REVIEW 3 major objections 6 minor 16 references
This thesis proves that error feedback with contractive compressors converges in O((L + L_AM ξ(α))/ε) rounds, replacing the quadratic mean of client smoothness constants with their arithmetic mean.
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 · deepseek-v4-flash
2026-08-04 21:14 UTC pith:5YREGSGB
load-bearing objection A solid, well-organized PhD thesis that consolidates the author's previously published work; the EF21-W chapter is mathematically sound and the engineering chapters are substantial, but the empirical speedup claims and reliance on known per-client smoothness constants deserve scrutiny. the 3 major comments →
Optimization Methods and Software for Federated Learning
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
For minimizing f(x)=1/n Σ f_i(x) under L-smoothness of f and L_i-smoothness of each f_i, the paper proves that EF21—the best-known error-feedback method for contractive compressors—needs only T = O((L + L_AM ξ(α))/ε) iterations to reach E‖∇f(x̂)‖² ≤ ε, where L_AM = (1/n)Σ L_i and ξ(α) depends only on the compressor's contraction parameter. Previously the best bound had L_QM = sqrt((1/n)Σ L_i²) in place of L_AM. The improvement is obtained three ways: by cloning each client N_i times with frequencies ⌈L_i/L_AM⌉ (a √2-approximate optimal scheme), by the weighted algorithm EF21-W with weights w_i = L_i/Σ_j L_j, and by a weighted analysis of unweighted EF21. The proof introduces a weighted gradi
What carries the argument
The carrying identity is w_i = L_i / Σ_j L_j: with these weights, EF21-W's server update is x^{t+1} = x^t − γ Σ_i w_i g_i^t, and the weighted distortion G^t = Σ_i w_i ‖g_i^t − ∇f_i(x^t)/(n w_i)‖² obeys a contraction whose residual coefficient is L_AM² rather than L_QM². The same weights inserted into the proof for vanilla EF21, via Jensen's inequality on the distortion, deliver the same rate without modifying the algorithm. Client cloning is the discovery route: replicating client i to N_i copies with rescaled functions reduces the quadratic mean to the arithmetic mean; integer cloning frequencies ⌈L_i/L_AM⌉ are a √2-approximation and need at most 2n total clients.
Load-bearing premise
The step size and the weights in EF21-W require exact knowledge of every client's smoothness constant L_i; if these constants are estimated or tuned imperfectly, the advertised step size bound and convergence complexity no longer hold.
What would settle it
Take a non-convex problem with n=1000 clients where one client has smoothness 100 times the others, run EF21 and EF21-W with the Top1 compressor and the theoretical steps from Theorem 8 versus the old bound, and record rounds to reach E‖∇f‖² ≤ ε. If EF21-W's rounds do not follow (L + L_AM ξ(α))/ε, or if perturbing the claimed optimal weights w_i by 10% drives the method to diverge at the theoretical step size, the central claim fails.
If this is right
- Communication complexity of EF21-style methods with contractive compressors becomes O((L + L_AM ξ(α))/ε), so heterogeneous smoothness between clients is less harmful than the earlier L_QM bound suggested.
- EF21-W permits theoretical step size γ ≤ 1/(L + L_AM ξ(α)), which is larger than the L_QM-based step whenever L_QM > L_AM, yielding fewer communication rounds in the experiments.
- The weighted analysis extends to EF21-SGD, EF21-PP, PL functions, and the rare-features regime, so the L_AM improvement is not limited to full-gradient full-participation EF21.
- Cloning shows that adding rescaling clients can improve rates: at most 2n virtual clients suffice to get within √2 of the optimal cloning objective.
- Experiments on non-convex logistic regression and synthetic linear problems show the largest gains when L_var = L_QM² − L_AM² is large.
Where Pith is reading between the lines
- If exact L_i are unavailable, the theory suggests an online estimator for w_i could still capture much of the benefit; a testable extension is to replace L_i with running estimates of local gradient Lipschitz constants and measure whether the step-size validity holds.
- The equivalence between cloning and weighting hints that the L_AM rate may extend to other biased compressors (RankK, natural compression) and to decentralized topologies where each node's 'clones' are weighted aggregations.
