Gradient Descent Qg Convex
wc_gradient_descent_qg_convex(L, gamma, n; solver=Clarabel.Optimizer, verbose=true)Problem statement
Compute a PEPit worst-case guarantee for wc_gradient_descent_qg_convex.
Consider the convex minimization problem
\[f_\star \triangleq \min_x f(x),\]
where $f$ is quadratically upper bounded ($\text{QG}^+$ [1]), i.e. $\forall x, f(x) - f_\star \leqslant \frac{L}{2} \|x-x_\star\|^2$, and convex.
Performance metric
This code computes a worst-case guarantee for gradient descent with fixed step-size $\gamma$. That is, it computes the smallest possible $\tau(n, L, \gamma)$ such that the guarantee
\[f(x_n) - f_\star \leqslant \tau(n, L, \gamma) \| x_0 - x_\star\|^2\]
is valid, where $x_n$ is the output of gradient descent with fixed step-size $\gamma$, and where $x_\star$ is a minimizer of $f$.
In short, for given values of $n$, $L$, and $\gamma$, $\tau(n, L, \gamma)$ is computed as the worst-case value of $f(x_n)-f_\star$ when $||x_0 - x_\star||^2 \leqslant 1$.
Algorithm
Gradient descent is described by
\[x_{t+1} = x_t - \gamma \nabla f(x_t),\]
where $\gamma$ is a step-size.
Theoretical guarantee
When $\gamma < \frac{1}{L}$, the lower theoretical guarantee can be found in [1, Theorem 2.2]:
\[f(x_n)-f_\star \leqslant \frac{L}{2}\max\left(\frac{1}{2n L \gamma + 1}, L \gamma\right) \|x_0-x_\star\|^2.\]
References
The detailed approach is available in [1, Theorem 2.2].
Arguments
L: smoothness or Lipschitz parameter, as used by the modeled class.gamma: step-size parameter.n: number of iterations.solver: JuMP optimizer constructor used to solve the generated SDP.verbose: print example and solver progress information when true.
Returns
pepit_tau: worst-case valuetheoretical_tau: theoretical value
Julia usage
pepit_tau, theoretical_tau = wc_gradient_descent_qg_convex(L, 0.2 / L, 4; verbose=true)
## Returns approximately: (pepit_tau, theoretical_tau) = (0.192308, 0.192308)