte-Carlo and empirical Fisher, and several (E)KFAC variants, and shows that a user can write the same style of code for Newton-CG, influence-function inversion, Fisher-weighted model merging, pruning, and Hessian spectral analysis. It also argues that the interface is extensible, because new curvature approximations are just new operators, and interoperable, because operators can be composed, transformed (for example inverted through iterative solvers), exported to established numerical linear-algebra routines, and connected to randomized linear algebra for estimating matrix properties at scale.

Load-bearing premise

The argument rests on the assumption that a user who cannot see the internal details can still choose the curvature matrix that is mathematically appropriate for their application, so that hiding complexity helps rather than silently misleads.

Editorial extensions

If this is right

  • Second-order optimizers and their approximate variants can be implemented once against the operator interface, with Newton-CG, Neumann-series inversion, and KFAC inverses selected by swapping operators.
  • Influence functions, bi-level optimization, and hyper-gradient methods can use full inverse curvature-vector products rather than being forced onto diagonal approximations.
  • Fisher-weighted model merging can move beyond diagonal Fishers to full or block-structured Fisher operators via conjugate-gradient solves.
  • Randomized linear algebra estimators for trace, diagonal, spectral density, and low-rank sketches become immediately applicable to any curvature matrix, not just one bespoke implementation.
  • Curvature estimators maintained inside optimizers can be exported as operators at any point of training and reused for pruning, uncertainty quantification, or landscape analysis.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A natural extension the paper leaves implicit is to attach a machine-readable 'curvature pedigree' to each operator—specifying whether it is a Hessian, GGN, type-I or type-II Fisher, empirical Fisher, its damping, and its data reduction—so that automated checks can veto an operator that is mathematically inappropriate for the requested application.
  • The same operator abstraction would transfer beyond curvature to any structured matrix used in deep learning, such as attention, kernel, or factorized weight matrices, potentially giving the broader field the same interoperability with randomized linear algebra.
  • If the interface becomes a standard, an empirical prediction follows: the dominant remaining source of user error in curvature-based applications will shift from implementing matrix-vector products to choosing the right curvature variant, which would make documentation and validation layers more important than the operator itself.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. This position paper argues that presenting curvature matrices such as the Hessian, GGN, Fisher variants, and KFAC as linear operators—i.e., as black-box matrix-vector product interfaces—provides a general, scalable, and user-friendly abstraction. The argument is supported by curvlinops, a PyTorch library that implements these operators, offers composition and transformation, exports to SciPy, and provides randomized linear algebra estimators. The manuscript illustrates the interface on five applications, reports runtime and memory benchmarks on ResNet50/ImageNet and nanoGPT/Shakespeare, validates estimators on toy problems, and includes a candid alternative-views section that discusses limitations.

Significance. If the position is accepted, a single linear-operator interface could unify exact and approximate curvature matrices across optimization, influence functions, pruning, merging, and spectral analysis, while connecting deep learning to mature randomized linear algebra and SciPy tooling. The manuscript ships a pip-installable library, reproducible code snippets and toy validations, and performance measurements that are broadly consistent with published HVP/GGNVP cost factors; the central claim is independently checkable and is not circular. The main gap is that the 'democratization' and 'user-friendly' wording claims more than the evidence establishes: the interface hides computational mechanics but leaves the consequential choice of curvature object to the user, and the paper's own Section 5 concedes that an inappropriate choice can be made unknowingly. There is also a concrete mathematical slip in the pseudo-loss derivation for Fisher-vector products (Section 2.1) that should be corrected. These issues are fixable, and the manuscript's strengths—transparency, reproducible software, and a clear design argument—make it a valuable position statement.

