Training Fundamentals and Optimization

Overview

Now that you have a solid foundation in NLP concepts, transformer architectures, and language model evolution from the fundamentals course, it's time to dive into the engineering and production aspects of working with these models. This lesson focuses on the practical aspects of training large language models—the critical skills needed to move from understanding models to actually building and deploying them.

We'll explore the entire training pipeline, from dataset preparation to distributed computing strategies and advanced optimization techniques. Understanding these fundamentals is essential whether you're fine-tuning existing models or building new ones from scratch.

Learning Objectives

After completing this lesson, you will be able to:

• Design and prepare datasets for pre-training and fine-tuning language models
• Understand the computational challenges of training large models and how to address them
• Implement distributed training strategies across multiple devices and machines
• Apply advanced optimization techniques to improve training stability and efficiency
• Diagnose and resolve common training issues
• Evaluate training progress and determine when a model is converged

Dataset Preparation: The Foundation of Model Quality

The Critical Role of Data

The quality, diversity, and scale of training data directly impact model performance—often more than architectural improvements. As the saying goes: "garbage in, garbage out."

Analogy: Training Data as Nutrition

Think of training data as the nutrition for an AI model:

• Quality: Just as an athlete needs clean, high-quality food, models need high-quality data
• Diversity: Like a balanced diet provides all necessary nutrients, diverse data provides broad knowledge
• Quantity: Both growing bodies and growing models need sufficient quantities of inputs
• Preparation: Raw ingredients must be processed appropriately, just as raw text needs to be processed

Pre-training Datasets: Scale and Diversity

For pre-training large language models, datasets typically include:

1. Web text: Filtered content from Common Crawl, WebText, etc.
2. Books: BookCorpus, Project Gutenberg, etc.
3. Scientific papers: arXiv, PubMed, etc.
4. Code: GitHub, StackOverflow, etc.
5. Wikipedia: Encyclopedic knowledge in multiple languages

Dataset Size Comparison

Note: Sizes represent processed, deduplicated data used for training.

Data Cleaning and Filtering

Raw data from the internet contains noise, duplicates, and potentially harmful content. Data cleaning involves:

1. Deduplication: Removing exact and near-duplicate content
2. Quality Filtering: Heuristics for content quality (e.g., punctuation ratio, word diversity)
3. Harmful Content Removal: Filtering toxic, illegal, or private information
4. PII Redaction: Removing personally identifiable information

The Cleaning-Coverage Trade-off

Fine-tuning Datasets

Fine-tuning datasets are typically smaller, task-specific, and often require:

• Labels or aligned pairs: For supervised learning
• High-quality curation: Often manually reviewed
• Balanced class distribution: For classification tasks
• Diverse samples: To prevent overfitting

Popular fine-tuning datasets include:

• GLUE/SuperGLUE: Benchmark suites for language understanding
• SQuAD: Question answering
• MNLI: Natural language inference
• WMT: Machine translation

Computational Challenges and Solutions

The Compute Equation: Memory, Speed, and Scale

Training large language models faces three main computational challenges:

1. Memory constraints: Model parameters, activations, and gradients
2. Computational intensity: FLOPs required for forward and backward passes
3. Training time: Epochs needed to achieve convergence

Analogy: Building a Skyscraper

Training a large language model is like building a skyscraper:

• Memory constraints are like the amount of land available for the foundation
• Computational intensity is like the number of workers and equipment needed
• Training time is like the construction schedule
• Distributed training is like coordinating multiple construction crews
• Optimization techniques are like improved building methods and materials

GPU Memory Anatomy

A typical training setup must fit:

• Model parameters: Weights and biases
• Optimizer states: Momentum terms, adaptive learning rates
• Activations: Forward pass outputs
• Gradients: Backward pass computations
• Temporary buffers: For operations like attention

Mixed Precision Training

Mixed precision leverages lower-precision formats to reduce memory usage and speed up computation on modern GPUs.

Implementation with PyTorch

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from torch.cuda.amp import autocast, GradScaler

# Create model and optimizer
model = TransformerModel().cuda()
optimizer = torch.optim.Adam(model.parameters())
scaler = GradScaler()

# Training loop
for epoch in range(num_epochs):
    for batch in dataloader:
        optimizer.zero_grad()
        
        # Forward pass with autocast (uses mixed precision)
        with autocast():
            outputs = model(batch)
            loss = compute_loss(outputs, batch)
        
        # Backward pass with gradient scaling
        scaler.scale(loss).backward()
        
        # Optimizer step with unscaling
        scaler.unscale_(optimizer)
        torch.nn.utils.clip_grad_norm_(model.parameters(), max_grad_norm)
        scaler.step(optimizer)
        scaler.update()

Gradient Accumulation

Gradient accumulation simulates larger batch sizes by accumulating gradients over multiple forward-backward passes.

