Debug your model (advanced)

Audience: Users who want to debug distributed models.

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Debug distributed models

To debug a distributed model, we recommend you debug it locally by running the distributed version on CPUs:

trainer = Trainer(accelerator="cpu", strategy="ddp", devices=2)

On the CPU, you can use pdb or breakpoint() or use regular print statements.

class LitModel(LightningModule):
    def training_step(self, batch, batch_idx):
        debugging_message = ...
        print(f"RANK - {self.trainer.global_rank}: {debugging_message}")

        if self.trainer.global_rank == 0:
            import pdb

            pdb.set_trace()

        # to prevent other processes from moving forward until all processes are in sync
        self.trainer.strategy.barrier()

When everything works, switch back to GPU by changing only the accelerator.

trainer = Trainer(accelerator="gpu", strategy="ddp", devices=2)