tokenization

For text training the bytes are trivial and the bottleneck is never bandwidth, so the usual advice about faster storage misses. What the loader actually has to deliver per second, the four things that consume the time instead, and the order to fix them with the measurement that proves each.

A tokenizer moves about a megabyte of text per second per core, which makes this a seven-hundred-core-day batch job rather than something to run during training. The throughput arithmetic per stage, the shard format the loader needs, and the determinism requirements that let you resume without corrupting a run.

Fifteen trillion tokens starts as a few petabytes of raw text and ends as 30 TB of shards a training job reads at 1.7 million tokens per second. The stages, the bytes at each boundary, the throughput each one needs to finish in two weeks, and the two stages where the pipeline actually spends its time.

A four-byte emoji split across three tokens will raise on every naive decode, and the fix is a buffer that holds the incomplete tail. The decoder in a dozen lines, the exception field that tells you exactly where to cut, and what to do when the stream ends mid-character.