RunTimeError while trying to use the new mixed precision from PyTorch's new AMP in fastai2

Issue on PyTorch’s GitHub.

I added a few lines to Learner and one_batch, which set up the autoscaler and updated the training process to use AMP without compiling apex. Here’s the tutorial on how to do this.

The code is here: new pytorch AMP in fastai2

Here’s how one_batch looks:

   # self.scaler = GradientScaler() -- defined in Learner.__init__
   def one_batch(self, i, b):
        self.iter = i
        try:
            self._split(b);                                  self('begin_batch')
            # AMP - Run the forward pass with autocasting
            with autocast():
                self.pred = self.model(*self.xb);                self('after_pred')
                if len(self.yb) == 0: return
                self.loss = self.loss_func(self.pred, *self.yb); self('after_loss')
            if not self.training: return
            # Scales loss.  Calls backward() on scaled loss to create scaled gradients.
            # Backward passes under autocast are not recommended.
            # Backward ops run in the same precision that autocast used for corresponding forward ops.
            self.scaler.scale(self.loss).backward();              self('after_backward')
            # self.loss.backward();                            self('after_backward')
            # self.scaler.step() first unscales the gradients of the optimizer's assigned params.
            # If these gradients do not contain infs or NaNs, optimizer.step() is then called,
            # otherwise, optimizer.step() is skipped.
            self.scaler.step(self.opt);                      self('after_step')
            # self.opt.step();                                 self('after_step')
            # Updates the scale for next iteration.
            self.scaler.update()
            self.opt.zero_grad()
        except CancelBatchException:                         self('after_cancel_batch')
        finally:                                             self('after_batch')

This hits several RuntimeErrors, which I cleaned up by sending data to the device directly instead of letting pytorch fail on the asserts here.

Finally failing at this point:

---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-21-94707bfc8cc1> in <module>
      4 xb,yb = learn.dls.one_batch()
      5 init_loss = learn.loss_func(learn.model(xb), yb)
----> 6 learn.fit(6)
      7 assert learn.loss < init_loss

<ipython-input-16-7932a3f1e613> in fit(self, n_epoch, lr, wd, cbs, reset_opt)
    131                     try:
    132                         self.epoch=epoch;          self('begin_epoch')
--> 133                         self._do_epoch_train()
    134                         self._do_epoch_validate()
    135                     except CancelEpochException:   self('after_cancel_epoch')

<ipython-input-16-7932a3f1e613> in _do_epoch_train(self)
    104         try:
    105             self.dl = self.dls.train;                        self('begin_train')
--> 106             self.all_batches()
    107         except CancelTrainException:                         self('after_cancel_train')
    108         finally:                                             self('after_train')

<ipython-input-16-7932a3f1e613> in all_batches(self)
     70     def all_batches(self):
     71         self.n_iter = len(self.dl)
---> 72         for o in enumerate(self.dl): self.one_batch(*o)
     73 
     74     def one_batch(self, i, b):

<ipython-input-16-7932a3f1e613> in one_batch(self, i, b)
     90         # If these gradients do not contain infs or NaNs, optimizer.step() is then called,
     91         # otherwise, optimizer.step() is skipped.
---> 92             self.scaler.step(self.opt);                      self('after_step')
     93             # self.opt.step();                                 self('after_step')
     94             # Updates the scale for next iteration.

~/anaconda3/envs/pytorch-nightly/lib/python3.7/site-packages/torch/cuda/amp/grad_scaler.py in step(self, optimizer, *args, **kwargs)
    278 
    279         if optimizer_state["stage"] == self.READY:
--> 280             self.unscale_(optimizer)
    281 
    282         assert len(optimizer_state["found_inf_per_device"]) > 0, "No inf checks were recorded for this optimizer."

~/anaconda3/envs/pytorch-nightly/lib/python3.7/site-packages/torch/cuda/amp/grad_scaler.py in unscale_(self, optimizer)
    229         found_inf = torch.full((1,), 0.0, dtype=torch.float32, device=self._scale.device)
    230 
--> 231         optimizer_state["found_inf_per_device"] = self._unscale_grads_(optimizer, inv_scale, found_inf, False)
    232         optimizer_state["stage"] = self.UNSCALED
    233 

~/anaconda3/envs/pytorch-nightly/lib/python3.7/site-packages/torch/cuda/amp/grad_scaler.py in _unscale_grads_(self, optimizer, inv_scale, found_inf, allow_fp16)
    180                         torch._amp_non_finite_check_and_unscale_(param.grad,
    181                                                                  per_device_found_inf.get(param.grad.device),
--> 182                                                                  per_device_inv_scale.get(param.grad.device))
    183 
    184         return per_device_found_inf._per_device_tensors

RuntimeError: Could not run 'aten::_amp_non_finite_check_and_unscale_' with arguments from the 'CPUTensorId' backend. 'aten::_amp_non_finite_check_and_unscale_' is only available for these backends: [CUDATensorId, VariableTensorId].
1 Like

PyTorch automatic mixed precision is not compatible with fastai. You should continue to use to_fp16 and to_fp32. I don’t think their abstractions are going to fit with our training loop so we will probably not use it since we already have something that works.

1 Like

Thank you. Will study it and learn how it works.

I am a little confused. You say it is not compatible but over here you have an idea for using it?:

Which statement inaccurate?

1 Like

If you notice the latter came 11hrs later after he had time to think about it :wink: so work in progress

2 Likes

Yep I hope so too…