- The same weighted-distortion technique could sharpen rates for second-order FL methods like FedNL by replacing worst-case client smoothness with an arithmetic-mean dependence.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The dissertation addresses five challenges in Federated Learning (FL) across seven chapters, each a previously published paper. The central theoretical contribution is in Chapter 3, where the author replaces the quadratic mean (L_QM) of client smoothness constants in the EF21 error-feedback method with the arithmetic mean (L_AM), yielding the communication complexity T = O((L + L_AM * xi(alpha))/epsilon) for non-convex smooth objectives. This is achieved through three routes: client cloning, a new weighted algorithm EF21-W (Algorithm 3), and a weighted analysis of the original EF21. The other chapters present FL_PyTorch, a simulation framework; DCGD/PermK/AES, a classical-cryptography approach to private FL; refined PAGE analysis; compressed personalized L2GD; a high-performance FedNL implementation; and BurTorch, a low-latency backpropagation engine. The thesis emphasizes the interplay between theory, systems, and practical implementation, with each chapter containing its own reproducibility appendix.
Significance. If the Chapter 3 result is correct, it is a genuine improvement over the best-known communication complexity for algorithms using contractive compressors, replacing a potentially much larger quadratic-mean constant with the arithmetic mean. The proofs are self-contained and the derivation of the weights w_i = L_i / sum_j L_j is structurally sound. The systems chapters provide substantial engineering contributions, and the thesis explicitly ships reproducibility statements and code links for several chapters. However, the practical applicability claims rest on assumptions that are not stress-tested: exact knowledge of per-client smoothness constants in EF21-W, no sensitivity analysis in the experiments, and performance claims (x1000 speedup, x3500 memory reduction) that are not fully verifiable from the manuscript alone. The significance is therefore real but tempered by the gap between the stated theoretical conditions and the claimed practical readiness.
major comments (3)
- [Chapter 3, Theorem 8 / Algorithm 3] The central practical claim of Section 3.3 relies on exact knowledge of each L_i to set w_i = L_i / sum_j L_j and gamma <= 1/(L + L_AM * xi(alpha)). The proof in Appendix C3.2 requires the inequality gamma^2 * beta * L_AM^2 / theta + L * gamma <= 1. If L_AM is underestimated, the implemented gamma can violate this bound and the contraction argument collapses. Overestimating preserves correctness but degrades the rate. The experiments report 'theoretical step sizes' but do not state how L_i were obtained or how sensitive the method is to misspecification. This is load-bearing for the thesis's applicability claims, and it conflicts with the thesis's own principle stated in Section 1.2.5 that estimation costs should be accounted for in final evaluation. I recommend adding a sensitivity analysis and discussing practical estimation of L_i.
- [Chapter 3, Theorem 9] The improved theory for the original EF21 also assumes knowledge of L_i to set the step size via L_AM and the analysis uses weights w_i = L_i / sum_j L_j in the Lyapunov argument (Appendix D3). The text (Section 3.2.4) suggests that EF21 inherits the L_AM rate automatically for positively homogeneous compressors, but in practice the step size still requires exact smoothness constants. The manuscript does not discuss how practitioners should obtain these constants in federated settings, nor does it analyze the robustness of Theorem 9 to inexact L_i. This is a gap between the stated theoretical improvement and the claimed practical impact.
- [Chapters 7 and 8] The performance claims - x1000 wall-clock improvement for FedNL (Section 7.5) and up to x2000/x3500 runtime/memory improvements for BurTorch (Chapter 8, Table 8.1) - are central to the software contributions. The manuscript provides detailed optimization narratives and reproducibility appendices, but the full experimental code is not included in the arXiv submission, and the baselines are compared against a specific reference implementation and specific framework versions. This makes independent verification difficult. Given that the thesis explicitly positions practical implementation as a key contribution, the lack of a complete, self-contained benchmark suite or measurement scripts in the manuscript is a significant gap.
minor comments (6)
- [Section 3.3] The phrase 'partial partial participation' appears twice (Figures 3.2 caption and text). The intended expression is likely 'partial participation' or 'random client participation'.
- [Table 7.4] The title contains a typo: 'Improvemens' should be 'Improvements'.
