Training an LLM From Scratch
Pre-training requires three components: a tokenizer, a dataset, and massive compute.
Tokenizer Selection
| Algorithm | Library | Used By |
| BPE (Byte Pair Encoding) | HF tokenizers, Tiktoken | GPT-4, Llama 3/4, most models |
| SentencePiece | sentencepiece | Multilingual models |
| FlashTokenizer | Custom C++/GPU | Emerging high-speed option |
Pre-Training Datasets (2026)
| Dataset | Size | Key Feature |
| Common Corpus | ~2T tokens | Largest truly open, copyright-compliant |
| RefinedWeb | ~5T tokens | Aggressive dedup & filtering |
| The Pile | 825GB | 22 diverse sources (books, code, papers) |
| RedPajama v2 | 30T tokens | Massive Common Crawl aggregation |
⚠️ Reality Check: Pre-training from scratch requires thousands of GPU-hours and millions in compute. For most use cases, continue pre-training or fine-tune an existing base model instead.