Switching to pretrained model leads to strange performance

Brief background:

I have some neural network code where I use the off-the-shelf AlexNet to classify pictures of galaxies according to their apparent shape. I use an outer loop to randomly split the data into train and test sets, and an inner loop to train for a certain number of epochs (using the given train-test split).

Below I show some performance snippets. When I use the pretrained AlexNet, I get performance as shown below.

Epoch 0/19
train Loss: 1.0289 Acc: 0.5794
val Loss: 0.8498 Acc: 0.6455

Epoch 1/19
train Loss: 0.7615 Acc: 0.6877
val Loss: 0.6023 Acc: 0.7675

Epoch 2/19
train Loss: 0.6124 Acc: 0.7541
val Loss: 0.5280 Acc: 0.7989

Epoch 3/19
train Loss: 0.5634 Acc: 0.7775
val Loss: 0.4999 Acc: 0.8088

Epoch 4/19
train Loss: 0.5335 Acc: 0.7915
val Loss: 0.4847 Acc: 0.8038

However, when I make the following two changes to my code:

  1. def initialize_model(model_name, num_classes, feature_extract, use_pretrained=True): becomes def initialize_model(model_name, num_classes, feature_extract, use_pretrained):
  2. model_ft, input_size = initialize_model(model_name, num_classes, feature_extract, use_pretrained=True) becomes model_ft, input_size = initialize_model(model_name, num_classes, feature_extract, use_pretrained=False)

Then I get performance like the following:

Epoch 0/19
train Loss: 1.3729 Acc: 0.4470
val Loss: 1.3457 Acc: 0.5472

Epoch 1/19
train Loss: 1.3219 Acc: 0.5496
val Loss: 1.2954 Acc: 0.5472

Epoch 2/19
train Loss: 1.2761 Acc: 0.5496
val Loss: 1.2544 Acc: 0.5472

Epoch 3/19
train Loss: 1.2391 Acc: 0.5496
val Loss: 1.2214 Acc: 0.5472

Epoch 4/19
train Loss: 1.2092 Acc: 0.5496
val Loss: 1.1948 Acc: 0.5472

I understand that the fresh (not pretrained) network will perform worse initially. What concerns me is that the accuracy scores are constant (except for one), even while the losses are decreasing.

I am thinking about re-running the pretrained job, but commenting out the lines where I use DistributedDataParallel. I have no idea if this is relevant, but I can’t think of anything else to suspect.

I have switched between pretrained and non-pretrained AlexNet before (although with no random train-test splits and no DistributedDataParallel) and the non-pretrained network behaved as I would have expected: it performed much worse initially but eventually (mostly) caught up.

Below I share some parts of my code (for the pretrained job).

Model training function: (this is mostly a direct paste from a PyTorch tutorial)

def train_model(model, dataloaders, criterion, optimizer, num_epochs=25, is_inception=False):
    since = time.time()
    val_acc_history = []
    train_acc_history = []
    best_model_wts = copy.deepcopy(model.state_dict())
    best_acc = 0.0
    for epoch in range(num_epochs):
        print('Epoch {}/{}'.format(epoch, num_epochs - 1))
        print('-' * 10)
        # Each epoch has a training and validation phase
        for phase in ['train', 'val']:
            if phase == 'train':
                model.train() # Set model to training mode
                model.eval() # Set model to evaluate mode
            running_loss = 0.0
            running_corrects = 0
            # Iterate over data.
            for inputs, labels in dataloaders[phase]:
                inputs = inputs.to(device)
                labels = labels.to(device)
                # zero the parameter gradients
                # forward
                # track history if only in train
                with torch.set_grad_enabled(phase == 'train'):
                    # Get model outputs and calculate loss
                    # Special case for inception because in training it has an
                    # auxiliary output. In train mode we calculate the loss by
                    # summing the final output and the auxiliary output but in
                    # testing we only consider the final output.
                    if is_inception and phase == 'train':
                        # From https://discuss.pytorch.org/t/how-to-optimize-inception-model-with-auxiliary-classifiers/7958
                        outputs, aux_outputs = model(inputs)
                        loss1 = criterion(outputs, labels)
                        loss2 = criterion(aux_outputs, labels)
                        loss = loss1 + 0.4*loss2
                        outputs = model(inputs)
                        loss = criterion(outputs, labels)
                    _, preds = torch.max(outputs, 1)
                    # backward + optimize only if in training phase
                    if phase == 'train':
                # statistics
                running_loss += loss.item() * inputs.size(0)
                running_corrects += torch.sum(preds == labels.data)
            epoch_loss = running_loss / len(dataloaders[phase].dataset)
            epoch_acc = running_corrects.double() / len(dataloaders[phase].dataset)
            print('{} Loss: {:.4f} Acc: {:.4f}'.format(phase, epoch_loss, epoch_acc))
            # deep copy the model
            if phase == 'val' and epoch_acc > best_acc:
                best_acc = epoch_acc
                best_model_wts = copy.deepcopy(model.state_dict())
            if phase == 'val':
            if phase == 'train':

