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Debug Tricks

Set the Dataset’s Length

During the process of debugging code, sometimes it is necessary to train for several epochs, such as debugging the validation process or checking whether the checkpoint saving meets expectations. However, if the dataset is too large, it may take a long time to complete one epoch, in which case the length of the dataset can be set. Note that only datasets inherited from BaseDataset support this feature, and the usage of BaseDataset can be found in the BaseDataset.

Take MMPretrain as an example (Refer to the documentation for installing MMPretrain).

Launch training

python tools/train.py configs/resnet/resnet18_8xb16_cifar10.py

Here is part of the training log, where 3125 represents the number of iterations to be performed.

02/20 14:43:11 - mmengine - INFO - Epoch(train)   [1][ 100/3125]  lr: 1.0000e-01  eta: 6:12:01  time: 0.0149  data_time: 0.0003  memory: 214  loss: 2.0611
02/20 14:43:13 - mmengine - INFO - Epoch(train)   [1][ 200/3125]  lr: 1.0000e-01  eta: 4:23:08  time: 0.0154  data_time: 0.0003  memory: 214  loss: 2.0963
02/20 14:43:14 - mmengine - INFO - Epoch(train)   [1][ 300/3125]  lr: 1.0000e-01  eta: 3:46:27  time: 0.0146  data_time: 0.0003  memory: 214  loss: 1.9858

Turn off the training and set indices as 5000 in the dataset field in configs/base/datasets/cifar10_bs16.py.

train_dataloader = dict(
    batch_size=16,
    num_workers=2,
    dataset=dict(
        type=dataset_type,
        data_prefix='data/cifar10',
        test_mode=False,
        indices=5000,  # set indices=5000,represent every epoch only iterator 5000 samples
        pipeline=train_pipeline),
    sampler=dict(type='DefaultSampler', shuffle=True),
)

Launch training again

python tools/train.py configs/resnet/resnet18_8xb16_cifar10.py

As we can see, the number of iterations has changed to 313. Compared to before, this can complete an epoch faster.

02/20 14:44:58 - mmengine - INFO - Epoch(train)   [1][100/313]  lr: 1.0000e-01  eta: 0:31:09  time: 0.0154  data_time: 0.0004  memory: 214  loss: 2.1852
02/20 14:44:59 - mmengine - INFO - Epoch(train)   [1][200/313]  lr: 1.0000e-01  eta: 0:23:18  time: 0.0143  data_time: 0.0002  memory: 214  loss: 2.0424
02/20 14:45:01 - mmengine - INFO - Epoch(train)   [1][300/313]  lr: 1.0000e-01  eta: 0:20:39  time: 0.0143  data_time: 0.0003  memory: 214  loss: 1.814

Training for a fixed number of iterations (epoch-based training)

During the process of debugging code, sometimes it is necessary to train for several epochs, such as debugging the validation process or checking whether the checkpoint saving meets expectations. However, if the dataset is too large, it may take a long time to complete one epoch. In such cases, you can configure the num_batch_per_epoch parameter of the dataloader.

Note

The num_batch_per_epoch parameter is not a native parameter of PyTorch dataloaders but an additional parameter added by MMEngine to achieve this functionality.

Let’s take the model defined in 5 minutes to get started with MMEngine as an example. By setting num_batch_per_epoch=5 in both train_dataloader and val_dataloader, you can ensure that one epoch consists of only 5 iterations.

train_dataloader = dict(
    batch_size=32,
    dataset=train_set,
    sampler=dict(type='DefaultSampler', shuffle=True),
    collate_fn=dict(type='default_collate'),
    num_batch_per_epoch=5)
val_dataloader = dict(
    batch_size=32,
    dataset=valid_set,
    sampler=dict(type='DefaultSampler', shuffle=False),
    collate_fn=dict(type='default_collate'),
    num_batch_per_epoch=5)
runner = Runner(
    model=MMResNet50(),
    work_dir='./work_dir',
    train_dataloader=train_dataloader,
    optim_wrapper=dict(optimizer=dict(type=SGD, lr=0.001, momentum=0.9)),
    train_cfg=dict(by_epoch=True, max_epochs=2, val_interval=1),
    val_dataloader=val_dataloader,
    val_cfg=dict(),
    val_evaluator=dict(type=Accuracy),
    launcher=args.launcher,
)
runner.train()

As we can see, the number of iterations has been reduced to 5. Compared to the original setting, this allows you to complete one epoch more quickly.

