Syllabus Map

• Study map: Syllabus Study Map

Overview

• This note covers optimisers, convergence behaviour, and regularisation for neural networks.
• Each section focuses on key ideas, equations, and practical choices that affect training stability and generalisation.

Optimisers

Core idea

• Optimisers update parameters $\theta$ to minimise a loss $\mathcal{L}(\theta)$.
• The basic update is:
  $\theta_{t+1} = \theta_t - \eta \nabla_\theta \mathcal{L}(\theta_t)$
• Different optimisers trade off speed, stability, and generalisation.

Gradient descent

• Uses the full dataset to compute the gradient.
• Stable but slow for large datasets.
• Update rule:

SGD

• Uses mini‑batches to approximate the gradient.
• Faster updates, introduces useful noise for generalisation.
• Update rule:

Momentum

• Accumulates a velocity to smooth updates:
  • The velocity acts like an exponential moving average of past gradients.
  • It builds inertia in consistent directions and filters out noisy updates.
  • A typical momentum coefficient is $\mu \in [0.9, 0.99]$.
• Helps traverse flat regions and damp oscillations.
• Update rule:

Adagrad

• Adapts the learning rate per parameter based on past gradients.
• Good for sparse features, but can decay too aggressively.
• Difference between $r_t$ and $\eta$:
  • $r_t$ is the accumulated sum of squared gradients for each parameter.
  • It grows over time, so parameters with large historical gradients get smaller effective steps.
  • $\eta$ is the global base learning rate, while $r_t$ is the per‑parameter history that scales it.
  • Effective step size is $\eta \over (\sqrt{r_t} + \epsilon)$, which shrinks as $r_t$ increases.
• Update rule:

RMSprop

• Fixes Adagrad’s aggressive decay with an exponential moving average.
• Works well for non‑stationary objectives.
• Function of $r_t$:
  • $r_t$ tracks the recent second moment (recent average of squared gradients) for each parameter.
  • Large $r_t$ means gradients were recently large, so RMSProp reduces that parameter’s effective step size.
  • Small $r_t$ means gradients were recently small, so RMSProp allows a larger effective step size.
  • This stabilizes updates and prevents one parameter with large gradients from dominating training.
• Update rule:

Adam

• Combines momentum and RMSprop ideas (first and second moments).
• Strong default for many tasks.
• Update rule:

• $m_t$ is the first moment estimate (EMA of gradients, i.e., mean direction).

• $v_t$ is the second moment estimate (EMA of squared gradients, i.e., gradient magnitude).

AdamW

• Decouples weight decay from the gradient update.
• More reliable regularisation than L2 inside Adam’s adaptive update.
• Common default for transformer‑style models.
• Update rule:

Decoupled weight decay vs L2 in Adam

Adam + L2 (coupled)

AdamW (decoupled)

• Think of weight decay as “gently shrinking” weights every step.
• In classic Adam + L2, the shrink happens inside the adaptive update, so each weight gets shrunk by a different amount.
• In AdamW, the shrink is separate and uniform, so every weight gets the same decay strength.
• This makes AdamW more predictable and usually easier to tune.

Practical Notes

Start with a strong optimizer baseline

• Adam is usually a fast default for early experimentation.

Compare with SGD + momentum for final quality

• On many vision tasks, SGD + momentum remains competitive for generalization.

Tune learning rate before optimizer switching

• Learning-rate choice usually has larger impact than changing optimizer family.

Convergence

What convergence means

• Training loss stabilises and gradients approach zero:
  $\|\nabla \mathcal{L}(\theta)\| \rightarrow 0$
• Validation loss should improve, then plateau without diverging.

Learning rate behaviour

• Too high $\eta$ causes divergence or oscillation.
• Too low $\eta$ causes slow training and poor minima.
• Schedules help balance exploration and refinement:
  • Step decay: drop $\eta$ at fixed epochs.
  • Cosine decay: smooth reduction over time.
  • Warmup: start small, then ramp to target $\eta$.

Learning rate schedulers

• The learning rate is a hyperparameter that strongly affects convergence.

Step-based schedulers

• Drops learning rate by a fixed amount every $k$ epochs.

Exponential schedulers

• Smooth exponential decay of learning rate.

Cosine annealing schedulers

• Smoothly decreases learning rate following a cosine curve.

Cyclical schedulers

• Learning rate oscillates between a set of values.
• wtf who designed this

Performance-based schedulers

• Adjust learning rate based on validation performance.
• e.g. ReduceLROnPlateau.
• Adjust LR only when improvement stagnates.
• Adaptive to validation metrics.

Loss performance on MNIST data

Warmup + decay (typical recipe)

• To stabilise early training, models usually include a warmup phase, starting with a small learning rate and gradually increasing it.
• Once warmup reaches the peak value, gradually decrease the learning rate using the schedulers above.

Diagnostics

• Plot loss curves for train and validation.
• Watch for overfitting (train loss down, val loss up).
• Monitour gradients to detect exploding/vanishing behaviour.

Regularisation

Core idea

• Regularisation adds constraints to reduce overfitting.
• It balances bias/variance by discouraging overly complex solutions.

Dropout

• Randomly zeroes activations during training:
  $h' = m \odot h, \quad m \sim \text{Bernoulli}(p)$
• Forces the network to learn redundant representations.
• Use typical rates: 0.1–0.5 depending on model size.

PyTorch example

import torch.nn as nn

model = nn.Sequential(
    nn.Linear(256, 128),
    nn.ReLU(),
    nn.Dropout(p=0.3),
    nn.Linear(128, 10),
)

Early stopping

• Stop training when validation loss stops improving.
• Saves computation and prevents overfitting.
• Use a patience window (e.g., 5–10 epochs).

PyTorch example

best_val = float("inf")
patience = 5
wait = 0

for epoch in range(100):
    # train ...
    val_loss = validate(model, val_loader)

    if val_loss < best_val:
        best_val = val_loss
        wait = 0
        best_state = {k: v.cpu() for k, v in model.state_dict().items()}
    else:
        wait += 1
        if wait >= patience:
            model.load_state_dict(best_state)
            break

Weight decay

• Adds L2 penalty to the loss:
  $\mathcal{L}' = \mathcal{L} + \lambda \|\theta\|^2$
• Shrinks weights and improves generalisation.
• Commonly paired with SGD or AdamW.

Label smoothing

• Replaces hard one-hot targets with slightly softened targets.
• Reduces overconfident predictions and can improve generalisation.
• For $K$ classes and smoothing factor $\alpha$:
  • True class target: $1-\alpha$
  • Each non-true class target: $\frac{\alpha}{K-1}$
• Example for $K=5$, $\alpha=0.1$:
  • True class target is $0.9$, others are $0.025$ each.
• Typical $\alpha$ values are small (for example 0.05 to 0.2).
• Too much smoothing can hurt class separation and calibration for some tasks.

Practical usage

• Combine data augmentation with regularisation for best results.
• Use dropout more in fully connected layers than in convolutional layers.
• Prefer AdamW when using weight decay with Adam-style optimisers.