microgpt

A GPT-style language model written from scratch in PyTorch, plus the inference techniques that matter in production LLM serving: a KV cache, grouped-query attention, continuous batching over variable-length requests, and speculative decoding. Every one of them is backed by tests that check correctness against a reference, not just "it runs".

No nn.Transformer, no Hugging Face. Attention, masking, caching, batching and sampling are all written out explicitly (~900 lines of library code) so every line can be reasoned about.

What's inside

Piece	Where	The interesting part
Causal attention, RoPE, RMSNorm, SwiGLU, tied embeddings	model.py	The Llama recipe at toy scale; mask offset by cache length
Grouped-query / multi-query attention	model.py	Cache stores only the KV heads; q is regrouped so matmul broadcasts instead of copying K/V
KV cache + sliding window	model.py	O(T) work per generated token instead of O(T^2)
Continuous batching	batching.py	Left-padded shared cache, per-row RoPE positions, key mask, mid-flight admission, column trimming
Speculative decoding	speculative.py	Exact target distribution (rejection sampling + residual), cache rollback
Sampling: temperature / top-k / top-p	sampling.py	logits_to_probs exposes the exact sampling distribution (greedy = one-hot)
Trainer	train.py	AdamW with decoupled decay, warmup + cosine LR, grad clipping

Quickstart

python3 -m venv .venv && source .venv/bin/activate
pip install -e ".[dev]"
pytest                                   # 49 tests, ~15s on CPU

mkdir -p data && curl -sSL -o data/shakespeare.txt \
  https://raw.githubusercontent.com/karpathy/char-rnn/master/data/tinyshakespeare/input.txt
python -m microgpt.train --data data/shakespeare.txt --steps 1200 --batch-size 24
python -m microgpt.train --data data/shakespeare.txt --n-layer 1 --n-embd 64 --n-head 2 \
    --steps 2000 --out checkpoints/draft.pt            # small draft model

python -m microgpt.generate --prompt "ROMEO:" --tokens 200 --top-k 40
python -m microgpt.generate --prompt "ROMEO:" --tokens 200 --draft checkpoints/draft.pt  # speculative

python scripts/benchmark.py              # KV cache vs no cache
python scripts/bench_gqa.py              # cache memory: MHA vs GQA vs MQA
python scripts/bench_batching.py         # continuous batching throughput
python scripts/bench_speculative.py      # speculative decoding speedup

How each feature is verified

KV cache. Prefill-then-decode reproduces the full forward pass to 1e-5, including multi-token continuation (which exercises the offset causal mask). Perturbing future tokens never changes earlier logits. RoPE preserves norms and its attention scores depend only on m - n.

Grouped-query attention. A GQA model must equal a multi-head model whose K/V heads are copies of the GQA ones (weights are copied across and logits compared, both full-forward and token-by-token). Cache bytes are asserted to scale exactly with the number of KV heads.

Continuous batching. Requests of different lengths share one cache. The tests require that greedy output for every request is identical to running it alone, across: queueing behind a full batch, requests joining mid-flight with longer and shorter prompts than the current batch, EOS stopping one row without disturbing others, and the block_size limit. A padded row must not produce NaN (more on that below).

Speculative decoding. At temperature 0 the output equals greedy target decoding token-for-token. For sampling, the test computes the target's exact distribution over the first two generated tokens and the marginal of the third by brute force on a 5-token vocabulary, then checks 2,500 speculative samples against it. A power check asserts the draft really is a different distribution, and I confirmed the test fails when the algorithm is deliberately broken (always accepting proposals, or resampling from p instead of the residual max(0, p - q)).

Results

All numbers were measured with this repo's scripts on an Intel iMac (6 CPU threads, no GPU). CPU timings are noisy; treat small differences as approximate.

Training (character-level Tiny Shakespeare)

0.81M parameters (4 layers, 128-dim), 1200 steps, about 5.6 minutes:

step	train loss	val loss
0	4.242	4.235
400	1.657	1.830
800	1.444	1.655
1200	1.348	1.596

Initial loss 4.24 is close to ln 65 = 4.17 (uniform over the 65-character vocabulary), as expected.

