[RFC] `Add Accelerator.is_available()` interface requirement

[ananthsub]

🚀 Feature

Motivation

Such functionality on the Accelerator abstraction would:

• Enable automatic hardware selection without duplicating code across Trainer & individual accelerator implementations.
• Simplify the accelerator connector logic and rewrite effort: Rewrite accelerator_connector #11448
• Enable automatic runtime checking of hardware availability during execution
• Provide consistency with how the Trainer auto-detects the cluster environments natively supported by the framework. The corollary here is ClusterEnvironment.detect

https://github.com/PyTorchLightning/pytorch-lightning/blob/9e63281a4c4a62f32cad9801a23b63454f8311be/pytorch_lightning/plugins/environments/cluster_environment.py#L43-L46

https://github.com/PyTorchLightning/pytorch-lightning/blob/9e63281a4c4a62f32cad9801a23b63454f8311be/pytorch_lightning/trainer/connectors/accelerator_connector.py#L810-L812

Pitch

class Accelerator(ABC):

    @staticmethod
    @abstractmethod
    def is_available() -> bool:
        """Detect if the hardware is available."""
    
    def setup_environment(self, root_device: torch.device) -> None:
        """Setup any processes or distributed connections.
        This is called before the LightningModule/DataModule setup hook which allows the user to access the accelerator
        environment before setup is complete.
        Raises:
            RuntimeError:
                If corresponding hardware is not found.
        """
        if not self.is_available():
            raise RuntimeError(f"{self.__class__.__qualname__} is not configured to run on this hardware.")

class CPUAccelerator(Accelerator):
    @staticmethod
    def is_available() -> bool:
        """CPU is always available for execution."""
        return True

class GPUAccelerator(Accelerator):
   @staticmethod
    def is_available() -> bool:
        return torch.cuda.is_available() and torch.cuda.device_count() > 0 

and so on

See a more-detailed implementation here: #11797 for what this looks like in practice

To support Trainer(accelerator="auto") this is what the logic simplifies to:

for acc_cls in (GPUAccelerator, TPUAccelerator, IPUAccelerator, CPUAccelerator):
    if acc_cls.is_available():
        return acc_cls()
return CPUAccelerator() # fallback to CPU

this could be even further simplified if we offered an AcceleratorRegistry, such that the Trainer/AcceleratorConnector didn't need to hardcode the list of accelerators to detect:

for acc_cls in AcceleratorRegistry.impls:
    if acc_cls.is_available():
        return acc_cls()
return CPUAccelerator() # fallback to CPU

Alternatives

Some other alternatives exist here:
#11799
#11798

Issues with these approaches:

• Also breaking changes: simply instantiating the accelerator could raise a runtime error if the device isn't available.
• The bigger issue to me is that it does not ease support for Trainer(accelerator="auto"). The accelerator connector needs to hardcode & re-implement each of the device checks to determine which Accelerator to even instantiate.

Additional context

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cc @Borda @tchaton @justusschock @awaelchli @akihironitta @rohitgr7

[tchaton]

Looks great, I like this.

[rohitgr7]

looks good!

one quick ques: why inside setup_environment here?

    def setup_environment(self, root_device: torch.device) -> None:
        """Setup any processes or distributed connections.
        This is called before the LightningModule/DataModule setup hook which allows the user to access the accelerator
        environment before setup is complete.
        Raises:
            RuntimeError:
                If corresponding hardware is not found.
        """
        if not self.is_available():
            raise RuntimeError(f"{self.__class__.__qualname__} is not configured to run on this hardware.")

and not inside init? I think we had a discussion regarding this in one of your PRs.

[ananthsub]

and not inside init? I think we had a discussion regarding this in one of your PRs.

@rohitgr7 great question. ultimately, the accelerator is mimicking torch device semantics.

PyTorch users today can create a torch.device("cuda") on a host without GPUs available without any error being raised.
An error only appears when someone tries to do CUDA operations, like moving the tensor to the device.

d = torch.device("cuda")  # this is fine
torch.tensor(0.0).to(d)  # this raises an error

To me, this is very similar to the Accelerator init (device creation) and Accelerator.setup_environment (moving data to the device).

To preserve this style, I don't raise an exception in init, and instead do it at setup_environment.
It also makes instantiating, mocking, and testing the different devices a lot easier to do

[justusschock]

@ananthsub I see the reasons for that, but on the other hand I usually am in favor of raising errors as early as possible. And for testing we will probably have to mock the torch.cuda package anyways.

Not sure which one has more pros or cons, just wanted to mention this :) But I definitely like the idea in general :)

[ananthsub]

@justusschock I agree, failing fast is very important.

To note:

• This proposal would not affect the current logic inside of the Trainer's accelerator connector for validating that the specified hardware is in fact available at Trainer construction time.
• And separately, we could move setup_environment to happen even sooner inside of Trainer._run: https://github.com/PyTorchLightning/pytorch-lightning/blob/b34d8673a9e77d25a826b7ada31d1204dfc818ab/pytorch_lightning/trainer/trainer.py#L1115

I'd argue that Strategy.setup_environment should happen before prepare_data, as currently other existing misconfiguration checks take place here that could happen sooner: https://github.com/PyTorchLightning/pytorch-lightning/blob/b34d8673a9e77d25a826b7ada31d1204dfc818ab/pytorch_lightning/accelerators/cpu.py#L28-L36

[awaelchli]

Makes sense

And separately, we could move setup_environment to happen even sooner inside of Trainer._run

Yes. Related but off-topic, there is a small misalignment in calling setup_environment() between ddp and ddp spawn #11073 but this can be easily resolved after #11643

[four4fish]

Yeah make sense! We can call accelerator.is_available() when we init_accelerator() in accel_connector. I will modify the rewrite PR after these get merged