08/18 20:27:22 - mmengine - INFO - Epoch(train) [1][5/5]  lr: 1.0000e-03  eta: 0:00:02  time: 0.4566  data_time: 0.0074  memory: 477  loss: 6.7576
08/18 20:27:22 - mmengine - INFO - Saving checkpoint at 1 epochs
08/18 20:27:22 - mmengine - WARNING - `save_param_scheduler` is True but `self.param_schedulers` is None, so skip saving parameter schedulers
08/18 20:27:23 - mmengine - INFO - Epoch(val) [1][5/5]    accuracy: 7.5000  data_time: 0.0044  time: 0.0146
08/18 20:27:23 - mmengine - INFO - Exp name: 20230818_202715
08/18 20:27:23 - mmengine - INFO - Epoch(train) [2][5/5]  lr: 1.0000e-03  eta: 0:00:00  time: 0.2501  data_time: 0.0077  memory: 477  loss: 5.3044
08/18 20:27:23 - mmengine - INFO - Saving checkpoint at 2 epochs
08/18 20:27:24 - mmengine - INFO - Epoch(val) [2][5/5]    accuracy: 12.5000  data_time: 0.0058  time: 0.0175

Find Unused Parameters

When using multiple GPUs training, if model’s parameters are involved in forward computation but are not used in producing loss, the program may throw the following error:

RuntimeError: Expected to have finished reduction in the prior iteration before starting a new one. This error indicates that your module has parameters that were not used in producing loss. You can enable unused parameter detection by passing the keyword argument `find_unused_parameters=True` to `torch.nn.parallel.DistributedDataParallel`, and by
making sure all `forward` function outputs participate in calculating loss.

Let’s take the model defined in 5 minutes to get started with MMEngine as an example:

class MMResNet50(BaseModel):
    def __init__(self):
        super().__init__()
        self.resnet = torchvision.models.resnet50()

    def forward(self, imgs, labels, mode):
        x = self.resnet(imgs)
        if mode == 'loss':
            return {'loss': F.cross_entropy(x, labels)}
        elif mode == 'predict':
            return x, labels

Modify it to:

class MMResNet50(BaseModel):

    def __init__(self):
        super().__init__()
        self.resnet = torchvision.models.resnet50()
        self.param = nn.Parameter(torch.ones(1))

    def forward(self, imgs, labels, mode):
        x = self.resnet(imgs)
        # self.param is involved in the forward computation,
        # but y is not involved in the loss calculation
        y = self.param + x
        if mode == 'loss':
            return {'loss': F.cross_entropy(x, labels)}
        elif mode == 'predict':
            return x, labels

Start training with two GPUs:

torchrun --nproc-per-node 2 examples/distributed_training.py --launcher pytorch

The program will throw the error mentioned above.

This issue can be resolved by setting find_unused_parameters=True:

cfg = dict(
    model_wrapper_cfg=dict(
        type='MMDistributedDataParallel', find_unused_parameters=True)
)
runner = Runner(
    model=MMResNet50(),
    work_dir='./work_dir',
    train_dataloader=train_dataloader,
    optim_wrapper=dict(optimizer=dict(type=SGD, lr=0.001, momentum=0.9)),
    train_cfg=dict(by_epoch=True, max_epochs=2, val_interval=1),
    val_dataloader=val_dataloader,
    val_cfg=dict(),
    val_evaluator=dict(type=Accuracy),
    launcher=args.launcher,
    cfg=cfg,
)
runner.train()

Restart training, and you can see that the program trains normally and prints logs.

However, setting find_unused_parameters=True will slow down the program, so we want to find these parameters and analyze why they did not participate in the loss calculation.

This can be done by setting detect_anomalous_params=True to print the unused parameters.

cfg = dict(
    model_wrapper_cfg=dict(
        type='MMDistributedDataParallel',
        find_unused_parameters=True,
        detect_anomalous_params=True),
)

Restart training, and you can see that the log prints the parameters not involved in the loss calculation.

08/03 15:04:42 - mmengine - ERROR - mmengine/logging/logger.py - print_log - 323 - module.param with shape torch.Size([1]) is not in the computational graph

Once these parameters are found, we can analyze why they did not participate in the loss calculation.

Important

find_unused_parameters=True and detect_anomalous_params=True should only be set when debugging.

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