KV cache

new tokens	no cache (tok/s)	KV cache (tok/s)	speedup	identical output
32	158.3	194.2	1.2x	yes
128	98.2	174.4	1.8x	yes
256	74.5	198.8	2.7x	yes
480	34.7	167.4	4.8x	yes

Cached throughput stays roughly flat as the sequence grows while uncached throughput decays.

Grouped-query attention

8 query heads, 6 layers, 256-dim, cache holding 496 tokens:

KV heads	variant	params	KV cache	vs MHA
8	MHA	4.84M	5.8 MiB	1.00x
4	GQA	4.45M	2.9 MiB	0.50x
2	GQA	4.25M	1.5 MiB	0.25x
1	MQA	4.16M	0.7 MiB	0.12x

Memory is exact. Decode speed on this CPU (126 / 176 / 136 / 133 tok/s for 8 / 4 / 2 / 1 KV heads) is not monotonic, which suggests noise, but I did not measure run-to-run variance, so no speed claim is made. The benefit of GQA shown here is cache size, which matters when serving long contexts or many concurrent sequences.

Continuous batching

16 requests (prompts of 8-47 tokens, 64 new tokens each), 6-layer 192-dim model:

strategy	tokens/s	speedup	mean batch
one at a time	240	1.0x	1.0
continuous, max_batch=2	303	1.3x	2.0
continuous, max_batch=4	575	2.4x	4.0
continuous, max_batch=8	1144	4.8x	8.0
continuous, max_batch=16	1767	7.4x	16.0

Caveat: every request here asks for the same number of tokens, so the batch stays full. Real traffic with mixed lengths would see lower occupancy; the scheduler refills slots as rows finish.

Speculative decoding

Target: the 0.81M model (val loss 1.60). Draft: 57.6K parameters, 1 layer (val loss 1.81). Temperature 0.8, 100 new tokens, 12 runs per row:

method	s / run	speedup	acceptance	tokens / target pass
target only	0.36	1.00x	-	1.0
speculative, gamma=2	0.26	1.38x	64%	2.26
speculative, gamma=4	0.22	1.65x	50%	2.96
speculative, gamma=6	0.22	1.68x	44%	3.59
speculative, gamma=8	0.25	1.42x	32%	3.50

The speedup peaks at moderate gamma and then falls: more proposals per round means each is less likely to be accepted, so the draft's extra work stops paying for itself. The output distribution is the target's in every row (see verification above), so this is a pure speed trade-off. Wall-clock gains depend on the draft being much cheaper than the target; with a GPU-scale target the same acceptance rates would give a larger speedup.

Pitfalls hit (and avoided) while building this

• Sliding window. An early version claimed cached and uncached generation were identical. They are, but only while the text fits in block_size; past it the two paths slide the window differently. Caught by seeded samples diverging at token 135; the docstring and README now say so.
• NaN from padding. In a left-padded batch, a padding query has no visible key, so softmax over all -inf gives NaN, and NaN * 0 in the value product poisons real tokens. Fix: every query may always attend to itself (its output is discarded anyway). Covered by a finite-logits assertion.
• Ties in parity tests (avoided). The default init gives near-uniform logits, where greedy argmax could flip on float noise between code paths. Parity tests therefore use a wider init (tests/helpers.py) so they compare confident outputs rather than ties.
• Wrong test, not wrong code. One batching test failed because it sampled cache width after a row had already been retired in the same step; the engine was right, the assertion was not.

Limitations

• Past block_size, the single-sequence cache path drops and rebuilds its cache from the latest half-window; the batching engine and speculative decoding instead require sequences to fit in block_size.
• Speculative decoding handles one sequence at a time and does not combine with the batch engine.
• The cache grows by torch.cat each step (simple and correct, but it reallocates); a production system would use preallocated or paged blocks.
• Attention is the plain O(T^2)-memory formulation: no FlashAttention.
• Character-level and small-scale: the point is correctness and clarity, not sample quality.

Roadmap

• BPE tokenizer trained from scratch, with round-trip tests
• Paged KV cache (fixed-size blocks, block table) to remove the torch.cat reallocation
• Speculative decoding inside the batch engine
• Load a public checkpoint into this architecture and match Hugging Face logits