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microjpt

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The most atomic way to train and run inference for a GPT in 100 lines of pure, dependency-free Julia.

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microjpt.jl
nanojpt.jl
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A pure Julia port of Karpathy's [microgpt](https://gist.github.com/karpathy/8627fe009c40f57531cb18360106ce95) - a single-file, zero-dependency GPT that trains and generates text from scratch. Built to explore Julia's mathematical expressiveness for ML: native matrix operations, manual backprop, no autograd, no frameworks.

### Files

| File            | Lines | Description                                         |
| --------------- | ----- | --------------------------------------------------- |
| `microjpt.jl` | 150   | Fully commented version - every operation annotated |
| `nanojpt.jl`  | 99    | Minimal version - same algorithm, no comments       |

Both files contain the complete algorithm: tokenizer, RMSNorm, multi-head causal attention, ReLU MLP, manual matrix backpropagation, Adam optimizer, and temperature-controlled inference.

### Benchmarks

Independent benchmarks by [@Entrpi](https://github.com/Entrpi) on an Apple M5 (single P-core), using the Karpathy names dataset with `n_head=4`.

**d16, names.txt, 10K training samples**

| Implementation                                                                                     | µs/sample   | Speedup vs CPython |
| -------------------------------------------------------------------------------------------------- | ------------ | ------------------ |
| [CPython 3.14](https://gist.github.com/karpathy/8627fe009c40f57531cb18360106ce95) (autograd, batch=1) | 49,000       | 1×                |
| [microgpt.cpp](https://github.com/Charbel199/microgpt.cpp) (autograd, batch=1)                        | 270          | 181×              |
| [rust-microgpt](https://github.com/mplekh/rust-microgpt) (autograd, batch=1)                         | 118          | 415×              |
| **microjpt (explicit, batch=1)**                                                             | **31** | **1,581×**  |
| [EEmicroGPT](https://github.com/Entrpi/eemicrogpt) (explicit, batch=16)                               | 3.0          | 16,333×           |

**d64, names.txt, 1K training samples**

| Implementation                                                                                     | µs/sample    | Speedup vs CPython |
| -------------------------------------------------------------------------------------------------- | ------------- | ------------------ |
| [CPython 3.14](https://gist.github.com/karpathy/8627fe009c40f57531cb18360106ce95) (autograd, batch=1) | 713,200       | 1×                |
| [rust-microgpt](https://github.com/mplekh/rust-microgpt) (autograd, batch=1)                          | 1,620         | 440×              |
| **microjpt (explicit, batch=1)**                                                             | **294** | **2,425×**  |
| [EEmicroGPT](https://github.com/Entrpi/eemicrogpt) SME2 (explicit, batch=16)                          | 36.8          | 19,380×           |

microjpt is the fastest batch=1 implementation with **3.8× faster than rust-microgpt at d16** and **5.5× faster at d64**. The explicit backprop collapses ~57K autograd tape nodes into ~20 matrix operations, the same insight driving EEmicroGPT's performance. The remaining gap to EEmicroGPT comes from batching (16 samples), f32 vs f64, and hand-optimized Neon/SME2 SIMD.

> 100 lines of dependency-free Julia, 1,581× faster than CPython, and only 7.6× off hand-tuned C.

### Setup & Installation

**Install Julia**

The recommended way is [juliaup](https://github.com/JuliaLang/juliaup), the official version manager:

```bash
# macOS / Linux
curl -fsSL https://install.julialang.org | sh

# Windows (PowerShell)
winget install julia -s msstore
```

Or download directly from [julialang.org](https://julialang.org/downloads/).

**Verify**

```bash
julia --version   # should print Julia 1.9 or later
```

**Run**

From a terminal:

```bash
julia microjpt.jl
```

Or from the Julia REPL:

```julia
include("microjpt.jl")
```

There are zero external packages to install. `Random` and `Downloads` are part of Julia's standard library and require no `Pkg.add`. The training dataset downloads automatically on first run.

---

### Why Julia?

**Math-literal syntax.** Julia has native Unicode identifier support, so variables are written as `β1`, `β2`, `ε`, `∑` - the code maps directly to the paper's notation without translation.

**JIT via LLVM.** Julia compiles to native machine code through LLVM. The first run is slow due to method compilation; subsequent runs execute at full speed. If the initial startup feels sluggish, that's precompilation and not the algorithm.

**BLAS with zero configuration.** The `*` operator on matrices dispatches to OpenBLAS (bundled with Julia) automatically. No `numpy`, no linking, no flags.

**No Python overhead.** No GIL, no interpreter loop, no framework abstraction layers. This is the direct reason batch=1 reaches 1,581× over CPython.

**Specialisation via the type system.** Julia's compiler generates specialised native code for each concrete type at dispatch time, eliminating virtual dispatch overhead that typically burdens dynamic languages.

---