REVIEW 3 major objections 6 minor 40 references
Numerical Design of Optimized First-Order Algorithms
T0 review · 3 major / 6 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read The paper's central claim: a sequential linearization method designs step-size schedules that beat a branch-and-bound benchmark and accelerate several first-order descent methods beyond classical rates.
desk verdict A genuinely useful numerical design method (SLM) for fixed-step first-order algorithms, with exciting but unproven acceleration claims and a load-bearing benchmark comparison that needs code and data before it can be believed. 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 central machinery is the Performance Estimation Problem (PEP), a reformulation of worst-case convergence analysis as a semidefinite program in the Gram matrix of iterates and gradients, together with the sequential linearization method (SLM). PEP replaces the infinite-dimensional choice of a worst-case function by interpolation conditions on a finite set of points, and the design problem is written as a min-max problem whose inner maximization is dualized into a nonlinear conic constraint $A(\tau,\lambda,\alpha)\succeq 0$. SLM iteratively linearizes that constraint around the current step sizes and dual multipliers, solves the resulting tangent SDP with a trust-region bound on the update, and accepts or rejects the update according to the ratio of actual to predicted improvement. This machinery converts algorithm design into a sequence of convex problems, which is why it can be applied to any SDP-representable function class.
What would settle it
Solve the PEP for the SLM-designed two-block memoryless cyclic coordinate descent at much larger $N$, say $N=200$, and for the cyclic gradient descent at many multiples of the 4-step cycle; if the coordinate upper bounds stop tracking $1/N^{1.145}$ or the per-cycle contraction factor rises above $0.14239$, the claimed acceleration is refuted.
Extended reading notes
Core claim
The central discovery is that a sequential linearization method can solve the nonconvex algorithm-design problem that arises when one dualizes a Performance Estimation Problem: at each iteration one linearizes the nonlinear matrix inequality $A(\tau,\lambda,\alpha)\succeq 0$ around the current step sizes and dual variables and solves a tangent semidefinite program inside a trust region. The paper reports that on the benchmark of optimizing memoryless gradient descent step sizes over L-smooth convex functions, SLM produces worst-case upper bounds that are at most the BNB-PEP values at every listed $N$ up to 50, including $f(x_{19})-f_*\le 0.004869$ against BNB-PEP's $0.004941$, and therefore that BNB-PEP's claimed optima up to $N=25$ are not optimal. For new settings, SLM-designed steps give fits $O(1/N^{1.145})$ for two-block memoryless cyclic coordinate descent, $O(1/N^{1.47})$ for full cyclic coordinate descent, rates from $O(1/N^{1.018})$ to $O(1/N^{1.116})$ for memoryless inexact gradient descent on the squared gradient norm, and a cyclic gradient descent that satisfies $\|x_{4N}-x_*\|^2\le 0.14239^N\|x_0-x_*\|^2$ on $1$-smooth $0.1$-strongly convex functions, improving the $0.2008^N$ bound of the optimal constant step size.
Load-bearing premise
The load-bearing premise is that the fitted formulas over a short range of iteration counts reveal the true long-run convergence rates; the paper also assumes, on numerical evidence only, that its reformulated design problems give exact worst-case values rather than loose relaxations.
Editorial extensions
If this is right
- For memoryless gradient descent over smooth convex functions, SLM produces worst-case upper bounds that beat BNB-PEP's published values at every listed $N$ up to 50, so those published values are not genuinely optimal as claimed up to $N=25$.
- Memoryless cyclic coordinate descent with SLM-tuned steps converges faster than $O(1/N)$ even with two blocks and no memory of past gradients; the numerical fit is $O(1/N^{1.145})$.
- On $1$-smooth $0.1$-strongly convex functions, cyclic gradient descent with SLM steps contracts the squared error by a factor $0.14239$ per 4-step cycle, against $0.2008$ per 4 steps for the optimal constant step size.
- Inexact gradient descent with optimized steps beats the $O(1/N)$ bound of constant-step inexact gradient descent for the squared gradient norm when inexactness is small, and using past inexact gradients increases the fitted exponent further.
- The design pipeline applies to any SDP-representable function class, so the same numerical method can be reused to tune fixed-step algorithms for other oracle models.
Reading between the lines
- If the finite-$N$ fits reveal true asymptotics, momentum-free acceleration is a general step-size phenomenon rather than a quirk of gradient descent; the same pipeline should be tried on proximal, randomized, and decentralized variants to see where the acceleration survives.
- The paper's Table 1 suggests that published BNB-PEP optimal values are systematically slightly too high; re-running other BNB-PEP-designed algorithms with SLM could uncover similar gaps in published benchmark tables.
