Move the module to the precision dtype

[yuvalkirstain]

🐛 Bug

@SeanNaren
When I use bf-16 and check the dtype of the model, it seems like the model's precision is fp32 (and I do not see the memory gains I expect). On other frameworks that support bf-16 (like fairseq) the model's dtype is torch.bfloat16. Is there a simple example that "proves" that this feature reduces the memory consumption as it should? I suspect that there might be something wrong (but of course, I might be wrong).
Thank you!

To Reproduce

launch any job with precision=bf16 and compare with precision=32.

Expected behavior

This feature should save 30-50% memory but I do not see such gains in lightning.

Environment

• CUDA:
  • GPU:
    • GeForce RTX 3090
  • available: True
  • version: 11.3
• Packages:
  • numpy: 1.21.2
  • pyTorch_debug: False
  • pyTorch_version: 1.11.0
  • pytorch-lightning: 1.6.0dev
  • tqdm: 4.62.3
• System:
  • OS: Linux
  • architecture:
    • 64bit
    • ELF
  • processor: x86_64
  • python: 3.8.12
  • version: Load fix #74-Ubuntu SMP Tue Sep 17 17:06:04 UTC 2019

Additional context

BF-16 is a very important feature. It is usually more stable than fp16 and lightning should support it effectively (models that are pretrained with bf-16 should not be used with fp-16) :)

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

[SeanNaren]

Thanks @yuvalkirstain!

I investigated this as it definitely seemed strange however I think there may be a misunderstanding of behaviour.

using AMP, the model remains in FP32 but operations that can happen in FP16 are auto-cast. This is the same with BF16 (see this example):

import os

import torch
from torch.utils.data import DataLoader, Dataset

from pytorch_lightning import LightningModule, Trainer


class RandomDataset(Dataset):
    def __init__(self, size, length):
        self.len = length
        self.data = torch.randn(length, size)

    def __getitem__(self, index):
        return self.data[index]

    def __len__(self):
        return self.len


class BoringModel(LightningModule):
    def __init__(self):
        super().__init__()
        self.layer = torch.nn.Linear(32, 2)

    def forward(self, x):
        assert self.layer.weight.dtype == torch.float32 # the weights are in 32!
        x = self.layer(x) 
        assert x.dtype == torch.bfloat16 # output was bfloat16 as the operation is bf16 compatible
        return x

    def training_step(self, batch, batch_idx):
        loss = self(batch).sum()
        self.log("train_loss", loss)
        return {"loss": loss}

    def configure_optimizers(self):
        return torch.optim.SGD(self.layer.parameters(), lr=0.1)


def run():
    train_data = DataLoader(RandomDataset(32, 64), batch_size=2)
    model = BoringModel()

    trainer = Trainer(
        default_root_dir=os.getcwd(),
        limit_train_batches=1,
        max_epochs=1,
        enable_model_summary=False,
        precision="bf16",
        gpus=1,
    )

    trainer.fit(model, train_dataloaders=train_data)


if __name__ == "__main__":
    run()

I then checked FairSeq, which converts the entire model into BF16 even in AMP mode: https://github.com/pytorch/fairseq/blob/main/fairseq/trainer.py#L105-L106

This might be why you see the memory improvements. If this is something you'd like to do, you can simply do the same in your __init__.

class BoringModel(LightningModule):
    def __init__(self):
        super().__init__()
        self.layer = torch.nn.Linear(32, 2).bfloat16()

Let me know if this helps! We might need to improve our documentation to explain this case.

[yuvalkirstain]

Thank you so much for the detailed answer! I will try it out and update :)

[SeanNaren]

Nope thank you @yuvalkirstain let me know how it goes! You do also bring up a great point which is if we should support converting the pl_module internally for users. I think it is a potentially good idea, however, I wonder what the API for this would be.

[carmocca]

This would be just a flag on the precision plugin (name up for debate)

from pytorch_lightning import Trainer
from pytorch_lightning.plugins import NativeMixedPrecisionPlugin

Trainer(plugins=NativeMixedPrecisionPlugin(convert_modules=True))

Another question is whether this should be enabled by default.

As per the test added by @rohitgr7 in https://github.com/PyTorchLightning/pytorch-lightning/pull/12508/files#diff-3e387bfea0892d3f7033341769075f9e8b17dbe2fbe4a44a68e3d450bdba2e0eR1223, the layers are also moved using DeepSpeed

[yuvalkirstain]

@SeanNaren Yes, doing so results in less memory on the GPU with identical results, thank you!

T5-XL (3B parameters) - inference on SQuAD dataset
8058MiB / 24268MiB (with model = model.bfloat16())
13196MiB / 24268MiB (without)

Regarding converting the pl_module internally for users, definitely, it makes more sense IMO that the trainer will take care of that rather than the model.

[SeanNaren]

@yuvalkirstain I'm glad it worked! Hopefully we'll get the feature in soon :)

[rohitgr7]

Another question is whether this should be enabled by default.

I think yes because:

1. less memory consumption
2. other frameworks like deepspeed does it too

issues:

1. BC

but if there are no other side effects, I think enabling it by default should be good.

[SeanNaren]

Another question is whether this should be enabled by default.

I think yes because:

1. less memory consumption
2. other frameworks like deepspeed does it too

issues:

1. BC

but if there are no other side effects, I think enabling it by default should be good.

Just to be clear, we're talking about just BF16 precision? DeepSpeed's handling of AMP is very different and should be treated as such.