- [Appendix A3.4] The heading '2-Suboptimal but simple step size rule' is unclear; the '2-' prefix is not explained. Presumably it means the step size is within a factor of 2 of optimal, but this should be stated.
- [Appendix E3.3, Theorem 13] The condition for choosing s and mu uses both 'mu' and 'nu' with inconsistent notation: the algorithm defines nu, but the condition states (1+s)(1+mu)<1/(1-alpha). This should be unified.
- [Appendix E3.2, Lemma 10 proof] Several intermediate lines contain garbled or duplicated symbols (e.g., '\\|\\nabla f_i(x^{t+1}) -' followed immediately by '\\|\\nabla f_i(x^t)\\|^2\\|'). These appear to be formatting errors from the LaTeX conversion; please correct.
- [Section 1.5.3, Definition 6] The phrase 'conically bounded variance' is used but not defined. If this is standard, a reference or definition is needed.
Circularity Check
No significant circularity; EF21-W's L_AM rate is a self-contained theorem from stated smoothness assumptions, not a fitted or self-citational reduction.
full rationale
The strongest claimed contribution, replacing L_QM by L_AM in EF21's communication complexity, is derived independently in Chapter 3. The proof of Theorem 8 (Appendix C3) starts from Assumptions 1-3 and the contractive-compressor definition, constructs the Lyapunov function G^t = \sum_i w_i ||g_i^t - (1/(n w_i))\nabla f_i(x^t)||^2, and obtains the coefficient L_AM^2 from the algebraic identity \sum_i L_i^2/w_i = n^2 L_AM^2 after setting w_i = L_i / \sum_j L_j (Lemma 7, Eq. 3.30). This is a direct calculation, not a parameter fitted to data or a renamed known result. Theorem 9 similarly re-weights the analysis of the original EF21 and proves the same identity (Appendix D3, Lemma 8). The step-size condition and the resulting T = O((L + L_AM \xi(\alpha))/\varepsilon) are consequences of the Lyapunov inequality, not assumptions equivalent to the conclusion. The paper's reliance on exact knowledge of L_i is a genuine practical limitation, and the thesis itself flags the estimation cost in Section 1.2.5, but this is a conditional guarantee, not circularity. Citations to Richtárik et al. (2021a) supply standard lemmas (descent inequality, Young's inequality, a scalar bound) that are elementary and not the target result; they do not carry the EF21-W conclusion. The experimental sections use theoretical step sizes and do not fit convergence rates to measured data. I therefore find no circular step that reduces a prediction to its inputs.
Axiom & Free-Parameter Ledger
free parameters (1)
- Regularization coefficient lambda in EF21-W experiments =
0.001 or 1000 (chosen by hand)
axioms (5)
- domain assumption f and f_i are L- and L_i-smooth (Assumptions 1 and 2, Chapter 3)
- domain assumption f is lower bounded (Assumption 3, Chapter 3)
- standard math Contractive compression operators satisfy E||C(x)-x||^2 <= (1-alpha)||x||^2 (Definition 6, Chapter 3)
- domain assumption Polyak-Lojasiewicz condition (Assumption 4, Chapter 3)
- domain assumption Stochastic gradient estimator bound (Assumption 5, Chapter 3 appendix)
read the original abstract
Federated Learning (FL) is a novel, multidisciplinary Machine Learning paradigm where multiple clients, such as mobile devices, collaborate to solve machine learning problems. Initially introduced in Kone{\v{c}}n{\'y} et al. (2016a,b); McMahan et al. (2017), FL has gained further attention through its inclusion in the National AI Research and Development Strategic Plan (2023 Update) of the United States (Science and on Artificial Intelligence, 2023). The FL training process is inherently decentralized and often takes place in less controlled settings compared to data centers, posing unique challenges distinct from those in fully controlled environments. In this thesis, we identify five key challenges in Federated Learning and propose novel approaches to address them. These challenges arise from the heterogeneity of data and devices, communication issues, and privacy concerns for clients in FL training. Moreover, even well-established theoretical advances in FL require diverse forms of practical implementation to enhance their real-world applicability. Our contributions advance FL algorithms and systems, bridging theoretical advancements and practical implementations. More broadly, our work serves as a guide for researchers navigating the complexities of translating theoretical methods into efficient real-world implementations and software. Additionally, it offers insights into the reverse process of adapting practical implementation aspects back into theoretical algorithm design. This reverse process is particularly intriguing, as the practical perspective compels us to examine the underlying mechanics and flexibilities of algorithms more deeply, often uncovering new dimensions of the algorithms under study.
