REVIEW 2 major objections 5 minor 41 references
Decentralized Min-Max Optimization with Gradient Tracking
T0 review · 2 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read Decentralized min-max optimization matches centralized convergence rates.
desk verdict A genuinely new per-agent-y formulation with a solid deterministic analysis, but the constant-batch-size stochastic claim is not supported by the paper's own bound. 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 engine is the gradient tracker, updated in DGTA by $g_i^{t+1} = \sum_{j \in \mathcal{N}_i} w_{ij}(g_j^t + \nabla_x f_i(x_i^{t+1}, y_i^{t+1}) - \nabla_x f_i(x_i^t, y_i^t))$, which lets every agent estimate the average of the local $x$-gradients without a coordinator. Around it, the paper builds the Lyapunov function $\mathcal{L}^t = \Phi(\bar{x}^t) - \Phi^* + \frac{300\kappa^2 L^2 \eta_x}{n(1-\lambda^2)}\|x^t - \bar{x}^t\|^2 + \frac{8\eta_x L^2}{n\eta_y \mu}\delta^t + \frac{2400\eta_x^3\kappa^2 L^2}{n(1-\lambda^2)^3}\|g^t - \bar{g}^t\|^2$. The coefficients are chosen so that Lemmas 3.1–3.4 combine into a single contraction (Lemma 3.5), turning the four coupled error terms into one step-size-dependent descent on the smoothed function $\Phi$.
What would settle it
Use Corollary 4.1.1 to compute the iteration bound on a family of graphs with shrinking spectral gap (e.g., rings of increasing size). If the number of iterations to reach $\|\nabla\Phi(\bar{x})\|^2 \le \varepsilon^2$ scales like $\kappa^2/\varepsilon^2$ times a constant independent of $n$, the headline claim is exactly right; if it scales like $n^2\kappa^2/\varepsilon^2$, then the graph dependence is a genuine factor, not a constant. This is a direct check from the displayed bounds in the paper.
Extended reading notes
Core claim
The central claim is that problem (1.1) — $\min_x \max_{\{y_i\}} \frac{1}{n} \sum_{i=1}^n f_i(x,y_i)$ with agent-specific $y_i$ and constraint sets $Y_i$ — can be solved by a single-loop decentralized method in which only $x$ is averaged over the network while each $y_i$ is updated by one projected gradient ascent step and never communicated. The paper proves this through a Lyapunov analysis: the gradient tracker keeps each agent's $g_i$ close to the average $x$-gradient, and the coupled recursions for the function value, consensus gap, maximizer gap $\delta^t = \sum_i \|\hat{y}_i(\bar{x}^t) - y_i^t\|^2$, and tracker disagreement all contract together. The resulting rates match the centralized GDA/SGDA results in [17] up to factors involving the spectral gap $1-\lambda$ of the mixing matrix, and the analysis requires no bounded-heterogeneity or bounded-second-moment assumptions.
Load-bearing premise
The load-bearing assumption is that every local constraint set $Y_i$ is convex, compact, and has a common finite diameter $D$; if any $Y_i$ were unbounded, the initial distance from $y_i^0$ to its argmax could be infinite and the finite-time bound would not close.
Editorial extensions
If this is right
- DGTA attains iteration complexity $O(\kappa^2\varepsilon^{-2})$ for full gradients, the same order as centralized GDA, with the network entering only through spectral-gap factors.
- DSGTA with batch size $b = \kappa\sigma^2/\varepsilon^2$ attains sample complexity $O(\kappa^3\varepsilon^{-4})$, matching centralized SGDA.
- With batch size $b=1$, DSGTA attains sample complexity $O(\kappa^2\varepsilon^{-4})$ for small $\varepsilon$, independent of the graph and better in $\kappa$ than the large-batch regime.
- The methods handle heterogeneous constraint sets $Y_i$ across agents without consensus constraints on $y$ and without bounded gradient dissimilarity.
- A trade-off emerges: large batches reduce iteration count, while small batches reduce dependence on the communication graph.
Reading between the lines
- I infer that the advertised 'match up to constant factors' should be read with the spectral gap in mind: Corollary 4.1.1 contains $1/(1-\lambda)^2$ terms, so on a sparse ring with $1-\lambda = O(1/n^2)$ the complexity can degrade like $n^2\kappa^2/\varepsilon^2$, which is more than a constant-factor gap.
