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REVIEW 5 major objections 4 minor 106 references

Accelerated Training of Federated Learning via Second-Order Methods

T0 review · 5 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash

Pith's one-line read The paper argues that second-order optimization—using Hessian curvature—generally accelerates federated learning, reducing communication rounds and improving accuracy, and organizes the field into six method categories.

desk verdict A useful survey map of second-order FL methods, but the blanket 'generally improve' claim is unsupported by the survey's own evidence. read the letter →

arxiv 2505.23588 v1 pith:4EBXRANI submitted 2025-05-29 cs.LG cs.DC

classification cs.LGcs.DC
keywords federatedlearningsecond-orderoptimizationHessianmatrixNewtonmethodcommunicationefficiencynon-IIDdatasurvey
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

Federated learning's main practical bottleneck is slow training of the global model: first-order methods such as FedAvg need many communication rounds, especially when client data is heterogeneous. This survey claims that second-order methods, which scale updates by curvature information from the Hessian rather than by a fixed learning rate, generally converge faster and reach higher test accuracy in fewer rounds. To make that claim usable, the paper proposes a taxonomy of six categories based on how the Hessian is obtained or approximated, and compares their computation, memory, transmission, and convergence profiles. The paper's conclusion is that curvature information is highly beneficial for FL training, provided the Hessian and its inverse are handled efficiently, and it positions the taxonomy as a selection guide for researchers.

What carries the argument

The load-bearing object is the Hessian matrix $H = \nabla^2 F(x)$ and the Newton update $x \leftarrow x - H^{-1}g$, which uses curvature to choose both direction and step size. Because computing, storing, inverting, and transmitting the full Hessian is prohibitive—$O(d^2)$ computation and memory per client and $O(d^3)$ for inversion—and because averaging local inverse Hessians does not equal the inverse of the global Hessian, the survey's taxonomy classifies methods by their approximation strategy. Each category is a different answer to the question 'where does curvature come from and who computes it': Hessian-vector products, compressed full Hessians, server-side BFGS, diagonal vectors, Nyström sketches, or rank-one matrices. The categories carry the argument because the survey claims that performance differences track these choices of curvature approximation and placement.

What would settle it

Run one controlled benchmark with identical data partitions, model architectures, and per-method tuning budgets on non-IID, partial-participation tasks, comparing FedAvg against representatives from each category (for example DANE, FedNL, FedSSO, and FAGH); if FedAvg matches or beats them in communication rounds and final test accuracy, the survey's central conclusion fails.

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Extended reading notes

Core claim

On the paper's own terms, the central discovery is that moving from the first-order update $x \leftarrow x - \eta \nabla F(x)$ to a Newton-type update $x \leftarrow x - H^{-1}\nabla F(x)$ lets the global model follow the curvature of the objective and reach the optimum with quadratic convergence. In federated settings this translates into fewer communication rounds or less wall-clock time to hit a target loss or accuracy. The survey groups current methods into six categories by how they obtain curvature: Hessian-free methods such as DANE, GIANT, and DONE; full local Hessian computation with compression (FedNL, SHED, FedNS); quasi-Newton methods (FedSSO, DQN-Fed); Hessian diagonal approximations (FopLAHD, LTDA, HWA, Fed-Sophia); Nyström-based approximations (FONN, Nys-FL); and one-rank approximations (FReNG, FAGH). Across the empirical results collected from these methods' papers, the survey observes that second-order methods generally reduce classification error, training loss, and test loss and increase test accuracy faster than first-order baselines, with the known exception of FedDANE, which underperforms under non-IID data and partial participation.

Load-bearing premise

The empirical conclusion assumes that 'method X beats method Y' results measured on different datasets, partitions, models, and baselines can be pooled into a general statement that second-order methods are better.

Editorial extensions

If this is right

  • If the survey's conclusion is right, a practitioner facing non-IID data and partial client participation can narrow options to the diagonal, Nyström, and one-rank categories, which keep computation close to FedAvg and still accelerate convergence.
  • The full-local-Hessian and quasi-Newton families become attractive mainly when a powerful server exists, since they shift the heavy $O(d^2)$–$O(d^3)$ work to the server side.
  • The Hessian-free family's four transmissions per round may erase its convergence advantage in high-latency networks, making two-transmission methods like LocalNewton and FedSSO better fits for bandwidth-constrained settings.
  • A convergence-rate hierarchy emerges from the tables: super-linear claims for full-Hessian methods, linear-quadratic claims for GIANT-style and Nyström/one-rank methods, and sub-linear claims for FedSSO, all stated for convex objectives.
  • The taxonomy itself is the practical output: it lets researchers pick an algorithm by matching constraints rather than reading every method paper.