major comments (3)
  1. [Section 2.1] In the paragraph 'Fisher-vector products (FVPs) via GGNVPs', the pseudo-loss is defined as \ell~(f_n,y_n) = f_n^T g_n g_n^T f_n. Its Hessian with respect to f_n is 2 g_n g_n^T, not g_n g_n^T as claimed immediately after the definition. Consequently, the GGN of the stated pseudo-loss is twice the desired Fisher, MC-Fisher, or empirical Fisher. Please correct the definition to \ell~(f_n,y_n) = (1/2) f_n^T g_n g_n^T f_n, or state explicitly that the implementation rescales by 1/2. This is a load-bearing mathematical error because a user implementing the described recipe would obtain incorrectly scaled Fisher-vector products.
  2. [Section 1 and Section 5] The central claim that users can apply curvature matrices 'without worrying about implementation complexity' and the title's 'Democratized' overreach what the interface actually provides. Section 2.1 and the Step 1 API in Section 3 require users to choose among Hessian, GGN, Fisher, MC-Fisher, empirical Fisher, and KFAC and to set definitional kwargs such as GGN split, Fisher type, and damping. Section 5 explicitly concedes that the user 'might unknowingly use a curvature linear operator that is inappropriate for their application.' The abstraction therefore does not free users from the conceptually consequential choice of curvature matrix, and a wrong choice yields silently wrong results rather than a warning. The paper should either narrow the claim throughout to 'without worrying about implementation complexity' in the narrow computational sense, or add safeguards and guidance—e.g., operator-application compatibility checks and warnings about definitional choices—to make the democratization claim credible.
  3. [Appendix A] The claim that HVP performance 'matches related work' is stated from ranges (4.5–5.5 gradient equivalents observed versus 4–5 reported by Dagréou et al.) that are close but not identical, with no confidence intervals, number of repeats, or matching of setup details such as batch size and model family. Since the scalability argument is part of the paper's support, please report dispersion and repeat counts, and soften the claim to 'is consistent with' rather than 'matches' if the conditions are not directly comparable.
minor comments (4)
  1. [Section 1] The sentence 'In this paper, we argue our position that;' is a grammatical fragment and should be rephrased, e.g., 'In this paper, we argue the following position: ...'
  2. [Section 2.1] The spelling 'Naïve' and 'naive' appear inconsistently; please choose one convention throughout.
  3. [Section 3, Application 3] In the model-merging snippet, the variable 'Fs' is constructed with GGNLinearOperator but described as 'Per-task Fisher matrices'. Please state the conditions under which the GGN equals the Fisher (or rename the variables) to avoid implying that the two are interchangeable unconditionally.
  4. [Figure 4 caption] The top two panels on spectral density do not state the number of Monte-Carlo repetitions used, while the bottom panels report medians over 200 runs; please add this information for reproducibility.

Circularity Check

0 steps flagged · score 2.0 of 10

No circular derivation: the paper advocates a design position backed by an installable library; the only mildly self-referential element is that the demonstrations use the authors' own curvlinops, which is independently checkable and not a reduction of the claim to its input.

full rationale

This is a position paper rather than a derivation of quantitative predictions, so the standard circularity patterns do not apply. The mathematical content in Section 2 (Hessian, GGN, type-I/II Fisher, MC/empirical Fisher, KFAC, and the pseudo-loss identity for FVPs) is stated from standard definitions or proven directly, e.g. the pseudo-loss construction yields the desired Fisher because ∇²_f ℓ̃ = g gᵀ, which is a definitional identity rather than a fitted assumption. Section 3's applications are code snippets that follow from the operator abstraction, not a fitted parameter renamed as a prediction. The main self-referential element is that the performance benchmarks in Appendix A and the randomized-linear-algebra sanity checks in Appendix C are produced with the authors' own curvlinops library, which instantiates the very interface whose benefits are claimed. However, the library is installable and the snippets are executable, so the support is independently checkable and does not reduce the position to its own input. Co-authored citations (e.g., Eschenhagen et al. 2023) appear in implementation and testing contexts, not as a load-bearing justification for the central claim, and there is no imported uniqueness theorem or ansatz smuggled in via self-citation. Section 5 explicitly concedes, 'In both cases, the user might unknowingly use a curvature linear operator that is inappropriate for their application,' which weakens the strong 'without worrying' wording but is an acknowledged design tradeoff, not circular reasoning. Overall, no significant circularity is present; the score of 2 reflects only the mildly self-referential demonstration.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

