How to Train Custom Language Models

If you’re an AI engineer, understanding how LLMs are trained and aligned is essential for building high-performance, reliable AI systems. Most large language models follow a 3-step training procedure: Step 1: Pretraining → Goal: Learn general-purpose language representations. → Method: Self-supervised learning on massive unlabeled text corpora (e.g., next-token prediction). → Output: A pretrained LLM, rich in linguistic and factual knowledge but not grounded in human preferences. → Cost: Extremely high (billions of tokens, trillions of FLOPs). → Pretraining is still centralized within a few labs due to the scale required (e.g., Meta, Google DeepMind, OpenAI), but open-weight models like LLaMA 4, DeepSeek V3, and Qwen 3 are making this more accessible. Step 2: Finetuning (Two Common Approaches) → 2a: Full-Parameter Finetuning - Updates all weights of the pretrained model. - Requires significant GPU memory and compute. - Best for scenarios where the model needs deep adaptation to a new domain or task. - Used for: Instruction-following, multilingual adaptation, industry-specific models. - Cons: Expensive, storage-heavy. → 2b: Parameter-Efficient Finetuning (PEFT) - Only a small subset of parameters is added and updated (e.g., via LoRA, Adapters, or IA³). - Base model remains frozen. - Much cheaper, ideal for rapid iteration and deployment. - Multi-LoRA architectures (e.g., used in Fireworks AI, Hugging Face PEFT) allow hosting multiple finetuned adapters on the same base model, drastically reducing cost and latency for serving. Step 3: Alignment (Usually via RLHF) Pretrained and task-tuned models can still produce unsafe or incoherent outputs. Alignment ensures they follow human intent. Alignment via RLHF (Reinforcement Learning from Human Feedback) involves: → Step 1: Supervised Fine-Tuning (SFT) - Human labelers craft ideal responses to prompts. - Model is fine-tuned on this dataset to mimic helpful behavior. - Limitation: Costly and not scalable alone. → Step 2: Reward Modeling (RM) - Humans rank multiple model outputs per prompt. - A reward model is trained to predict human preferences. - This provides a scalable, learnable signal of what “good” looks like. → Step 3: Reinforcement Learning (e.g., PPO, DPO) - The LLM is trained using the reward model’s feedback. - Algorithms like Proximal Policy Optimization (PPO) or newer Direct Preference Optimization (DPO) are used to iteratively improve model behavior. - DPO is gaining popularity over PPO for being simpler and more stable without needing sampled trajectories. Key Takeaways: → Pretraining = general knowledge (expensive) → Finetuning = domain or task adaptation (customize cheaply via PEFT) → Alignment = make it safe, helpful, and human-aligned (still labor-intensive but improving) Save the visual reference, and follow me (Aishwarya Srinivasan) for more no-fluff AI insights ❤️ PS: Visual inspiration: Sebastian Raschka, PhD

