Extreme single thread cpu kernel usage while training on GPU

Hello there,
Just into deep learning, and currently I am facing some weird issues regarding the model I am working on. As the title and images below suggest, during training, one of the cpu cores is experiencing extreme kernel usage while other cores barely moves. Despite I am using GPU as the accelerator.

I had read quite a few discussions regarding similar issues, but none fixed my problem.
e.g. CPU usage extremely high
and Very high CPU utilization with pin_memory=True and num_workers > 0 · Issue #25010 · pytorch/pytorch · GitHub
I had tried to set the num_workers in the dataloader to 2 or more, make no difference. Set pin_memory to True or False doesn’t seem to do anything as well.
torch.get_num_threads() returns 6, but torch.set_num_threads() = 6 doesn’t help with the issue.
If I switch the accelerator from cuda to cpu, the model also only uses one cpu core for training, which is extremely inefficient.
The model itself is a Half-UNet for image segmentation, 16bit grey scale image data are converted into tensors in the Dataset, before a variety of transformations got applied and send into the model for training.

Computer Specs:
CPU: Ryzen 5500
GPU: 1050ti
Disks: Data are stored in two 1TB Barracuda HDD in raid 1 using AMD Raid, Operating system and Pycharm are stored in a SSD
Memory: 32GB

Pytorch Version: 1.13.1
Cuda Version: 11.7
Operating System: Linux mint cinnamon

Something interesting I noticed is when I train with my laptop, which has an 11th gen intel i5 (with no discrete graphics) and is running on Windows. The same piece of codes will correctly uses all of the cpu cores of the laptops.

Here is the Dataset class I used:

class Train_Dataset(torch.utils.data.Dataset):
    def __init__(self, images_dir, labels_dir):
        # Get a list of file paths for images and labels
        self.file_list = make_dataset_train(images_dir, labels_dir)
        self.num_files = len(self.file_list)
        self.img_tensors = []
        self.lab_tensors = []
        # Pre-allocating the memory for img_tensors and lab_tensors
        for idx in range(self.num_files):
            # Convert the image and label path to tensors
            img_tensor = path_to_tensor(self.file_list[idx][0], label=False)
            lab_tensor = path_to_tensor(self.file_list[idx][1], label=True)
            lab_tensor = lab_tensor.long()
            # Append the tensors to the list

    def __len__(self):
        return self.num_files

    def __getitem__(self, idx):
        # Get the image and label tensors at the specified index
        img_tensor = self.img_tensors[idx]
        lab_tensor = self.lab_tensors[idx]

        # Decide whether to apply each random transformation
        random_transforms = [random.random() < 0.5 for i in range(6)]

        # Apply vertical flip
        if random_transforms[0]:
            img_tensor, lab_tensor = T_F.vflip(img_tensor), T_F.vflip(lab_tensor)
        # Apply horizontal flip
        if random_transforms[1]:
            img_tensor, lab_tensor = T_F.hflip(img_tensor), T_F.hflip(lab_tensor)
        # Apply Gaussian blur
        if random_transforms[2]:
            img_tensor = gaussian_blur_3d(img_tensor, random.randint(1, 2) * 2 + 1)
        # Apply padding and cropping
        if random_transforms[3]:
            # Randomly generate padding sizes
            left_pad_size = random.randint(2, 64)
            right_pad_size = random.randint(2, 64)
            up_pad_size = random.randint(2, 64)
            down_pad_size = random.randint(2, 64)
            img_tensor = img_tensor[:, up_pad_size:-down_pad_size, left_pad_size:-right_pad_size]
            lab_tensor = lab_tensor[:, up_pad_size:-down_pad_size, left_pad_size:-right_pad_size]
            padding = (left_pad_size, right_pad_size,
                       up_pad_size, down_pad_size,
                       0, 0)
            img_tensor = F.pad(img_tensor, padding, mode="constant")
            lab_tensor = F.pad(lab_tensor, padding, mode="constant")
        # Apply rotations
        if random_transforms[4]:
            rotation_angle = random.uniform(-90, 90)
            img_tensor = T_F.rotate(img_tensor, rotation_angle)
            lab_tensor = T_F.rotate(lab_tensor, rotation_angle)
        # Apply contrast adjustments
        if random_transforms[5]:
            img_tensor = adjust_contrast_3d(img_tensor)
        img_tensor = img_tensor[None, :].to(torch.float16)
        return img_tensor, lab_tensor