            'epoch': epoch,
            'model_state_dict': model.state_dict(),
            'optimizer_state_dict': optimizer.state_dict(),
            # 'best_model_accuracy': best_acc,
            # 'best_model_weights': best_model_wts,
            'loss': loss,
            # 'train_hist': train_acc_history,
            # 'val_hist': val_acc_history,
        }, f'/project/rrg-lelliott/jsa378/model_1_output/checkpoint_run_{run}_epoch_{epoch}.tar')
        # }, str(os.getenv('location3'))+f'/checkpoint_run_{run}_epoch_{epoch}.tar')  

            'epoch': epoch,
            'model_state_dict': model.state_dict(),
            'optimizer_state_dict': optimizer.state_dict(),
            'best_model_accuracy': best_acc,
            'best_model_weights': best_model_wts,
            'loss': loss,
            'train_hist': train_acc_history,
            'val_hist': val_acc_history,
        }, f'/project/rrg-lelliott/jsa378/model_1_output/checkpoint_run_{run}.tar')
        # }, str(os.getenv('location3'))+f'/checkpoint_run_{run}.tar')

    time_elapsed = time.time() - since
    print('Training complete in {:.0f}m {:.0f}s'.format(time_elapsed // 60, time_elapsed % 60))
    print('Best val Acc: {:4f}'.format(best_acc))
    # load best model weights
    return model, val_acc_history, train_acc_history

Model initializer: (again mostly pasted from a tutorial)

def initialize_model(model_name, num_classes, feature_extract, use_pretrained):
    # Initialize these variables which will be set in this if statement.
    # Each of these variables is model specific.
    model_ft = None
    input_size = 0
    if model_name == "alexnet":
        """ Alexnet
        model_ft = models.alexnet(pretrained=use_pretrained)
        set_parameter_requires_grad(model_ft, feature_extract)
        num_ftrs = model_ft.classifier[6].in_features
        model_ft.classifier[6] = nn.Linear(num_ftrs,num_classes)
        input_size = 224

    elif model_name == "resnet":
        """ Resnet18
        model_ft = models.resnet152(pretrained=use_pretrained)
        set_parameter_requires_grad(model_ft, feature_extract)
        num_ftrs = model_ft.fc.in_features
        model_ft.fc = nn.Linear(num_ftrs, num_classes)
        input_size = 224

        print("Invalid model name, exiting...")

    return model_ft, input_size

DistributedDataParallel usage and model initialization:

def main():

    rank = os.environ.get("SLURM_LOCALID")

    current_device = 0
    print('From Rank: {}, ==> Initializing Process Group...'.format(rank))

    dist.init_process_group(backend="mpi", init_method='tcp://') # Use backend="mpi" or "gloo". NCCL does not work on a single GPU due to a hard-coded multi-GPU topology check.
    print("process group ready!")

    print('From Rank: {}, ==> Making model..'.format(rank))
    model_ft, input_size = initialize_model(model_name, num_classes, feature_extract, use_pretrained=False)
    model_ft = model_ft.to(device)
    model_ft = torch.nn.parallel.DistributedDataParallel(model_ft, device_ids=[current_device]) # Wrap the model with DistributedDataParallel

(There is much more code in the main function that I’m not showing.)

Thanks for any help.

Update: I took an older version of my code (that does not use DistributedDataParallel), modified it to use the non-pretrained version of AlexNet, and I have the same problem:

Epoch 0/9
train Loss: 1.2660 Acc: 0.5396
val Loss: 1.1646 Acc: 0.5615

Epoch 1/9
train Loss: 1.1342 Acc: 0.5472
val Loss: 1.0960 Acc: 0.5615

Epoch 2/9
train Loss: 1.0965 Acc: 0.5472
val Loss: 1.0793 Acc: 0.5615

Epoch 3/9
train Loss: 1.0847 Acc: 0.5472
val Loss: 1.0764 Acc: 0.5615

Epoch 4/9
train Loss: 1.0830 Acc: 0.5472
val Loss: 1.0755 Acc: 0.5615

So I can probably rule out DistributedDataParallel as the problem. I guess I have to go back to an older Python file that did successfully use the non-pretrained network, and try to modify it to do what I want now. I cannot find any errors in my code (though I am not an expert).

I should also mention that I’m using Python 3.9.6, PyTorch 1.10.0 and Torchvision 0.11.1. I know that the Boolean pretrained variable is being deprecated in newer versions, but I somehow don’t think that should affect me.