# Data equilibration

Data scale has a large impact on the convergence of first-order algorithms like SCS in practice. With that in mind SCS implements a heuristic data equilibration (or normalization) scheme that attempts to whiten the data which is enabled by the normalize setting.

At a high level the equilibration scheme (which is performed once, right at the start of the solve) produces diagonal matrices $$E \in \mathbf{R}^n$$, $$D \in \mathbf{R}^m$$ and scalar $$\sigma > 0$$ that rescale the data in-place, so that the tuple $$(P, A, b, c)$$ becomes $$(\hat P, \hat A, \hat b, \hat c)$$ where

$\hat P = EPE,\quad \hat A = DAE,\quad \hat c = \sigma Ec,\quad \hat b = \sigma Db$

Essentially:

$\begin{split}\begin{bmatrix} \hat P & \hat A^\top & \hat c\\ \hat A & 0 & \hat b \\ \hat c^\top & \hat b^\top & 0 \end{bmatrix} = \begin{bmatrix} E & 0 & 0 \\ 0 & D & 0 \\ 0 & 0 & \sigma \end{bmatrix} \begin{bmatrix} P & A^\top & c\\ A & 0 & b \\ c^\top & b^\top & 0 \end{bmatrix} \begin{bmatrix} E & 0 & 0 \\ 0 & D & 0 \\ 0 & 0 & \sigma \end{bmatrix} = \begin{bmatrix} EPE & EA^\top D & \sigma Ec\\ DAE & 0 & \sigma Db \\ \sigma c^\top E & \sigma b^\top D & 0 \end{bmatrix}\end{split}$

In other words $$D$$ rescales the rows of $$A, b$$ and $$E$$ rescales the columns of $$A$$ and the rows / cols of $$P, c$$. Equilibrating matrices is a classic problem in linear algebra, and SCS simply implements NUM_RUIZ_PASSES (default 25) steps of Ruiz equilibration followed by NUM_L2_PASSES (default 1) steps of $$\ell_2$$ equilibration on the matrix

$\begin{split}\begin{bmatrix} P & A^\top & c\\ A & 0 & b \\ c^\top & b^\top & 0 \end{bmatrix}\end{split}$

The main complication of the equilibration is that $$D$$ has to maintain cone memberships, ie, it needs to satisfy for each sub-cone $$\mathcal{K}$$

$s \in \mathcal{K} \Leftrightarrow D_{\mathcal{K}} s \in \mathcal{K}$

To do this we restrict $$D_\mathcal{K} = d_\mathcal{K} I_\mathcal{K}$$, ie, $$D$$ is constant diagonal within each cone. We take the scalar value to be the $$\ell_\infty$$ norm of the calculated in-cone values for Ruiz equilibration and the average $$\ell_2$$ norm of the in-cone values for $$\ell_2$$ equilibration.

Originally the code supported separate primal_scale and dual_scale parameters scaling $$c$$ and $$b$$, but the latest version of SCS has set both of these to the same value of $$\sigma$$.

## Solution

This equilibration changes the fixed point of SCS, but we can recover the solutions to the original problem from the equilibrated solution. Denote by $$(\hat x, \hat y, \hat s)$$ the solution to the equilibrated problem, then the solution to the original problem is given by

$x = E\hat x / \sigma, \quad y = D \hat y / \sigma, \quad s = D^{-1} \hat s / \sigma$

with objective terms

$c^\top x = \hat c^\top \hat x / \sigma^2, \quad b^\top y = \hat b^\top \hat y / \sigma^2, \quad x^\top P x = \hat x^\top \hat P \hat x / \sigma^2$

The same modifications hold for the certificates of infeasibility.

## Residuals

The equilibration also changes the residuals and when running SCS we are interested in detecting convergence using the original problem residuals, rather than the equilibration problem residuals. Using the above results we can compute the residuals of the original problem as follows (under any norm).

Primal residual:

$r_p = \|A x + s - b\| = (1/\sigma) \| D^{-1} (\hat A \hat x + \hat s + \hat b)\|$

Dual residual:

$r_d = \|P x + A^\top y + c\| = (1/\sigma) \|E^{-1} (\hat P \hat x + \hat A^\top \hat y + \hat c) \|$

Duality gap:

$r_g = |x^\top P x + b^\top y + c^\top x| = (1/\sigma^2) |\hat x^\top \hat P \hat x + \hat b^\top \hat y + \hat c^\top \hat x|$

## Infeasibility certificates

Similar relationships hold for the infeasibility certificate terms:

$\|Ax + s\| = (1/\sigma) \|D^{-1} (\hat A \hat x + \hat s) \|, \quad \|Px\| = (1/\sigma) \|E^{-1} \hat P \hat x \|, \quad \|A^\top y \| = (1/\sigma) \|E^{-1} \hat A^\top \hat y \|$