No free parameters are fitted; the paper makes no numerical claims requiring parameter fitting. The axioms listed are background assumptions: the importance of curvature, the sufficiency of matvec queries, the correctness of the library, and the representativeness of the benchmarks. No new theoretical entities are postulated; the library is the paper's artifact, not a hypothesis. Overall, the paper's contribution rests on engineering assumptions rather than on unstated fitted parameters.

assumptions (4)
  • domain assumption Curvature matrices are essential to the cited applications (second-order optimization, uncertainty, pruning, influence, spectra) and challenging to compute.
    Motivates the position in Sections 1 and 2; not proven in the paper but widely accepted in the community.
  • domain assumption Matrix-vector products are sufficient for a broad class of curvature applications.
    Supporting examples in Section 3 all consume matvecs; the paper does not address applications that need submatrices or full factors.
  • domain assumption The curvlinops implementations compute the intended curvature matrices correctly.
    Correctness is backed by sanity checks and equivalence tests for KFAC (Section 2.2), but no formal proof; the benchmarking claims inherit this assumption.
  • ad hoc to paper The two benchmark settings (ResNet50 on ImageNet, nanoGPT on Shakespeare) are representative for the scalability claim.
    Appendix A uses these with chosen batch sizes and excludes some layers from KFAC; generalizing beyond them is an assumption.

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Pith. "Pith review of Position: Curvature Matrices Should Be Democratized via Linear Operators." pith.science (2026). https://pith.science/paper/AXJ2DQ24

@misc{pith2026250119183,
  author       = {Pith},
  title        = {Pith review of: Position: Curvature Matrices Should Be Democratized via Linear Operators},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/AXJ2DQ24}},
  note         = {Machine review of arXiv:2501.19183}
}
abstract

Structured large matrices are prevalent in machine learning. A particularly important class is curvature matrices like the Hessian, which are central to understanding the loss landscape of neural nets (NNs), and enable second-order optimization, uncertainty quantification, model pruning, data attribution, and more. However, curvature computations can be challenging due to the complexity of automatic differentiation, and the variety and structural assumptions of curvature proxies, like sparsity and Kronecker factorization. In this position paper, we argue that linear operators -- an interface for performing matrix-vector products -- provide a general, scalable, and user-friendly abstraction to handle curvature matrices. To support this position, we developed $\textit{curvlinops}$, a library that provides curvature matrices through a unified linear operator interface. We demonstrate with $\textit{curvlinops}$ how this interface can hide complexity, simplify applications, be extensible and interoperable with other libraries, and scale to large NNs.

Figures

Figures reproduced from arXiv: 2501.19183 by the authors.

Figure 2
Figure 2. Visual tour of curvature matrices. White lines separate parameters into layers. We consider a synthetic classification task with a small convolutional neural net (three convolutional and one dense layer with ReLU and sigmoid activations, D = 683). We also demonstrate scalability to larger architectures like nanoGPT on Shakespeare and ResNet50 on ImageNet (§A). Related software. Various Python packages provide a line… view at source ↗
Figure 3
Figure 3. Performance analysis: Run time (left column) and peak memory (right column) of linear operators benchmarked on ResNet50 on ImageNet (top row) and nanoGPT on Shakespeare (bottom row) on an A40 GPU with 40 GiB of RAM (the code used to generate these results is here). Details: For ImageNet, we use a batch size of 64 and images of shape (3, 224, 224); for Shakespeare, we use a batch size of 4 and context length 1024. Al… view at source ↗
Figure 4
Figure 4. Estimating linear operator properties with curvlinops. We implement various estimation algorithms from the literature and evaluate them on toy problems. Top: Spectral density estimation with the algorithms and toy matrices from Papyan et al. (2020). The left panel estimates a spectral density, the right panel the spectral density of the matrix logarithm log(|A| + ϵI) with ϵ = 10−5 . Code to reproduce these figures i… view at source ↗

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Pith tools

Reviewed August 9, 2026 · model on record in the stance chip above.