- The SLM step sizes in Table 2 resemble but do not match the composed schedules discussed in [4,20], so finding an analytic description of these schedules could turn a numerical discovery into a proof.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This manuscript studies the computer-assisted design of fixed-step first-order optimization methods through the Performance Estimation Problem (PEP) framework. It develops a generic SDP formulation for evaluating a fixed algorithm and a nonconvex design formulation, then proposes three numerical heuristics: Alternating Minimization, a first-order method, and a Sequential Linearization Method (SLM). The methods are benchmarked on the task of optimizing step sizes for memoryless gradient descent over smooth convex functions, with Table 1 claiming that SLM outperforms the published BNB-PEP values on many horizons and is computationally more efficient. The paper then applies SLM to design step sizes for cyclic coordinate descent, inexact gradient descent, full-memory variants, and cyclic gradient descent on strongly convex functions, reporting accelerated numerical rates such as O(1/N^1.145) for two-block memoryless cyclic coordinate descent and a linear contraction factor of 0.14239^N for a four-step cyclic gradient method. The authors are explicit that the rates in Section 5 are numerical fits over finite N and label several of them as conjectures, but the abstract and Section 4 make stronger claims of superiority over BNB-PEP.
Significance. If the results are correct, the paper offers a flexible numerical toolbox for algorithm design that goes beyond known analytical constructions and can produce nontrivial step-size schedules for settings where no analytic optimal schedule is known. The PEP upper bounds for smooth strongly convex functions are grounded in exact interpolation conditions, and the manuscript is honest in labeling the Section 5 rates as fitted numerical evidence rather than proven theorems. The applications to cyclic coordinate descent and inexact gradient descent are interesting and likely to stimulate further work. However, the strongest claims are numerical and are not independently reproducible: no code, data, solver settings, or certificates are provided, and the Section 4 comparison against BNB-PEP is based on published values rather than a re-run. The reported upper bounds are also the same quantities minimized during design, which does not invalidate them as bounds but means the experiments do not independently certify optimality. These issues must be addressed before the paper's more ambitious claims can be accepted.
major comments (3)
- [Section 4, Table 1 and following paragraph] The claim that SLM outperforms BNB-PEP and that BNB-PEP's published values are not optimal is load-bearing but is not supported by reproducible evidence. No code, data, solver versions, tolerances, or certificates are provided, and BNB-PEP is not re-run; Table 1 compares an unreleased SLM implementation against values read from [21]. If both methods solve the same exact PEP, a local method cannot beat a correctly solved global method, so the discrepancy must be explained; if the formulations differ, the equivalence must be established. The manuscript attributes the discrepancy to 'numerical issues/bugs due to Gurobi branch-and-bound' without evidence. To make the claim stand, the authors should provide reproducibility artifacts and either a primal SDP certificate for the SLM bound below the BNB-PEP value or a corrected BNB-PEP run with reported tolerances.
- [Section 5, Remark 12 and Tables 6-9] The accelerated-rate claims (O(1/N^1.145) for memoryless cyclic coordinate descent, the rates in Tables 6-9, and the cyclic contraction 0.14239^N) are obtained by fitting the model 1/(alpha N^nu + beta) to PEP upper bounds over a small finite range of N, with no error bars, no sensitivity analysis, and no asymptotic verification. A three-parameter fit over N up to 30 or 40 can readily produce nu > 1 even when the true asymptotic rate is O(1/N), possibly with logarithmic factors. Since these rates are the main conclusions of Section 5, the authors should either provide evidence that the fitted exponents are stable as N grows and across fitting windows, or explicitly reframe the claims as finite-horizon numerical observations rather than convergence rates. This also affects the abstract's statement that accelerated convergence rates are reported in all cases.
- [Section 4, Tables 3-5 and Figure 3] The runtime and efficiency comparison conflates BNB-PEP with its local IPOPT first step. Tables 3-5 and Figure 3 compare SLM against the local QCQP step of BNB-PEP using IPOPT, not against the full branch-and-bound procedure, while the conclusion describes the comparison as being with the BNB-PEP approach. The full BNB-PEP runtimes cited from [21] are not measured in this paper, so the claim of computational advantage over BNB-PEP is not established by these experiments. The tables should be labeled as a local-solver comparison, and the runtime-related conclusions should be narrowed accordingly.
minor comments (6)
- [Section 3, Theorem 3.2] The gradient formula uses G, which Theorem 3.1 identifies as a solution of Problem (SDP-PEP-Generic), but the text says 'the unique (see remark 7) solution of Problem (dual-PEP-generic)'; this appears to be a typo and should be corrected.
- [Figures 6 and 7] The legends refer to 'Theorem 5.19' but the relevant statement is Theorem 5.3; the numbering should be fixed.
