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Changing only the learning-rate schedule can reverse which data schedule wins

AspireAI community · arXiv
Mid-training pipelines usually start on pretraining data and then switch completely to higher-value mid-training data. This paper argues that schedule cannot be chosen in isolation: with a decaying learning rate, introducing mid-training data earlier and mixing it with pretraining data lowers final mid-training loss, but the same result reverses under a constant learning rate. The practical prescription is to decay the learning rate only after mid-training data enters training, which the authors confirm holds for mixture-of-experts models as well as dense ones, and connect to a router-imbalance effect where abrupt data switches create load-balancing spikes that gradual entry avoids. The caveat is experimental scale: the runs use 630M dense and 2.99B mixture-of-experts models, so the optimal decay durations may shift at larger budgets.
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