---
license: cdla-sharing-1.0
tags:
  - text-generation
  - gpt
  - from-scratch
  - pytorch
  - mps
  - tinystories
datasets:
  - roneneldan/TinyStories
pipeline_tag: text-generation
library_name: pytorch
language:
  - en
---

# mini-LLM — a 30M-parameter transformer trained overnight on a laptop

A decoder-only GPT written from scratch in plain PyTorch. No `transformers`, no `accelerate`, no
`fastai` — the model, the tokenizer training and the training loop are all in the repository.

**Code: https://github.com/vous99/mini-llm**

Trained on one MacBook Pro (M5 Pro, 24 GB, MPS) in a single overnight run.

## Results

| Metric | Value |
|---|---|
| **Best val loss** | **1.1527** (perplexity 3.17) |
| Loss at initialisation | 9.01 = ln(8192), the uniform-over-vocabulary baseline |
| Steps | 7,561 |
| Tokens seen | 0.74B (1.4 epochs of TinyStories V2) |
| Training time | ~9 hours of compute at 23–25K tokens/s |
| Parameters | 29,893,120 (25.7M non-embedding) |

## Architecture

Modern rather than 2017: what separates this from the original transformer paper is where most of
the interest lies.

| | |
|---|---|
| Layers | 8 |
| Model width | 512 |
| Heads | 8 (head_dim 64) |
| Context | 512 tokens |
| Vocabulary | 8,192, byte-level BPE trained on the corpus itself |
| Positional encoding | RoPE (rotary), not learned embeddings |
| Normalisation | RMSNorm, pre-norm |
| Feed-forward | SwiGLU, hidden 1408 (8/3 × width, rounded to a multiple of 64) |
| Attention | `F.scaled_dot_product_attention`, causal |
| Embeddings | tied — one matrix serves both input and output |
| Dropout | 0.0 |

The 8,192-token vocabulary is a deliberate choice: GPT-2's 50,257 would put a 25M-parameter
embedding table inside a 30M-parameter model.

## Usage

```bash
git clone https://github.com/vous99/mini-llm && cd mini-llm
pip install -r requirements.txt

python -c "
from huggingface_hub import hf_hub_download
import shutil, os
os.makedirs('ckpt', exist_ok=True); os.makedirs('data', exist_ok=True)
shutil.copy(hf_hub_download('vous99/mini-llm', 'best.pt'), 'ckpt/best.pt')
shutil.copy(hf_hub_download('vous99/mini-llm', 'tokenizer.json'), 'data/tokenizer.json')
"

python sample.py
python sample.py --prompt "Once upon a time, a little robot" --n 3
python chat.py                # terminal REPL
python serve.py               # browser playground on 127.0.0.1:8890
```

**`tokenizer.json` is required.** It is the BPE trained alongside the model; without it the
weights emit token ids, not text.

## Training details

| | |
|---|---|
| Optimizer | AdamW, lr 6e-4, betas (0.9, 0.95), grad clip 1.0 |
| Weight decay | 0.1 on matrices, 0 on 1-D parameters (norms) |
| LR schedule | linear warmup 200 steps → cosine decay to 10% of peak |
| Batch | 24 × 8 gradient accumulation × 512 tokens = 98,304 tokens per step |
| Precision | bfloat16 autocast |

Train and validation loss stayed within 0.01–0.03 of each other for the whole run — no
overfitting, which is why dropout is 0.

## Limitations

- **TinyStories only.** The corpus is synthetic children's stories using a deliberately small
  vocabulary. The model writes fluent, coherent short stories in that register and nothing else.
  It has no world knowledge, cannot answer questions and cannot follow instructions.
- **512-token context.**
- **No instruction tuning, no RLHF.** This is a base model in the most literal sense.
- **Not safe for production.** It is a study artifact for understanding how a transformer trains.

## Checkpoint contents

`best.pt` is a `torch.save` dict with `model`, `cfg` (the `GPTConfig`), `iter`, `best_val` and
`val_loss`. Load it with `sample.py` from the GitHub repository.