Pytorch model running out of memory on both CPU and GPU, can’t figure out what I’m doing wrong
Trying to implement a simple multi-label image classifier using Pytorch Lightning. Here's the model definition:
import torch
from torch import nn

# creates network class
class Net(pl.LightningModule):
    def __init__(self):
        super().__init__()

        # defines conv layers
        self.conv_layer_b1 = nn.Sequential(
            nn.Conv2d(in_channels=3, out_channels=32,
                      kernel_size=3, padding=1),
            nn.ReLU(),
            nn.MaxPool2d(kernel_size=2, stride=2),
            nn.Flatten(),
        )

        # passes dummy x matrix to find the input size of the fc layer
        x = torch.randn(1, 3, 800, 600)
        self._to_linear = None
        self.forward(x)

        # defines fc layer
        self.fc_layer = nn.Sequential(
            nn.Linear(in_features=self._to_linear,
                      out_features=256),
            nn.ReLU(),
            nn.Linear(256, 5),
        )

        # defines accuracy metric
        self.accuracy = pl.metrics.Accuracy()
        self.confusion_matrix = pl.metrics.ConfusionMatrix(num_classes=5)

    def forward(self, x):
        x = self.conv_layer_b1(x)

        if self._to_linear is None:
            # does not run fc layer if input size is not determined yet
            self._to_linear = x.shape[1]
        else:
            x = self.fc_layer(x)
        return x

    def cross_entropy_loss(self, logits, y):
        criterion = nn.CrossEntropyLoss()
        
        return criterion(logits, y)

    def training_step(self, train_batch, batch_idx):
        x, y = train_batch
        logits = self.forward(x)

        train_loss = self.cross_entropy_loss(logits, y)
        train_acc = self.accuracy(logits, y)
        train_cm = self.confusion_matrix(logits, y)

        self.log('train_loss', train_loss)
        self.log('train_acc', train_acc)
        self.log('train_cm', train_cm)

        return train_loss

    def validation_step(self, val_batch, batch_idx):
        x, y = val_batch
        logits = self.forward(x)

        val_loss = self.cross_entropy_loss(logits, y)
        val_acc = self.accuracy(logits, y)

        return {'val_loss': val_loss, 'val_acc': val_acc}

    def validation_epoch_end(self, outputs):
        avg_val_loss = torch.stack([x['val_loss'] for x in outputs]).mean()
        avg_val_acc = torch.stack([x['val_acc'] for x in outputs]).mean()

        self.log(&quot;val_loss&quot;, avg_val_loss)
        self.log(&quot;val_acc&quot;, avg_val_acc)

    def configure_optimizers(self):
        optimizer = torch.optim.Adam(self.parameters(), lr=0.0008)

        return optimizer

The issue is probably not the machine since I'm using a cloud instance with 60 GBs of RAM and 12 GBs of VRAM. Whenever I run this model even for a single epoch, I get an out of memory error. On the CPU it looks like this:
RuntimeError: [enforce fail at CPUAllocator.cpp:64] . DefaultCPUAllocator: can't allocate memory: you tried to allocate 1966080000 bytes. Error code 12 (Cannot allocate memory)
and on the GPU it looks like this:
RuntimeError: CUDA out of memory. Tried to allocate 7.32 GiB (GPU 0; 11.17 GiB total capacity; 4.00 KiB already allocated; 2.56 GiB free; 2.00 MiB reserved in total by PyTorch)

Clearing the cache and reducing the batch size did not work. I'm a novice so clearly something here is exploding but I can't tell what. Any help would be appreciated.
Thank you!

Indeed, it's not a machine issue; the model itself is simply unreasonably big. Typically, if you take a look at common CNN models, the fc layers occur near the end, after the inputs already pass through quite a few convolutional blocks (and have their spatial resolutions reduced).
Assuming inputs are of shape (batch, 3, 800, 600), while passing the conv_layer_b1 layer, the feature map shape would be (batch, 32, 400, 300) after the MaxPool operation. After flattening, the inputs become (batch, 32 * 400 * 300), ie, (batch, 3840000).
The immediately following fc_layer thus contains nn.Linear(3840000, 256), which is simply absurd. This single linear layer contains ~983 million trainable parameters! For reference, popular image classification CNNs roughly have 3 to 30 million parameters on average, with larger variants reaching 60 to 80 million. Few ever really cross the 100 million mark.
You can count your model params with this:
def count_params(model):
    return sum(map(lambda p: p.data.numel(), model.parameters()))

My advice: 800 x 600 is really a massive input size. Reduce it to something like 400 x 300, if possible. Furthermore, add several convolutional blocks similar to conv_layer_b1, before the FC layer. For example:
def get_conv_block(C_in, C_out):
    return nn.Sequential(
            nn.Conv2d(in_channels=C_in, out_channels=C_out,
                      kernel_size=3, padding=1),
            nn.ReLU(),
            nn.MaxPool2d(kernel_size=2, stride=2)
        )

class Net(pl.LightningModule):
    def __init__(self):
        super().__init__()

        # defines conv layers
        self.conv_layer_b1 = get_conv_block(3, 16)
        self.conv_layer_b2 = get_conv_block(16, 32)
        self.conv_layer_b3 = get_conv_block(32, 64)
        self.conv_layer_b4 = get_conv_block(64, 128)
        self.conv_layer_b5 = get_conv_block(128, 256)

        # passes dummy x matrix to find the input size of the fc layer
        x = torch.randn(1, 3, 800, 600)
        self._to_linear = None
        self.forward(x)

        # defines fc layer
        self.fc_layer = nn.Sequential(
            nn.Flatten(),
            nn.Linear(in_features=self._to_linear,
                      out_features=256),
            nn.ReLU(),
            nn.Linear(256, 5)
        )

        # defines accuracy metric
        self.accuracy = pl.metrics.Accuracy()
        self.confusion_matrix = pl.metrics.ConfusionMatrix(num_classes=5)

    def forward(self, x):
        
        x = self.conv_layer_b1(x)
        x = self.conv_layer_b2(x)
        x = self.conv_layer_b3(x)
        x = self.conv_layer_b4(x)
        x = self.conv_layer_b5(x)
        
        if self._to_linear is None:
            # does not run fc layer if input size is not determined yet
            self._to_linear = nn.Flatten()(x).shape[1]
        else:
            x = self.fc_layer(x)
        return x

Here, because more conv-relu-pool layers are applied, the input is reduced to a feature map of a much smaller shape, (batch, 256, 25, 18), and the overall number of trainable parameters would be reduced to about ~30 million parameters.
