Debug your model (advanced)

:orphan:

.. _debugging_advanced:

########################### Debug your model (advanced) ########################### Audience: Users who want to debug distributed models.

Debug distributed models

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

.. code-block:: python

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

On the CPU, you can use pdb <https://docs.python.org/3/library/pdb.html>_ or breakpoint() <https://docs.python.org/3/library/functions.html#breakpoint>_ or use regular print statements.

.. testcode::

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.

.. code-block:: python

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