Comparison of Optimization Algorithms on the Rosenbrock Function

This script demonstrates the performance of various optimization algorithms in minimizing the Rosenbrock function, a well-known test problem in optimization.

Optimization Algorithms from DLL.DeepLearning.Optimisers:

• LBFGS (Limited-memory BFGS)

• SGD (Stochastic Gradient Descent)

• ADAM (Adaptive Moment Estimation)

• ADAGRAD (Adaptive Gradient Algorithm)

• ADADELTA (Adaptive Delta)

• RMSPROP (Root Mean Square Propagation)

import torch
import matplotlib.pyplot as plt
from scipy.optimize import minimize
import numpy as np

from DLL.DeepLearning.Optimisers import LBFGS, ADAM, SGD, ADAGRAD, ADADELTA, RMSPROP


torch.set_printoptions(sci_mode=False)

def f(x):
    return (1 - x[0]) ** 2 + 100 * (x[1] - x[0] ** 2) ** 2

def f_prime(x):
    return torch.tensor([-2 * (1 - x[0]) - 400 * x[0] * (x[1] - x[0] ** 2), 200 * (x[1] - x[0] ** 2)], dtype=x.dtype)

initial_point = [-0.11, 2.5]
x1 = torch.tensor(initial_point, dtype=torch.float32)
x2 = torch.tensor(initial_point, dtype=torch.float32)
x3 = torch.tensor(initial_point, dtype=torch.float32)
x4 = torch.tensor(initial_point, dtype=torch.float32)
x5 = torch.tensor(initial_point, dtype=torch.float32)
x6 = torch.tensor(initial_point, dtype=torch.float32)
optimiser1 = LBFGS(lambda: f(x1), history_size=10, maxiterls=20, learning_rate=0.1)
optimiser2 = SGD(learning_rate=0.001)
optimiser3 = ADAM(learning_rate=1, amsgrad=True)
optimiser4 = ADAGRAD(learning_rate=2)
optimiser5 = ADADELTA(learning_rate=10)
optimiser6 = RMSPROP(learning_rate=0.1, momentum=0.5)
optimiser1.initialise_parameters([x1])
optimiser2.initialise_parameters([x2])
optimiser3.initialise_parameters([x3])
optimiser4.initialise_parameters([x4])
optimiser5.initialise_parameters([x5])
optimiser6.initialise_parameters([x6])

# with 30 iterations, LBFGS algorithms (my and scipy) converge, while ADAM and SGD converge with around 1000 iterations using initial_point == [-0.11, 2.5]. ADAGRAD takes around 5000.
max_iter = 30
max_iter = 5000
points1 = [x1.clone()]
points2 = [x2.clone()]
points3 = [x3.clone()]
points4 = [x4.clone()]
points5 = [x5.clone()]
points6 = [x6.clone()]
for epoch in range(max_iter):
    optimiser1.zero_grad()
    optimiser2.zero_grad()
    optimiser3.zero_grad()
    optimiser4.zero_grad()
    optimiser5.zero_grad()
    optimiser6.zero_grad()
    x1.grad += f_prime(x1)
    x2.grad += f_prime(x2)
    x3.grad += f_prime(x3)
    x4.grad += f_prime(x4)
    x5.grad += f_prime(x5)
    x6.grad += f_prime(x6)
    optimiser1.update_parameters()
    optimiser2.update_parameters()
    optimiser3.update_parameters()
    optimiser4.update_parameters()
    optimiser5.update_parameters()
    optimiser6.update_parameters()
    points1.append(x1.clone())
    points2.append(x2.clone())
    points3.append(x3.clone())
    points4.append(x4.clone())
    points5.append(x5.clone())
    points6.append(x6.clone())
    # x = x1
    # print(f"Epochs: {epoch + 1}, f(x): {f(x)}, ||f'(x)||_2: {torch.linalg.norm(f_prime(x))}, x: {[round(num, 3) for num in x.tolist()]}")

scipy_points = []
def scipy_func(x):
    scipy_points.append(x)
    return (1 - x[0]) ** 2 + 100 * (x[1] - x[0] ** 2) ** 2

minimize(scipy_func, initial_point, method="L-BFGS-B", options={"maxiter": max_iter})  # options={"maxls": 1, "maxiter": max_iter}


colors = plt.cm.gist_rainbow(np.linspace(0, 1, 6))
plt.rcParams['axes.prop_cycle'] = plt.cycler(color=colors)

points1 = torch.stack(points1)
points2 = torch.stack(points2)
points3 = torch.stack(points3)
points4 = torch.stack(points4)
points5 = torch.stack(points5)
points6 = torch.stack(points6)
scipy_points = np.stack(scipy_points)
plt.figure(figsize=(10, 8))
line_style = (0, (1, 3))  # (0, (1, 20))
marker_size = 10
plt.plot(points1[:, 0], points1[:, 1], linestyle=line_style, markersize=marker_size, label="path LBSGF")
plt.plot(points2[:, 0], points2[:, 1], linestyle=line_style, markersize=marker_size, label="path SGD")
plt.plot(points3[:, 0], points3[:, 1], linestyle=line_style, markersize=marker_size, label="path ADAM")
plt.plot(points4[:, 0], points4[:, 1], linestyle=line_style, markersize=marker_size, label="path ADAGRAD")
plt.plot(points5[:, 0], points5[:, 1], linestyle=line_style, markersize=marker_size, label="path ADADELTA")
plt.plot(points6[:, 0], points6[:, 1], linestyle=line_style, markersize=marker_size, label="path RMSPROP")
plt.plot(scipy_points[:, 0], scipy_points[:, 1], linestyle=line_style, markersize=marker_size, label="path SCIPY LBSGF")
plt.plot(1, 1, "o", markersize=15, label="optimum")
plt.plot(*points1[0], "o", markersize=15, label="start")

x_vals = torch.linspace(-2, 2, 100)
y_vals = torch.linspace(-2.5, 3, 100)
X, Y = torch.meshgrid(x_vals, y_vals, indexing="ij")
Z = f([X, Y])

contour = plt.contourf(X.numpy(), Y.numpy(), Z.numpy(), levels=100, cmap="viridis")
plt.colorbar(contour)
plt.legend()
plt.xlim(-2, 2)
plt.ylim(-2.5, 3)
plt.show()