Reading between the lines

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

  • Inference: the survey's pooled comparison likely overstates the generality of 'second-order beats first-order,' because each method was tested on different datasets, partitions, and baselines; a direct head-to-head benchmark is needed before the superiority claim can be trusted.
  • Inference: the theory in the surveyed papers suggests the gap between first- and second-order methods should grow as the problem's condition number worsens, so a natural controlled experiment would vary conditioning while holding data and model fixed.
  • Inference: the pattern points toward a research target the survey leaves implicit: a method that combines server-side curvature approximation with FedAvg-level transmission and partial-participation support would dominate the design space.
  • Inference: because all empirical comparisons report only target-accuracy or final-loss results at a chosen round count, the field would gain from reporting full convergence curves and per-round wall-clock time, which would make the 'fewer rounds or less time' claim testable across methods.
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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

5 major / 4 minor

Summary. This survey reviews second-order optimization methods for federated learning (FL). It groups 22 methods into six categories (Hessian-free, full local Hessian, quasi-Newton, Hessian diagonal, Nyström, and one-rank approximations), describes each method's algorithm, reported empirical behavior, and complexity, and summarizes the results in four tables. The paper's central claim, stated in Section IV-B, is that second-order methods generally improve FL training by reducing classification error, training loss, and test loss and by increasing test accuracy in fewer communication rounds or less time compared with first-order methods. The survey is positioned as filling a gap in FL survey literature that has focused on heterogeneity, privacy, and security rather than on optimization speed.

Significance. If the central claim were adequately supported, the paper would provide a useful organizational reference for researchers choosing among second-order FL methods. Its strengths are the breadth of coverage, the grouping of methods into a clear taxonomy, and the systematic reporting of local and server computation, memory, transmission, convergence, and applicability conditions in Tables I, II, and IV. The paper also usefully collects empirical claims from the original papers into a single table. However, the survey does not include new experiments or controlled comparisons, and its main empirical conclusion is drawn from heterogeneous, qualitative self-reported results. The taxonomy and tables are valuable as a catalogue, but the generalized superiority claim in Section IV-B is not supported by the evidence presented in the paper itself, and several internal inconsistencies in the method descriptions and tables need correction.

major comments (5)
  1. [Section IV-B, Tables III and IV] The central claim that 'second-order methods generally improve FL training' is contradicted by the survey's own evidence. Table III records FedDANE as underperforming FedAvg and FedProx in reducing training loss, and Table IV states that the entire Hessian-free category, which includes 7 of the 22 surveyed methods, is 'not suitable for Non-IID or partial client scenarios.' The generalization should be restricted to the specific categories and settings for which the reported evidence is consistent, or reformulated as a conditional conclusion.
  2. [Section IV-B, first paragraph] The empirical comparison is explicitly based on 'experimental information from the respective papers,' and Table III pools results across different datasets, data partitions, models, loss functions, baselines, numbers of communication rounds, and metrics. The table entries are qualitative verdicts such as 'faster reduction' with no shared unit of measurement. Such heterogeneous self-reported results cannot support a general superiority claim about second-order methods relative to first-order methods; at most they support a statement that several second-order algorithms were reported to perform well in their original papers.
  3. [Section III-A-6 (DONE)] There is an internal contradiction in the description of DONE. The main text states that DONE 'can be applicable to the non-iid data across clients,' but the Cons paragraph states that DONE is designed for a strongly convex global objective with 'homogeneous data across clients.' Table II also labels DONE as non-iid. This contradiction affects the reliability of the taxonomy and should be resolved, and the same check should be applied to other rows where the design conditions in Table II differ from the empirical conditions in Table III (for example, FedNL is listed as non-iid with FCP+PCP in Table II but as iid with FCP in Table III).
  4. [Section II-A-2, Eq. (4), and Section IV-A] The paper repeatedly attributes 'quadratic convergence' to second-order optimization, but Table II shows that most surveyed methods achieve only linear, sub-linear, or linear-quadratic convergence, and several methods (FopLAHD, LTDA, HWA, Fed-Sophia, FedNew) are listed without a stated convergence rate. The convergence discussion should distinguish the idealized Newton method from the practical approximations used in FL and should not overstate the rates that the survey itself documents.
  5. [Section III-B-1 (FedNL) versus Table I] The complexity text for FedNL reports client-to-server transmission cost O(r+d) and server-to-client transmission cost O(r), while Table I lists O(2d) and O(d), respectively. These numbers should be reconciled or the discrepancy explained, since the communication cost is one of the paper's main comparison criteria.
minor comments (4)
  1. [Throughout] There are numerous typographical and grammatical errors, including 'Exiting' for 'Existing' in Sections III-B, III-C, and III-D, 'demostrate' in Section III-A-6, 'phishig' in Section III-B-5, 'long with strong' in Section III-A-3, and 'FPC' for 'FCP' in the Table III header. A careful proofread is needed.
  2. [Title and header] The manuscript header still reads 'Manuscript received April 19, 2021; revised August 16, 2021,' which conflicts with the arXiv submission date of May 2025 and should be updated.
  3. [Reference [40]] Reference [40] is incomplete: it gives the title but no publication venue, date, or page information.
  4. [Section III-D-1] The method name is spelled inconsistently as FopLAHD, FopLADH, and FOPLAHD in different places; one standard spelling should be used throughout.

