How does DDP work under the hood in Lightning?

[davidgill97]

I'm trying to make a pipeline with hydra+lightning, but it seems lightning spawns multiple processes and runs the script multiple times (like using torchrun), causing unwanted side effects. So i want to understand how it works under the hood to fix the problem. (Not likely caused by lightning/hydra)

I know how it works with plain pytorch, so tried to dig into the source code.. which ended up with more confusion

Does anyone know how lightning (or fabric) handles DDP, especially without torchrun?