- I infer the heterogeneous-$y_i$ formulation is a natural fit for personalized federated learning and multi-domain GANs, where a single shared discriminator variable is unrealistic; the single-loop projected ascent on $y_i$ is what makes the method practical.
- A testable extension is to replace the deterministic projection step with variance-reduced stochastic updates for $y_i$; the Lyapunov recursion suggests $O(\varepsilon^{-3})$ sample complexity might be reachable.
- Because the paper's $b=1$ sample complexity loses the $1/(1-\lambda)$ factor, I infer that in communication-heavy regimes small or moderate batch sizes are preferable even if iteration counts rise.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the decentralized min-max problem (1.1), in which agents jointly minimize a shared variable x while maximizing their own local variables y_i over possibly heterogeneous constraint sets Y_i, under nonconvex-strongly-concave (NC-SC) smooth objectives. Two algorithms are proposed: DGTA, which uses full gradients with gradient tracking, and DSGTA, which uses mini-batched stochastic gradients. The authors claim that DGTA reaches an ε-first-order stationary point in O(κ^2 ε^{-2}) iterations and that DSGTA has sample complexity O(κ^3 ε^{-4}) with large batches and O(κ^2 ε^{-4}) with constant batch size b=1, matching centralized GDA/SGDA up to network-dependent constants. The analysis is based on a Lyapunov function combining the primal descent, the errors δ^t = sum_i ||\hat y_i(\bar x^t) - y_i^t||^2, the consensus error, and the gradient-tracking error. Numerical experiments on an empirical Wasserstein robustness model with MNIST data compare DGTA with GT/DA and study DSGTA under different step sizes. The appendix contains detailed proofs of the lemmas and corollaries.
Significance. The paper's formulation with heterogeneous y_i and Y_i is a genuine extension of the common decentralized min-max formulation, and the proposed algorithms avoid bounded-heterogeneity assumptions on stochastic gradients. The Lyapunov analysis is systematic, and the appendix provides full finite-time proofs for the deterministic and large-batch stochastic results. If the advertised constant-batch (b=1) complexity were correct, it would be a notable improvement in the dependence on the network spectral gap. However, the proof of Corollary 4.1.2 contains a substantial gap: a variance term that is asymptotically a positive constant is bounded as if it were O(1/T). In addition, the abstract's κ-only rates omit explicit spectral-gap factors that appear in the paper's own tables and corollaries. These issues affect central claims and must be fixed before the paper is suitable for publication.
major comments (2)
- [Corollary 4.1.2 / Theorem 4.1] The claimed O(κ^2 ε^{-4}) sample complexity with b=1 is not established. Under the step sizes stated in Corollary 4.1.2, for large T one has η_y ≈ D/√(3σ^2 T) and η_x ≈ √(n Δ_Φ/(L κ σ^2 T)). Substituting these into the first additive variance term of (4.1), namely (1/(n η_y^2 μ^2)) · 64 η_x^2 κ^2 L^2 σ^2, gives an asymptotic constant of order Δ_Φ κ L σ^2/(μ^2 D^2), not a vanishing term. Hence the right-hand side of (4.1) has a positive limit as T→∞, so the bound cannot imply E‖∇Φ(\bar x^t)‖^2 ≤ ε^2 with T = O(κ^2 ε^{-4}). In the proof of Corollary 4.1.2 this term is replaced by a multiple of n Δ_Φ κ L / T; that replacement requires 1/(η_y^2 μ^2) = O(1), whereas the chosen η_y satisfies η_y^2 μ^2 = O(1/T). Because the b=1 row of Table 2 and Remark 4.3 depend directly on this step, the proof must be corrected or the step-size choices revised.
- [Abstract / Remark 4.2 / Table 1] The abstract and Section 1.2 state that DGTA achieves O(κ^2 ε^{-2}) and that both algorithms match centralized counterparts up to constant factors related to the communication network. However, Corollary 4.1.1 and Remark 4.2, as well as Table 1, give rates containing explicit factors such as 1/(1−λ)^2 and 1/√(1−λ). For sparse graphs 1/(1−λ) can grow as O(n^2), so these factors are not bounded by a constant independent of the network size. The headline complexity statements should either explicitly include the spectral-gap dependence or clearly specify the class of graphs over which the stated constants are uniform.
minor comments (5)
- [Lemma 3.5 and Appendix A.5] In the statement of Lemma 3.5, Eq. (3.4), and the final display of Appendix A.5, the expression ∇Φ(\bar x^2) should be ∇Φ(\bar x^t); the superscript t is missing.