The model itself and training process:
(I know it’s handled by pytorch lightning but just in case it’s indeed a bug at pytorch end)

class HalfNet(nn.Module):

    def __init__(self):
        super(HalfNet, self).__init__()
        channel_base = 18
        self.inc = ModuleComponents.DoubleConv3D(1, channel_base, ghost=False, kernel_size=(1, 3, 3))
        self.down1 = ModuleComponents.Down3D(channel_base, channel_base, ghost=False, kernel_size=(1, 3, 3))
        self.merge1 = ModuleComponents.Merge3D(2)
        self.down2 = ModuleComponents.Down3D(channel_base, channel_base, ghost=False, kernel_size=(1, 3, 3))
        self.merge2 = ModuleComponents.Merge3D(4)
        self.down3 = ModuleComponents.Down3D(channel_base, channel_base, ghost=False, kernel_size=(1, 3, 3))
        self.merge3 = ModuleComponents.Merge3D(8)
        self.down4 = ModuleComponents.Down3D(channel_base, channel_base, ghost=False, kernel_size=(1, 3, 3))
        self.merge4 = ModuleComponents.Merge3D(16)
        self.down5 = ModuleComponents.Down3D(channel_base, channel_base, ghost=False, kernel_size=(1, 3, 3))
        self.merge5 = ModuleComponents.Merge3D(32)
        self.z_work = ModuleComponents.DoubleConv3D(channel_base, channel_base, ghost=False, kernel_size=(3, 1, 1))
        self.outc = ModuleComponents.OutConv3D(channel_base, 2)

    def forward(self, x):
        x = self.inc(x)
        x2 = self.down1(x)
        x = self.merge1(x, x2)
        x2 = self.down2(x2)
        x = self.merge2(x, x2)
        x2 = self.down3(x2)
        x = self.merge3(x, x2)
        x2 = self.down4(x2)
        x = self.merge4(x, x2)
        x2 = self.down5(x2)
        x = self.merge5(x, x2)
        x = self.z_work(x)
        x = self.outc(x)
        return x

class HalfNetPL(pl.LightningModule):

    def __init__(self, learning_rate=0.001):
        super(HalfNetPL, self).__init__()
        self.model = HalfNet()
        self.learning_rate = learning_rate
        #self.automatic_optimization = False

    def forward(self, image):
        return self.model(image)

    def training_step(self, batch, batch_idx):
        #loss = self._step(batch, batch_idx)
        #self.log("train_loss", loss, on_step=False, on_epoch=True, prog_bar=True, logger=True)
        return {'loss': self._step(batch, batch_idx)}

    def validation_step(self, batch, batch_idx):
        loss = self._step(batch, batch_idx)
        self.log("val_loss", loss, on_step=False, on_epoch=True, prog_bar=False, logger=False)
        return {'val_loss': loss}

    def configure_optimizers(self):
        optimizer = torch.optim.Adam(self.parameters(), lr=self.learning_rate)
        scheduler = torch.optim.lr_scheduler.ReduceLROnPlateau(optimizer, mode='min', factor=0.5, patience=30,
                                                               threshold=0.0001, threshold_mode='rel',
                                                               cooldown=0, min_lr=0.00025, verbose=True)
        return {"optimizer": optimizer,
                "lr_scheduler": {"scheduler": scheduler, "monitor": "val_loss"},}

    def lr_scheduler_step(self, scheduler, optimizer_idx, metric):
        if metric is None:

    def predict_step(self, batch, batch_idx: int, dataloader_idx: int = 0):
        x = batch
        return torch.argmax(self(x), dim=1)

