What Is a Warmup Period in Training? Starting Slow on Purpose
Most training runs deliberately start with a very small learning rate and ramp it up gradually — here's why jumping straight to full speed tends to backfire.
Starting deliberately slow
A warmup period is the early phase of training where the learning rate starts very small and gradually increases up to its intended full value, rather than starting at full strength from the very first training step.
Why jumping straight to full speed causes problems
At the very start of training, a model's parameters are randomly initialized and the estimated gradient direction can be unreliable and noisy. Taking large steps immediately, before training has stabilized at all, risks destabilizing training entirely — sometimes causing a loss spike or outright divergence early on.
How long warmup typically lasts
Warmup is usually a relatively small fraction of total training — just long enough for the model's initial parameters and gradient estimates to stabilize — before the schedule transitions into its main phase, where the learning rate then gradually decreases for the remainder of the run.
Frequently Asked Questions
What is a warmup period in training?
The early phase of training where the learning rate starts very small and gradually ramps up to its full intended value, rather than starting at full strength from the very first step.
Why is warmup necessary?
At the start of training, a model's randomly initialized parameters make gradient estimates unreliable. Taking large steps immediately risks destabilizing training, sometimes causing a loss spike or divergence — warmup avoids that risk.