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accumulation_steps = 8  # Effectively multiplies batch size by 8
model.zero_grad()

for i, batch in enumerate(dataloader):
    # Forward pass
    outputs = model(batch)
    loss = compute_loss(outputs, batch)
    
    # Normalize loss to account for accumulation
    loss = loss / accumulation_steps
    
    # Backward pass
    loss.backward()
    
    # Optimizer step every accumulation_steps
    if (i + 1) % accumulation_steps == 0:
        optimizer.step()
        model.zero_grad()

Distributed Training Strategies

The Need for Distribution

As models grow, single-device training becomes impractical:

• GPT-3 (175B parameters) would require ~700GB for FP32 parameters alone
• Training time on a single device would be prohibitively long

Parallel Training Paradigms

Data Parallelism

In data parallelism, the model is replicated across devices, but each processes different data.

Pipeline Parallelism

Pipeline parallelism combines aspects of both data and model parallelism.

Hybrid Parallelism: The 3D Approach

Modern training systems like Megatron-LM combine multiple parallelism strategies:

• Data Parallelism: Across nodes
• Pipeline Parallelism: Across GPU groups
• Tensor Parallelism: Within GPU groups

Advanced Optimization Techniques

Learning Rate Scheduling

Learning rate scheduling is crucial for stable and effective training.

Common Schedules

What Each Metric Tells You

Common Training Issues and Solutions

Learning Rate Finder

Finding optimal learning rates automatically:

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from torch_lr_finder import LRFinder

model = TransformerModel()
optimizer = torch.optim.AdamW(model.parameters())
criterion = torch.nn.CrossEntropyLoss()
lr_finder = LRFinder(model, optimizer, criterion, device="cuda")
lr_finder.range_test(train_dataloader, end_lr=10, num_iter=100)
lr_finder.plot()  # Visually inspect to find optimal LR
lr_finder.reset()  # Reset model and optimizer to continue training

A Complete Training Pipeline

Putting It All Together

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import torch
import torch.distributed as dist
from torch.nn.parallel import DistributedDataParallel as DDP
from torch.cuda.amp import autocast, GradScaler
from transformers import get_scheduler

def train(config):
    # Initialize distributed environment
    dist.init_process_group(backend='nccl')
    local_rank = dist.get_rank()
    torch.cuda.set_device(local_rank)
    
    # Create model, optimizer, and scheduler
    model = TransformerModel(config).cuda()
    model = DDP(model, device_ids=[local_rank])
    
    # Optimizer with parameter groups
    optimizer = torch.optim.AdamW([
        {'params': model.module.embedding.parameters(), 'weight_decay': 0.0},
        {'params': model.module.encoder.parameters()},
        {'params': model.module.decoder.parameters()},
    ], lr=config.learning_rate, weight_decay=config.weight_decay)
    
    # Learning rate scheduler
    num_training_steps = len(train_dataloader) * config.num_epochs
    lr_scheduler = get_scheduler(
        name="linear", 
        optimizer=optimizer,
        num_warmup_steps=int(0.1 * num_training_steps),
        num_training_steps=num_training_steps
    )
    
    # Grad scaler for mixed precision
    scaler = GradScaler()
    
    # Training loop
    for epoch in range(config.num_epochs):
        model.train()
        train_dataloader.sampler.set_epoch(epoch)
        
        for step, batch in enumerate(train_dataloader):
            # Move batch to device
            batch = {k: v.cuda() for k, v in batch.items()}
            
            # Zero gradients
            optimizer.zero_grad()
            
            # Gradient accumulation loop
            for micro_step in range(config.gradient_accumulation_steps):
                # Get micro-batch
                micro_batch = get_micro_batch(batch, micro_step, config.gradient_accumulation_steps)
                
                # Forward pass with mixed precision
                with autocast():
                    outputs = model(**micro_batch)
                    loss = outputs.loss / config.gradient_accumulation_steps
                
                # Backward pass with gradient scaling
                scaler.scale(loss).backward()
            
            # Gradient clipping
            scaler.unscale_(optimizer)
            torch.nn.utils.clip_grad_norm_(model.parameters(), config.max_grad_norm)
            
            # Optimizer step
            scaler.step(optimizer)
            scaler.update()
            lr_scheduler.step()
            
            # Log metrics
            if step % config.logging_steps == 0 and local_rank == 0:
                log_metrics(loss, lr_scheduler.get_last_lr()[0], step, epoch)
            
            # Save checkpoint
            if step % config.save_steps == 0 and local_rank == 0:
                save_checkpoint(model, optimizer, lr_scheduler, epoch, step)
        
        # Evaluation at end of epoch
        if local_rank == 0:
            evaluate(model, eval_dataloader)
    
    # Final model saving
    if local_rank == 0:
        model.module.save_pretrained(config.output_dir)

Future Directions in Training Optimization

Emergent Techniques

1. Mixture of Experts (MoE): Training larger models with conditional computation
2. Efficient Attention Mechanisms: Linear and sub-quadratic attention variants
3. Neural Architecture Search (NAS): Automated discovery of efficient architectures
4. Lifelong Learning: Continuous training with new data without forgetting

Mixture of Experts (MoE) Approach