Training a Large Language Model (LLM) involves more than just scaling up data and compute. It requires a disciplined approach across multiple layers of the ML lifecycle to ensure performance, efficiency, safety, and adaptability. This visual framework outlines eight critical pillars necessary for successful LLM training, each with a defined workflow to guide implementation: 𝟭. 𝗛𝗶𝗴𝗵-𝗤𝘂𝗮𝗹𝗶𝘁𝘆 𝗗𝗮𝘁𝗮 𝗖𝘂𝗿𝗮𝘁𝗶𝗼𝗻: Use diverse, clean, and domain-relevant datasets. Deduplicate, normalize, filter low-quality samples, and tokenize effectively before formatting for training. 𝟮. 𝗦𝗰𝗮𝗹𝗮𝗯𝗹𝗲 𝗗𝗮𝘁𝗮 𝗣𝗿𝗲𝗽𝗿𝗼𝗰𝗲𝘀𝘀𝗶𝗻𝗴: Design efficient preprocessing pipelines—tokenization consistency, padding, caching, and batch streaming to GPU must be optimized for scale. 𝟯. 𝗠𝗼𝗱𝗲𝗹 𝗔𝗿𝗰𝗵𝗶𝘁𝗲𝗰𝘁𝘂𝗿𝗲 𝗗𝗲𝘀𝗶𝗴𝗻: Select architectures based on task requirements. Configure embeddings, attention heads, and regularization, and then conduct mock tests to validate the architectural choices. 𝟰. 𝗧𝗿𝗮𝗶𝗻𝗶𝗻𝗴 𝗦𝘁𝗮𝗯𝗶𝗹𝗶𝘁𝘆 and 𝗢𝗽𝘁𝗶𝗺𝗶𝘇𝗮𝘁𝗶𝗼𝗻: Ensure convergence using techniques such as FP16 precision, gradient clipping, batch size tuning, and adaptive learning rate scheduling. Loss monitoring and checkpointing are crucial for long-running processes. 𝟱. 𝗖𝗼𝗺𝗽𝘂𝘁𝗲 & 𝗠𝗲𝗺𝗼𝗿𝘆 𝗢𝗽𝘁𝗶𝗺𝗶𝘇𝗮𝘁𝗶𝗼𝗻: Leverage distributed training, efficient attention mechanisms, and pipeline parallelism. Profile usage, compress checkpoints, and enable auto-resume for robustness. 𝟲. 𝗘𝘃𝗮𝗹𝘂𝗮𝘁𝗶𝗼𝗻 & 𝗩𝗮𝗹𝗶𝗱𝗮𝘁𝗶𝗼𝗻: Regularly evaluate using defined metrics and baseline comparisons. Test with few-shot prompts, review model outputs, and track performance metrics to prevent drift and overfitting. 𝟳. 𝗘𝘁𝗵𝗶𝗰𝗮𝗹 𝗮𝗻𝗱 𝗦𝗮𝗳𝗲𝘁𝘆 𝗖𝗵𝗲𝗰𝗸𝘀: Mitigate model risks by applying adversarial testing, output filtering, decoding constraints, and incorporating user feedback. Audit results to ensure responsible outputs. 🔸 𝟴. 𝗙𝗶𝗻𝗲-𝗧𝘂𝗻𝗶𝗻𝗴 & 𝗗𝗼𝗺𝗮𝗶𝗻 𝗔𝗱𝗮𝗽𝘁𝗮𝘁𝗶𝗼𝗻: Adapt models for specific domains using techniques like LoRA/PEFT and controlled learning rates. Monitor overfitting, evaluate continuously, and deploy with confidence. These principles form a unified blueprint for building robust, efficient, and production-ready LLMs—whether training from scratch or adapting pre-trained models.

Understanding LLM Adaptation: Full Training vs Fine-Tuning vs LoRA/QLoRA 🚀 Which approach really moves the needle in today’s AI landscape? As large language models (LLMs) become mainstream, I frequently get asked: “Should we train from scratch, full fine-tune, or use LoRA/QLoRA adapters for our use case?” Here’s a simple breakdown based on real-world considerations: 🔍 1. Full Training from Scratch What: Building a model from the ground up with billions of parameters. Who: Only major labs/Big Tech (OpenAI, Google, etc.) Cost: 🏦 Millions—requires massive clusters and huge datasets. Why: Needed ONLY if you want a truly unique model architecture or foundation. 🛠️ 2. Full Fine-Tuning What: Take an existing giant model and update ALL its weights for your task. Who: Advanced companies with deep pockets. Cost: 💰 Tens of thousands to millions—need multiple high-end GPUs. Why: Useful if you have vast domain data and need to drastically “re-train” the model’s capabilities. ⚡ 3. LoRA/QLoRA (Parameter-Efficient Tuning) What: Plug low-rank adapters into a model, updating just 0.5-5% of weights. Who: Startups, researchers, almost anyone! Cost: 💡 From free (on Google Colab) to a few hundred dollars on cloud GPUs. Why: Customize powerful LLMs efficiently—think domain adaption, brand voice, or private datasets, all without losing the model’s general smarts. 🤔 Which one should YOU use? For most organizations and projects, LoRA/QLoRA is the optimal sweet spot Fast: Results in hours, not weeks Affordable: Accessible to almost anyone Flexible: Update or revert adapters with ease Full fine-tuning and from-scratch training make sense only for the biggest players—99% of AI innovation today leverages parameter-efficient tuning! 💬 What’s your experience? Are you using full fine-tunes, or has LoRA/QLoRA met your business needs? Share your project (or frustrations!) in comments.

