How do you choose the data mixture for pretraining an LLM, and what does domain reweighting buy you?
The proportions of web, code, books, and math in pretraining quietly set downstream skills. The signal is knowing how mixtures get chosen, why upsampling helps, and how methods like DoReMi automate it.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
The proportions of web, code, books, and math in pretraining quietly set downstream skills. The signal is knowing how mixtures get chosen, why upsampling helps, and how methods like DoReMi automate it.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
