RuntimeError: Expected all tensors to be on the same device, but found at least two devices, cuda:0 and cpu! when resuming training

Question

I saved a checkpoint while trainig on gpu. after reloading the checkpoint and continue training i get the following error.

Traceback (most recent call last):
  File "main.py", line 140, in <module>
    train(model,optimizer,train_loader,val_loader,criteria=args.criterion,epoch=epoch,batch=batch)
  File "main.py", line 71, in train
    optimizer.step()
  File "/opt/conda/lib/python3.7/site-packages/torch/autograd/grad_mode.py", line 26, in decorate_context
    return func(*args, **kwargs)
  File "/opt/conda/lib/python3.7/site-packages/torch/optim/sgd.py", line 106, in step
    buf.mul_(momentum).add_(d_p, alpha=1 - dampening)
RuntimeError: Expected all tensors to be on the same device, but found at least two devices, cuda:0 and cpu!

my training code is:

def train(model,optimizer,train_loader,val_loader,criteria,epoch=0,batch=0):
    batch_count = batch
    if criteria == 'l1':
        criterion = L1_imp_Loss()
    elif criteria == 'l2':
        criterion = L2_imp_Loss()
    if args.gpu and torch.cuda.is_available():
        model.cuda()
        criterion = criterion.cuda()

    print(f'{datetime.datetime.now().time().replace(microsecond=0)} Starting to train..')
    
    while epoch <= args.epochs-1:
        print(f'********{datetime.datetime.now().time().replace(microsecond=0)} Epoch#: {epoch+1} / {args.epochs}')
        model.train()
        interval_loss, total_loss= 0,0
        for i , (input,target) in enumerate(train_loader):
            batch_count += 1
            if args.gpu and torch.cuda.is_available():
                input, target = input.cuda(), target.cuda()
            input, target = input.float(), target.float()
            pred = model(input)
            loss = criterion(pred,target)
            optimizer.zero_grad()
            loss.backward()
            optimizer.step()
            ....

the saving proccess happend after finishing each epoch.

torch.save({'epoch': epoch,'batch':batch_count,'model_state_dict': model.state_dict(),'optimizer_state_dict':
                    optimizer.state_dict(),'loss': total_loss/len(train_loader),'train_set':args.train_set,'val_set':args.val_set,'args':args}, f'{args.weights_dir}/FastDepth_Final.pth')

I cant figure why i get this error. args.gpu == True , and Im passing the model, all data, and loss function to cuda, somehow there is still a tensor on cpu, could anyone figure out whats wrong?

Thanks.

Answer 1

There might be an issue with the device parameters are on:

If you need to move a model to GPU via .cuda() , please do so before constructing optimizers for it. Parameters of a model after .cuda() will be different objects with those before the call.
In general, you should make sure that optimized parameters live in consistent locations when optimizers are constructed and used.