Pretraining

The first of three training phases. See docs/training/index.md for the overview, docs/training/sft.md and docs/training/dpo.md for what comes next.

What it does

Given an enormous stream of raw text, predict the next token everywhere. Loss is averaged over every position; gradients flow through every token. The model learns grammar, vocabulary, world facts, and a distribution over what humans tend to write.

Math

L_pretrain = - (1 / N) * Σ_t  log P(x_{t+1} | x_1 … x_t)

where x_1 … x_N are the corpus tokens. Averaging over a single contiguous corpus makes loss directly comparable across runs.

What it needs

Thing	Why	Where it lives
Large text corpus	~5-20× more tokens than the model has parameters (Hoffmann/Chinchilla)	Streamed from HuggingFace via `python_ref/fetch_hf_corpus.py`
BPE tokenizer	Byte-level wastes ~4× the compute at the same coverage	`--tokenizer <hf-dir>` pointing at any HF model directory
Long-run infrastructure	A crash at hour 22 of 26 shouldn’t lose 22 hours	Tier 0 safety nets in `posttrainllm train`: resume, atomic save-every, SIGINT-flushes-final
bf16 training	2× memory savings → 2× larger effective batch. See `docs/memory_tradeoffs.md`.	`--dtype bfloat16`
Gradient accumulation	Effective batch larger than memory budget. See `docs/memory_tradeoffs.md`.	`--accum N`

Reproduce

# 1. Stream ~500M tokens of high-quality educational web text.
source python_ref/.venv/bin/activate
python python_ref/fetch_hf_corpus.py \
    --dataset HuggingFaceFW/fineweb-edu --config sample-10BT \
    --split train --target-tokens 500M \
    --out /tmp/fineweb-edu-500M.txt

# 2. Pretrain Mega-bf16 (76M body + 25M token embedding = ~100M total).
#    B=4 × accum=4 × ctx=1024 = effective batch 16 at ~2 GB GPU memory.
#    ~23 hours on M5 Pro / 48 GB.
cd native-mac
caffeinate -di .xcode-build/Build/Products/Debug/posttrainllm train \
    --preset mega \
    --tokenizer /tmp/smollm2 \
    --corpus /tmp/fineweb-edu-500M.txt \
    --out /tmp/mega-fineweb.tinygpt \
    --dtype bfloat16 \
    --batch 4 --accum 4 --ctx 1024 \
    --steps 30500 \
    --lr-schedule cosine --warmup 1000 \
    --max-lr 6e-4 --min-lr 6e-5 \
    --val-split 0.005 --val-every 500 --save-every 1000

Expected outcome at our scale

Tokens trained on	Predicted val loss	What it looks like
5 M (Tiny demo)	4.9	gibberish, fragments
50 M	4.0	real words, broken grammar
500 M (this run)	3.0-3.5	coherent fragments, GPT-2-124M-class
1.5 B (Chinchilla floor)	2.5	useful base, post-trainable
5 B	2.0	Pythia-1.4B-class base

A “good pretrain” is anywhere from loss ~2.5 to ~3.5. Below that, the base is becoming useful on its own; above it, post-training is doing nearly all the work.

Background reading

Hoffmann et al., 2022 (Chinchilla scaling laws).
Kaplan et al., 2020 (Kaplan scaling laws).