LLM Pretraining Helper

LLM Pre-training Helper

A skill for training language models from scratch using PyTorch, following best practices from the "LLMs from Scratch" methodology.

What this skill does

This skill helps you:

• Set up GPT model architectures with proper configuration
• Prepare training data with tokenization and data loaders
• Configure training loops with loss monitoring and evaluation
• Implement text generation with sampling strategies
• Save and load model checkpoints
• Visualize training progress (loss, perplexity)

When to use this skill

Use this skill when:

• You want to train an LLM from scratch on your own dataset
• You need to understand the pre-training workflow
• You're setting up GPT model configurations
• You want to monitor training metrics (loss, perplexity)
• You need to save/load model checkpoints
• You're implementing text generation with temperature/top-k sampling

Quick Start

1. Set up model configuration

GPT_CONFIG = {
    "vocab_size": 50257,      # GPT-2 vocabulary size
    "context_length": 256,    # Context window (adjust based on data)
    "emb_dim": 768,           # Embedding dimension
    "n_heads": 12,            # Attention heads
    "n_layers": 12,           # Transformer layers
    "drop_rate": 0.1,         # Dropout rate
    "qkv_bias": False         # Query-key-value bias
}

2. Prepare your data

# Load your text data
text_data = "your training text here"

# Split into train/validation (90/10 is common)
train_ratio = 0.90
split_idx = int(train_ratio * len(text_data))
train_data = text_data[:split_idx]
val_data = text_data[split_idx:]

# Create data loaders
train_loader = create_dataloader_v1(
    train_data,
    batch_size=2,
    max_length=GPT_CONFIG["context_length"],
    stride=GPT_CONFIG["context_length"],
    shuffle=True,
    drop_last=True
)

val_loader = create_dataloader_v1(
    val_data,
    batch_size=2,
    max_length=GPT_CONFIG["context_length"],
    stride=GPT_CONFIG["context_length"],
    shuffle=False,
    drop_last=False
)

3. Initialize model and start training

import torch

# Set seed for reproducibility
torch.manual_seed(123)

# Initialize model
model = GPTModel(GPT_CONFIG)

# Select device
if torch.cuda.is_available():
    device = torch.device("cuda")
elif torch.backends.mps.is_available():
    device = torch.device("mps")
else:
    device = torch.device("cpu")

model.to(device)

# Setup optimizer
optimizer = torch.optim.AdamW(
    model.parameters(),
    lr=0.0004,
    weight_decay=0.1
)

# Train
num_epochs = 10
train_losses, val_losses, tokens_seen = train_model_simple(
    model, train_loader, val_loader, optimizer, device,
    num_epochs=num_epochs,
    eval_freq=5,           # Evaluate every 5 steps
    eval_iter=5,           # Use 5 batches for evaluation
    start_context="Your starting phrase",
    tokenizer=tokenizer
)

Core Components

Model Architecture

The GPT model consists of:

• Token embeddings: Convert token IDs to vectors
• Positional embeddings: Add position information
• Transformer blocks: Multi-head attention + feed-forward
• Output head: Maps embeddings back to vocabulary

Training Loop Structure

For each epoch:
  For each batch:
    1. Zero gradients
    2. Forward pass → get logits
    3. Calculate loss (cross-entropy)
    4. Backward pass → compute gradients
    5. Optimizer step → update weights
    6. (Optional) Evaluate and log metrics

Loss Functions

• Cross-entropy loss: Measures difference between predicted and actual token distributions
• Perplexity: exp(loss) - represents model uncertainty (lower is better)

Text Generation Strategies

Training Parameters Guide

Learning Rate

• Small (1e-5 to 1e-4): Precise convergence, slower training
• Large (1e-3 to 1e-2): Faster training, risk of overshooting
• Recommended: Start with 4e-4 for AdamW

Batch Size

• Small (1-4): More frequent updates, noisier gradients
• Large (8-32): Smoother gradients, more memory
• Recommended: 2-4 for CPU, 8-16 for GPU

Context Length

• Short (128-256): Faster training, less context
• Long (512-1024): More context, slower training
• Recommended: Match your use case, start with 256

Number of Epochs

• Few (5-10): Quick iteration, may underfit
• Many (20-50): Better convergence, risk of overfitting
• Recommended: Monitor validation loss, stop when it plateaus

Monitoring Training

Key Metrics to Track

1. Training Loss: Should decrease over time
2. Validation Loss: Should decrease, watch for overfitting
3. Perplexity: exp(loss), lower is better
4. Tokens Seen: Track progress through dataset

Signs of Good Training

• Training loss steadily decreases
• Validation loss follows training loss
• Generated text becomes more coherent
• Perplexity drops significantly

Signs of Problems

• Overfitting: Training loss ↓, Validation loss ↑
• Underfitting: Both losses stay high
• Exploding gradients: Loss becomes NaN or inf
• Vanishing gradients: Loss stops decreasing

Saving and Loading Models

Save Full Checkpoint (for resuming training)

torch.save({
    "model_state_dict": model.state_dict(),
    "optimizer_state_dict": optimizer.state_dict(),
    "epoch": current_epoch,
    "loss": current_loss
}, "checkpoint.pth")

Load Full Checkpoint

checkpoint = torch.load("checkpoint.pth", map_location=device)
model.load_state_dict(checkpoint["model_state_dict"])
optimizer.load_state_dict(checkpoint["optimizer_state_dict"])
model.train()

Save Model Only (for inference)

torch.save(model.state_dict(), "model.pth")

Load Model Only

model = GPTModel(GPT_CONFIG)
model.load_state_dict(torch.load("model.pth", map_location=device))
model.eval()

Common Issues and Solutions

"Not enough tokens for training"

• Solution: Reduce context_length or increase training data
• Check: total_tokens * train_ratio >= context_length

"CUDA out of memory"

• Solution: Reduce batch size or context length
• Alternative: Use gradient accumulation

"Loss not decreasing"

• Check: Learning rate (try 1e-4 to 1e-3)
• Check: Data quality and tokenization
• Check: Model is in training mode (model.train())

"Validation loss increasing"

• Solution: Early stopping, reduce epochs
• Alternative: Add regularization (dropout, weight decay)

Advanced Techniques (Not in Base Code)

Learning Rate Scheduling

• Linear Warmup: Start small, increase to max LR
• Cosine Decay: Gradually reduce LR after warmup

Gradient Clipping

• Prevents exploding gradients
• Set max_norm in optimizer or use torch.nn.utils.clip_grad_norm_

Top-p Sampling (Nucleus)

• More adaptive than top-k
• Sums probabilities until threshold (e.g., 0.9)

Beam Search

• Explores multiple sequences simultaneously
• Better quality than greedy, more expensive

Next Steps

After training:

1. Evaluate: Test on held-out data
2. Fine-tune: Adapt to specific tasks
3. Deploy: Use for inference or as base model
4. Iterate: Adjust hyperparameters and retrain

References

• LLMs from Scratch
• rasbt/LLMs-from-scratch
• GPT-2 Architecture