Figures
Reference graph
Works this paper leans on
-
[5]
EF21 with Bells & Whistles: Practical Algorithmic Extensions of Modern Error Feedback.CoRRabs/2110.03294 (2021). arXiv:2110.03294 https: //arxiv.org/abs/2110.03294 Ilyas Fatkhullin, Alexander Tyurin, and Peter Richtárik. 2023. Momentum Prov- ably Improves Error Feedback!. InAdvances in Neural Information Processing Systems 36: Annual Conference on Neural ...
Pith/arXiv arXiv 2021
-
[7]
MLX: Efficient and flexible machine learning on Apple silicon.https: //github.com/ml-explore Filip Hanzely and Peter Richtárik. 2020. Federated Learning of a Mixture of Global and Local Models.CoRRabs/2002.05516 (2020). arXiv:2002.05516 https://arxiv.org/abs/2002.05516 Andrew Hard, Kanishka Rao, Rajiv Mathews, Françoise Beaufays, Sean Au- genstein, Hubert...
Pith/arXiv arXiv 2020
-
[8]
Densely Connected Convolutional Networks. In2017 IEEE Conference on Computer Vision and Pattern Recognition, CVPR 2017, Honolulu, HI, USA, July 21-26, 2017. IEEE Computer Society, 2261–2269.https://doi.org/10. 1109/CVPR.2017.243 Junxian Huang, Feng Qian, Yihua Guo, Yuanyuan Zhou, Qiang Xu, Zhuoqing Mor- ley Mao, Subhabrata Sen, and Oliver Spatscheck. 2013...
arXiv 2017
-
[11]
Randomized Automatic Differentiation. (2021). https://openreview. net/forum?id=xpx9zj7CUlY Ceylan Onay and Elif Öztürk. 2018. A review of credit scoring research in the age of Big Data.Journal of Financial Regulation and Compliance26, 3 (2018), 382–405. Owkin. 2020. Story of the 1st Federated Learning Model at Owkin. https: //owkin.com/federated-learning/...
work page 2021
-
[13]
70), Doina Precup and Yee Whye Teh (Eds.)
DistributedMeanEstimationwithLimitedCommunication.InProceedings 434 of the 34th International Conference on Machine Learning, ICML 2017, Sydney, NSW, Australia, 6-11 August 2017 (Proceedings of Machine Learning Research, Vol. 70), Doina Precup and Yee Whye Teh (Eds.). PMLR, 3329–3337.http: //proceedings.mlr.press/v70/suresh17a.html Rafal Szlendak, Alexand...
-
[14]
Sharper Rates and Flexible Framework for Nonconvex SGD with Client and Data Sampling.Trans. Mach. Learn. Res.2023 (2023). https: //openreview.net/forum?id=zKgJ6TWAFE C. H. van Berkel. 2009. Multi-core for mobile phones. InDesign, Automation and Test in Europe, DATE 2009, Nice, France, April 20-24, 2009, Luca Benini, Giovanni De Micheli, Bashir M. Al-Hashi...
-
[15]
BatchCrypt: Efficient Homomorphic Encryption for Cross-Silo Feder- ated Learning. InProceedings of the 2020 USENIX Annual Technical Con- ference, USENIX ATC 2020, July 15-17, 2020, Ada Gavrilovska and Erez Zadok (Eds.). USENIX Association, 493–506. https://www.usenix.org/ conference/atc20/presentation/zhang-chengliang Hantian Zhang, Jerry Li, Kaan Kara, D...
work page 2020
-
[16]
Personalized Federated Learning with First Order Model Optimization. In 9th International Conference on Learning Representations, ICLR 2021, Virtual Event, Austria, May 3-7, 2021. OpenReview.net.https://openreview.net/ forum?id=ehJqJQk9cw Sixin Zhang, Anna Choromanska, and Yann LeCun. 2015. Deep learning with Elastic Averaging SGD. InAdvances in Neural In...
