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Distributed Sparse Regression via Penalization
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Studies sparse linear regression over a network via a penalized consensus LASSO solved by distributed proximal gradient (DGD-like), proving near-optimal centralized sample rate O(s log d/N) with linear convergence whose rate scales as O(d), i.e., a speed-accuracy dilemma.
Problem
High-dimensional sparse linear regression over a decentralized network without a central node: statistical and computational guarantees of penalty-based consensus LASSO and its distributed proximal gradient method were unknown.
Approach
Formulates the network LASSO as a quadratic consensus-penalized problem, proves a restricted-strong-convexity-like in-network RE condition holds w.h.p. under Gaussian design so the averaged local estimation error matches centralized LASSO rate O(s log d/N) without local sample conditions, and shows the proximal DGD iterates converge linearly to a statistical-precision tolerance at rate scaling wit
Key results
- The averaged local estimation error of the penalized network LASSO matches the O(s log d/N) centralized rate under suitable lambda and gamma, with no condition on local sample size.
- Proximal gradient iterates converge linearly up to a tolerance of the order of the centralized statistical error, with iteration count scaling as O(d m log m/(1-rho)) per the stated bound.
- Experiments show a careful gamma (5e-4 for fully connected, 1e-4 for sparser ER graph with N=220, m=20, d=400) is needed to recover centralized error, and gamma scales roughly as 1/d.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use a sufficiently small penalty parameter γ | Penalized LASSO on the fully connected synthetic network · Synthetic data, N=220, m=20, d=400 | squared estimation error: γ=5×10^{-4} | Helped |
| Use γ=5×10^{-4} rather than a smaller value on the weakly connected graph | Penalized LASSO on the weakly connected synthetic network · Synthetic data, p=0.1, ρ≈0.973 | squared estimation error: 5×10^{-4} | Hurt |
| Decrease γ to 1×10^{-4} on the weakly connected graph | Penalized LASSO on the weakly connected synthetic network · Synthetic data, p=0.1, ρ≈0.973 | squared estimation error: 1×10^{-4} | Helped |
| Decrease γ as dimension d grows | Penalized LASSO · Synthetic data, dimensions 360, 800, and 1560 | squared estimation error: \mathcal{O}(1/d) | Helped |
| Use a smaller γ for a weaker-connected graph | Penalized LASSO · Synthetic data, fully connected versus weakly connected graphs | squared estimation error: 1/\gamma=\mathcal{O}(d/(1-\rho)) | Helped |
| Apply the distributed proximal-gradient algorithm to the penalized formulation | Penalized LASSO · Synthetic data | optimization error: \mathcal{O}(d) | Helped |
| Increase network size while holding total samples and dimension fixed | Distributed proximal gradient · Complete and Erdős–Rényi graphs | communication rounds: \widetilde{\mathcal{O}}(m) | Mixed |
| Use the distributed method on real data with a suitable γ | Penalized LASSO · eyedata test set | MSE | Helped |
| Decrease γ to improve statistical accuracy | Distributed proximal gradient on real data · eyedata test set, weakly connected graph | MSE | Mixed |
Surprising findings
- Achieving centralized statistical accuracy requires gamma to shrink with dimension, causing the linear convergence rate to degrade as O(d), a speed-accuracy dilemma.
- Naive decentralization of divide-and-conquer methods via local consensus fails to achieve centralized statistical consistency.
Limitations
- Linear convergence rate degrades with dimension d (roughly O(d) iterations), leading to total communication cost O(d^2).
- Theoretical guarantees rely on Gaussian random design and RSC-type conditions; the in-network RE condition is assumed/derived under these assumptions.
- Experiments use modest sizes (d up to 1560) and Monte Carlo with 30 repetitions.
Experimental setup
Datasets: synthetic Gaussian design data eyedata (NormalBeta-Prime package)
Metrics: average l2 estimation error optimization error MSE on test set communication rounds
Baselines: centralized LASSO local LASSO Avg-Debias CSL EDSL
Builds on: LASSO decentralized gradient descent (DGD) restricted eigenvalue condition
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