In early 2024 Andrej Karpathy stood up an llm.c repo to train GPT-2 (124M), which took an equivalent of 45 minutes on 8xH100 GPUs to reach 3.28 cross entropy loss. By Jan 2025, collaborators of modded-nanogpt brought that time down to 3 minutes. It sat near 3 minutes until July 2025, having a large swath of optimization already applied: RoPE, value embeddings, reduce scatter grad updates, Muon, QK Norm, Relu^2, a custom FP8 head, skip connections, flex attention, short-long windows, attention window warmup, linear lr cooldown, and more. Yet, in the last 3 months the record has fallen by another 20% to 2 minutes and 20 seconds.

Many of the improvements in the last 20% have not yet been published outside of the modded-nanogpt repo. This post summarizes those improvements. Not everything will generalize to larger scales, but there are some core concepts that I believe are promising. Improvements [...]

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Outline:

(02:02) ML Improvements

(02:06) #1: Document Alignment

(03:26) #2: Dynamic Attention Window Management by Layer

(04:50) #3 Heterogenous Batch Sizes

(05:31) #4 Backout: Enabling a model to back out context for predictions

(06:18) #5 Polar Express

(06:36) #6 Smear Module

(07:20) #7 Sparse Attention Gate

(07:58) #8 More Bfloat16

(08:36) #9 Softmax Skip Gate

(09:04) #10 Drop MLP Layer

(09:19) Engineering Improvements

(09:23) #1 Flash Attention 3

(09:50) #2 Parameter reshaping for shared reduce scatter

(11:15) #3 Async Data Fetch and Index

(11:44) #4 Vectorized Optimizer Step

(12:03) #5 Triton Kernel for Symmetric Matmul

(12:41) #6 Resize Lambda Parameters

(13:36) Takeaways from the Journey

The original text contained 4 footnotes which were omitted from this narration.

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First published:
October 5th, 2025

Source:
https://www.lesswrong.com/posts/j3gp8tebQiFJqzBgg/how-the-nanogpt-speedrun-wr-dropped-by-20-in-3-months

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Narrated by TYPE III AUDIO.

“How the NanoGPT Speedrun WR dropped by 20% in 3 months” by larry-dial

LessWrong (30+ Karma)