Circularity Check

0 steps flagged · score 0.0 of 10

No circular derivation: the survey aggregates independent published results; the presence of self-citations does not make the empirical claim circular.

full rationale

The paper is a survey, not a derivation: there are no fitted parameters, no theorem derived from the paper's own definitions, and no prediction generated from a model built on the target data. The central claim in Section IV-B ('second-order methods generally improve FL training...') is an inductive generalization from the surveyed papers' reported experiments, as the paper itself states: 'The empirical comparisons are made by using experimental information from the respective papers of the methods.' Several surveyed methods (FOPLAHD, LTDA, FReNG, FAGH, Nys-FL, FONN) are authored by the survey authors, which creates a mild self-interest signal, but the survey does not invoke a uniqueness theorem, an ansatz, or a definitional identity to force that conclusion. Each cited method is a separate publication with its own external benchmarks, and the survey does not use its own conclusions to validate those methods. The 'generally improve' claim is better evaluated as an evidence-quality or correctness risk: Table III includes FedDANE underperforming, and Table IV restricts the entire Hessian-free category to IID/full-participation settings, so the generalization is stronger than the pooled evidence supports. That is an empirical over-claim, not a circular reduction. Accordingly, no circular step is identified.

Assumptions & free parameters 0 free parameters · 2 assumptions · 0 invented entities

The survey introduces no free parameters or invented entities. Its conclusions rest on the reported results of the cited papers and on the validity of its six-way categorization of second-order FL methods.

assumptions (2)
  • domain assumption The convergence rates and empirical results attributed to each surveyed method are accurately reported in the original papers.
    The survey provides no new experiments or proofs; its conclusions depend entirely on the claims of the cited papers (Sections III-A to III-F).
  • domain assumption The six taxonomy categories (Hessian-free, full local Hessian, quasi-Newton, diagonal, Nyström, one-rank) form a valid and useful partition of second-order FL methods.
    The paper does not justify why these six categories are exhaustive or mutually exclusive, and some methods could arguably fit multiple categories.

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Cite this review

Pith. "Pith review of Accelerated Training of Federated Learning via Second-Order Methods." pith.science (2026). https://pith.science/paper/4EBXRANI

@misc{pith2026250523588,
  author       = {Pith},
  title        = {Pith review of: Accelerated Training of Federated Learning via Second-Order Methods},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/4EBXRANI}},
  note         = {Machine review of arXiv:2505.23588}
}
read the original abstract

This paper explores second-order optimization methods in Federated Learning (FL), addressing the critical challenges of slow convergence and the excessive communication rounds required to achieve optimal performance from the global model. While existing surveys in FL primarily focus on challenges related to statistical and device label heterogeneity, as well as privacy and security concerns in first-order FL methods, less attention has been given to the issue of slow model training. This slow training often leads to the need for excessive communication rounds or increased communication costs, particularly when data across clients are highly heterogeneous. In this paper, we examine various FL methods that leverage second-order optimization to accelerate the training process. We provide a comprehensive categorization of state-of-the-art second-order FL methods and compare their performance based on convergence speed, computational cost, memory usage, transmission overhead, and generalization of the global model. Our findings show the potential of incorporating Hessian curvature through second-order optimization into FL and highlight key challenges, such as the efficient utilization of Hessian and its inverse in FL. This work lays the groundwork for future research aimed at developing scalable and efficient federated optimization methods for improving the training of the global model in FL.

Figures

Figures reproduced from arXiv: 2505.23588 by the authors.

Figure 1
Figure 1. This figure compares first-order and second-order optimization [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗

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