Pith/arXiv arXiv 2021
-
[1999]
Cache-Oblivious Algorithms. In40th Annual Symposium on Foundations of Computer Science, FOCS ’99, 17-18 October, 1999, New York, NY, USA. IEEE Computer Society, 285–298. https://doi.org/10.1109/SFFCS.1999. 814600 Rishikesh R. Gajjala, Shashwat Banchhor, Ahmed M. Abdelmoniem, Aritra Dutta, Marco Canini, and Panos Kalnis. 2020. Huffman Coding Based Encoding...
-
[2017]
QSGD: Communication-Efficient SGD via Gradient Quantization and Encoding. (2017), 1709–1720. https://proceedings.neurips.cc/paper/ 2017/hash/6c340f25839e6acdc73414517203f5f0-Abstract.html Dan Alistarh, Torsten Hoefler, Mikael Johansson, Nikola Konstantinov, Sarit Khirirat, and Cédric Renggli. 2018. The Convergence of Sparsified Gradient Methods. InAdvance...
Pith/arXiv arXiv 2017
-
[2019]
arXiv:1901.09269 http://arxiv.org/abs/1901
Distributed Learning with Compressed Gradient Differences.CoRR abs/1901.09269 (2019). arXiv:1901.09269 http://arxiv.org/abs/1901. 09269 Konstantin Mishchenko, Grigory Malinovsky, Sebastian U. Stich, and Peter Richtárik. 2022. ProxSkip: Yes! Local Gradient Steps Provably Lead to Communication Acceleration! Finally!. InInternational Conference on Machine Le...
Pith/arXiv arXiv 1901
-
[2020]
Secure, privacy-preserving and federated machine learning in medical imaging.Nat. Mach. Intell.2, 6 (2020), 305–311. https://doi.org/10.1038/ S42256-020-0186-1 Sai Praneeth Karimireddy, Satyen Kale, Mehryar Mohri, Sashank J. Reddi, Se- bastian U. Stich, and Ananda Theertha Suresh. 2020. SCAFFOLD: Stochastic Controlled Averaging for Federated Learning. InP...
Pith/arXiv arXiv 2020
-
[2021]
ACM, 1–7.https://doi.org/10.1145/3488659.3493775 Konstantin Burlachenko and Peter Richtárik. 2024. Unlocking FedNL: Self- Contained Compute-Optimized Implementation.CoRRabs/2410.08760 (2024). https://doi.org/10.48550/ARXIV.2410.08760arXiv:2410.08760 Konstantin Burlachenko and Peter Richtárik. 2025. BurTorch: Revisiting Training from First Principles by Co...
-
[2022]
RevEAL: Single-Trace Side-Channel Leakage of the SEAL Homomorphic Encryption Library. In2022 Design, Automation & Test in Europe Conference & Exhibition, DATE 2022, Antwerp, Belgium, March 14-23, 2022, Cristiana Bolchini, Ingrid Verbauwhede, and Elena-Ioana Vatajelu (Eds.). IEEE, 1527– 1532.https://doi.org/10.23919/DATE54114.2022.9774724 Boaz Barak, Oded ...
arXiv 2022
-
[2023]
Personalized Federated Learning with Communication Compression. Trans. Mach. Learn. Res.2023 (2023).https://openreview.net/forum?id= dZugyhbNFY James Bergstra, Olivier Breuleux, Frédéric Bastien, Pascal Lamblin, Razvan Pascanu, Guillaume Desjardins, Joseph P. Turian, David Warde-Farley, and Yoshua Bengio. 2010. Theano: A CPU and GPU Math Compiler in Pytho...
Pith/arXiv arXiv 2023
-
[2024]
FedSHE: privacy preserving and efficient federated learning with adaptive segmented CKKS homomorphic encryption.Cybersecur.7, 1 (2024), 40.https: //doi.org/10.1186/S42400-024-00232-W Adam Paszke, Sam Gross, Francisco Massa, Adam Lerer, James Bradbury, Gregory Chanan, Trevor Killeen, Zeming Lin, Natalia Gimelshein, Luca Antiga, Alban Desmaison, Andreas Köp...
arXiv 2024
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.