    def validation_epoch_end(self, outputs):
        avg_loss = torch.stack([x['val_loss'] for x in outputs]).mean()
        self.logger.experiment.add_scalar("Loss/Val", avg_loss, self.current_epoch)
        self.val_loss = avg_loss
    def training_epoch_end(self, outputs):
            #sampleImg = torch.rand((1,1,5,128,128))
            #self.logger.experiment.add_graph(HalfNet(), sampleImg)
        avg_loss = torch.stack([x['loss'] for x in outputs]).mean()
        self.logger.experiment.add_scalar("Loss/Train", avg_loss, self.current_epoch)

    def test_step(self, batch, batch_idx):
        loss = self._step(batch, batch_idx)
        self.log("test_loss", loss, on_step=False, on_epoch=True, prog_bar=True, logger=True)
        return {'test_loss': loss}
    def _step(self, batch, batch_idx):
        x, y = batch
        y_hat = self.forward(x)
        #weight = torch.tensor(data=(1, 1), device=self.device)
        #weight = weight.to(torch.float)
        loss = F.cross_entropy(input=y_hat, target=y)
        return loss

if __name__ == "__main__":
    train_dataset = DataComponents.Train_Dataset('datasets/train/img',
    train_loader = torch.utils.data.DataLoader(dataset=train_dataset, batch_size=2, shuffle=True, num_workers=2, pin_memory=False)
    # Setting up training parameters
    trainer = pl.Trainer(max_epochs=200, log_every_n_steps=1, logger=logger,
                         accelerator="gpu", enable_checkpointing=False,
                         precision=16, auto_lr_find=True, gradient_clip_val=0.5,)
    model = HalfNetPL()
    # Train the model

Thank you.

1 Like

I would check if changing OMP_NUM_THREADS changes the behavior. Additionally, it may be useful to check if e.g., running the model on the same (already transferred to GPU) data over and over again changes the behavior to rule out other potential issues.

Hello eqy,
I tired export OMP_NUM_THREADS=6, doesn’t seem to do anything. I also tried transferring all the data in the dataloader to cuda device in the __init__ function as well, it marginally improved the rate of training (if any), but doesn’t help with my issue.

Could you check your plain PyTorch model (i.e. without the Lightning wrapper) and see if you are still using a single CPU core?

I honestly don’t know how to use a plain pytorch model for training… Guess I will look into it.

You could also try to opposite and check if all Lightning models suffer from this issue in your setup or in case it’s only a specific set of models what might be the common pattern.

Just check with the lightning MNIST example, the issue persists, so I guess all Lightning models will suffer from this issue…

Tried with the pure pytorch mnist example from MNIST Handwritten Digit Recognition in PyTorch - Nextjournal , this one correctly utilise all my cpu cores, so I guess this issue is more on pytorch lightning side.
PS: Issue submitted on Lightning’s Github as Extreme single thread cpu kernel usage while training on GPU · Issue #16737 · Lightning-AI/lightning (github.com)

1 Like

Thanks for sharing the related GitHub issue as I would also be interested in the root cause.

I converted @Yuyao_Huang’s code from Lightning to raw PyTorch and am able to reproduce the observations on CPU usage. See my comment here plus the full code: Extreme single thread cpu kernel usage while training on GPU · Issue #16737 · Lightning-AI/lightning · GitHub

The difference in CPU behavior shows when enabling and disabling the autocast context manager. Not an expert, but I would guess mixed precision on CPU is perhaps not as optimized as on CUDA and exists more for debugging purposes, so perhaps it isn’t so surprising to see this happen?

1 Like

That’s an interesting observation!
Based on your comment:

When using the autocast (mixed precision) context manager, we can see that the CPU usage is mainly concentrated in a single core. When setting use_autocast=False however, the CPU usage is more spread across several cores.

it seems you are using autocast on the CPU and then observe this behavior?
Also, it seems you are “mainly” seeing a single core usage but other cores are still used?

Sorry for the delayed response. Yes @ptrblck, that’s exactly right. That single core heavy behavior isn’t visible when removing the autocast and the load is more spread out across cores.

1 Like

Hi all,
I’m encountering the same issue. How would one disable autocast using only the Lightning API?


@Yuyao_Huang I have the same issue. Have you solved the problem?

In the end, I just endured it and finished my project with it.