- [End of Section 3] The sentence before Lemma 3.4 says 'whose recursion is stated in Lemma 3.3' but should refer to Lemma 3.4.
- [Corollary 4.1.1] The heading of Corollary 4.1.1 says 'DSGT' instead of 'DGTA'.
- [Section 5] In the discussion of Figure 1b, the sentence 'since the full gradient computation requires.' is incomplete and should be finished.
- [Remark 4.3, Eq. (4.9)] The claim that (4.9) reduces to O(Δ_Φ L κ σ^2/(n ε^4) + D^2 σ^2 L^2 κ^2/ε^4) when ε is small should state the threshold condition on ε relative to the other terms in the bound.
Circularity Check
No circular derivation: the convergence and complexity claims follow from self-contained Lyapunov recursions; only minor, non-load-bearing self-citations appear.
full rationale
The paper's derivation chain is self-contained. Lemmas 3.1-3.4 establish recursions for Phi(bar x^t), delta^t, ||x^t - bar x^t||^2, and ||g^t - bar g^t||^2 using Assumptions 2.1-2.5, and Lemma 3.5 combines them into a Lyapunov function with explicit constants. The constants in the Lyapunov function are chosen algebraically in Appendix A.5 to satisfy inequalities (A.28a)-(A.28c), not fitted to any target complexity or empirical data. Theorem 4.1 then averages the Lyapunov recursion, and Corollaries 4.1.1-4.1.3 substitute explicit stepsizes and batch sizes and solve for T. Thus the claimed O(kappa^2 epsilon^{-2}) and O(kappa^3 epsilon^{-4}) rates are consequences of the stated bounds, not restatements of the desired rates. The abstract's omission of spectral-gap factors is an imprecise presentation of Corollary 4.1.1, which explicitly contains 1/(1-lambda) and 1/(1-lambda)^2 terms; this is a presentation concern, not circularity. The support of the b=1 sample complexity in Corollary 4.1.2 may be questionable because the variance floor in Theorem 4.1 need not vanish with the stated T-dependent stepsizes, but that is a proof-gap/correctness issue, not a circularity: the claim is not obtained by defining a quantity in terms of itself or by renaming a fitted parameter. Self-citations appear, for example [27] and [11], for gradient tracking and variance assumptions, but the proofs in the appendix do not rely on those citations; they are background references, so they are not load-bearing. Hence no specific circular step can be exhibited, and the circularity score is minimal.
Assumptions & free parameters
assumptions (6)
- domain assumption Assumption 2.1: The graph is undirected, connected, and the mixing matrix W is nonnegative, symmetric, and doubly stochastic, with spectral gap 1-λ > 0.
- domain assumption Assumption 2.2: Each f_i is L-smooth and μ-strongly concave in y.
- domain assumption Assumption 2.3: Each Y_i is convex and compact with diameter D.
- domain assumption Assumption 2.4: Φ is lower bounded.
- domain assumption Assumption 2.5: Stochastic gradients are unbiased with bounded variance σ².
- standard math Lemma 2.1: Φ_i is 2κL-smooth and the argmax y_i(x) is κ-Lipschitz.
Cite this review
Pith. "Pith review of Decentralized Min-Max Optimization with Gradient Tracking." pith.science (2026). https://pith.science/paper/2TYIJWHC
@misc{pith2026250510631,
author = {Pith},
title = {Pith review of: Decentralized Min-Max Optimization with Gradient Tracking},
year = {2026},
howpublished = {\url{https://pith.science/paper/2TYIJWHC}},
note = {Machine review of arXiv:2505.10631}
}
abstract
This paper presents a novel distributed formulation of the min-max optimization problem. Such a formulation enables enhanced flexibility among agents when optimizing their maximization variables. To address the problem, we propose two distributed gradient methods over networks, termed Distributed Gradient Tracking Ascent (DGTA) and Distributed Stochastic Gradient Tracking Ascent (DSGTA). We demonstrate that DGTA achieves an iteration complexity of $\mathcal{O}(\kappa^2\varepsilon^{-2})$, and DSGTA attains a sample complexity of $\mathcal{O}(\kappa^3\varepsilon^{-4})$ for nonconvex strongly concave (NC-SC) objective functions. Both results match those of their centralized counterparts up to constant factors related to the communication network. Numerical experiments further demonstrate the superior empirical performance of the proposed algorithms compared to existing methods.
Figures
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