- [Algorithm 1] The output is written as alpha^{(N)}_{i,k} although the loop runs for T iterations; the output should be alpha^{(T)}_{i,k}.
- [Section 3 and Section 4] Remark 12 uses the symbol alpha both for the step sizes and for the coefficient in the fitted model 1/(alpha N^nu + beta); a different symbol, such as a or c, would avoid ambiguity.
- [Section 4 and Section 5] There are several typographical errors: 'initilizations' in the discussion of AM/FOM, 'T able' in several table captions, 'four our' in the full inexact gradient descent subsection, and 'forgoing' should be 'foregoing' in Remark 10.
- [Theorem 2.4] The statement says 'For any d in Z', but the dimension d of the ambient space should be a positive integer; if the bound is intended to be dimension-independent, that should be stated explicitly.
Circularity Check
No significant circularity: the reported rates are explicit numerical fits, and the design/evaluation loop is transparent rather than a derivation that reduces to its own inputs.
full rationale
The paper's derivation chain is a standard optimization loop: worst-case performance is formulated as a convex SDP (Theorem 2.4), dualized (dual-PEP-generic), and step sizes are designed by minimizing the dual upper bound (Algo-design-generic), solved by SLM. The same PEP bound is later used to evaluate the designed steps. This is self-referential only in the weak sense that the evaluation metric is the same as the design objective; it is not circular, because the PEP bound is a valid worst-case upper bound for the algorithm, and comparing it with the PEP bound of a classical schedule is a comparison of two legitimate bounds. The accelerated-rate claims (e.g., O(1/N^1.145) for cyclic coordinate descent) are not presented as first-principles predictions: Remark 12 explicitly states that the rates are obtained by solving the PEP over a range of N and fitting a model of the form 1/(αN^ν + β), and the statements are labeled as Conjectures 5.1 and 5.2 rather than theorems. The Section 4 claim that SLM outperforms BNB-PEP and that BNB-PEP's published values are therefore not optimal is not circular, but it depends on an unreproduced comparison against published numbers and on an asserted Gurobi branch-and-bound bug; that is a reproducibility/correctness risk, not a circularity. Self-citations to [38] and [23] supply background PEP interpolation conditions and coordinate-descent PEP formulations; they are technical tools with independent published or stated content, and no load-bearing conclusion reduces to an unverified self-citation. The paper also acknowledges its limitations (Remarks 1, 2, 3, 6), including that the SDP may be a relaxation, that strong duality is only observed experimentally, and that local methods have no global convergence guarantee, which further supports that no hidden circular construction is being used.
Assumptions & free parameters
free parameters (4)
- Optimized step sizes alpha_{i,k} =
Tables 2 and optimized schedules for Sections 5
- Fitted convergence exponents nu =
1.145, 1.47, 1.116, 1.384, 1.171, 1.450, 1.342, 1.190, 1.052, 1.275, 1.151, etc.
- Fitted model coefficients alpha and beta =
Not reported
- Cycle length N* =
4
assumptions (4)
- domain assumption The functional classes considered are SDP-representable and the interpolation conditions used give exact worst-case PEP values when the paper claims improved performance.
- domain assumption Strong duality holds for every explicit PEP formulation used.
- ad hoc to paper Finite-N PEP upper bounds follow the model 1/(alpha N^nu + beta), so fitted exponents represent asymptotic convergence rates.
- domain assumption For cyclic gradient descent, the contraction bound computed over one optimized cycle transfers to every subsequent cycle under the same schedule.
Cite this review
Pith. "Pith review of Numerical Design of Optimized First-Order Algorithms." pith.science (2026). https://pith.science/paper/C2DUP6UJ
@misc{pith2026250720773,
author = {Pith},
title = {Pith review of: Numerical Design of Optimized First-Order Algorithms},
year = {2026},
howpublished = {\url{https://pith.science/paper/C2DUP6UJ}},
note = {Machine review of arXiv:2507.20773}
}
read the original abstract
We derive several numerical methods for designing optimized first-order algorithms in unconstrained convex optimization settings. Our methods are based on the Performance Estimation Problem (PEP) framework, which casts the worst-case analysis of optimization algorithms as an optimization problem itself. We benchmark our methods against existing approaches in the literature on the task of optimizing the step sizes of memoryless gradient descent (which uses only the current gradient for updates) over the class of smooth convex functions. We then apply our methods to numerically tune the step sizes of several memoryless and full (i.e., using all past gradient information for updates) fixed-step first-order algorithms, namely coordinate descent, inexact gradient descent, and cyclic gradient descent, in the context of linear convergence. In all cases, we report accelerated convergence rates compared to those of classical algorithms.
Figures
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Reference graph
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