The bottleneck isn't GPUs or architecture. It's your dataset. Three ways to customize an LLM: 1. Fine-tuning: Teaches behavior. 1K-10K examples. Shows how to respond. Cheapest option. 2. Continued pretraining: Adds knowledge. Large unlabeled corpus. Extends what model knows. Medium cost. 3. Training from scratch: Full control. Trillions of tokens. Only for national AI projects. Rarely necessary. Most companies only need fine-tuning. How to collect quality data: For fine-tuning, start small. Support tickets with PII removed. Internal Q&A logs. Public instruction datasets. For continued pretraining, go big. Domain archives. Technical standards. Mix 70% domain, 30% general text. The 5-step data pipeline: 1. Normalize. Convert everything to UTF-8 plain text. Remove markup and headers. 2. Filter. Drop short fragments. Remove repeated templates. Redact PII. 3. Deduplicate. Hash for identical content. Find near-duplicates. Do before splitting datasets. 4. Tag with metadata. Language, domain, source. Makes dataset searchable. 5. Validate quality. Check perplexity. Track metrics. Run small pilot first. When your dataset is ready: All sources documented. PII removed. Stats match targets. Splits balanced. Pilot converges cleanly. If any fail, fix data first. What good data does: Models converge faster. Hallucinate less. Cost less to serve. The reality: Building LLMs is a data problem. Not a training problem. Most teams spend 80% of time on data. That's the actual work. Your data is your differentiator. Not your model architecture. Found this helpful? Follow Arturo Ferreira.

How to Build a LLM from Scratch: A Complete Step-by-Step Guide Using Historical Data 🏛️ Over the past few months, I've been building something I'm genuinely excited to share: a complete end-to-end pipeline for training Large Language Models from scratch using historical London texts (1500-1850). 🚀 This isn't just another AI project—it's a comprehensive 4-part series that teaches you every step of LLM development, from data collection through deployment. Here's what makes it special: What I Built: 🎯 • Two identical models (117M & 354M params) trained from scratch—not fine-tuned • Custom historical tokenizer with 30k vocabulary + 150+ special tokens for archaic English • Complete data pipeline processing 218+ historical sources (500M+ characters) • Production-ready training with multi-GPU support, WandB integration, and checkpointing • Published models on Hugging Face ready for immediate use The Learning Journey: • Part 1 (just published): Quick start, inference, and complete overview 👉 https://lnkd.in/gkAQNVqq Coming next: • Part 2: Deep dive into data collection and custom tokenization • Part 3: Model architecture, GPU optimization, and training infrastructure • Part 4: Evaluation frameworks and deployment strategies Why This Matters: 🔧 Most LLM guides focus on fine-tuning existing models. This series shows you how to build from the ground up—eliminating modern biases and creating models that truly understand historical language patterns, cultural contexts, and period-specific knowledge. Ready to Use: 🚀 The London Historical SLM is already published and working. You can generate authentic 18th-century London text in minutes, or follow the complete guide to build your own. Full Resources: 📖 • Blog Series: Building LLMs from Scratch - Part 1 -- https://lnkd.in/gkAQNVqq • Complete Codebase (GitHub): https://lnkd.in/g8BH7H7B • Published Models: https://lnkd.in/giSX4SEC Whether you're a developer, researcher, or just curious about how LLMs work under the hood, this series provides the hands-on experience most guides miss. #AI #LLM #MachineLearning #HistoricalAI #OpenSource #TechEducation #GenerativeAI