# 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](mmengine.dataset.BaseDataset) support this feature, and the usage of BaseDataset can be found in the [BaseDataset](../advanced_tutorials/basedataset.md).

Take MMPretrain as an example (Refer to the [documentation](https://mmpretrain.readthedocs.io/en/latest/get_started.html) for installing MMPretrain).

Launch training

```bash
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](https://github.com/open-mmlab/mmpretrain/blob/main/configs/_base_/datasets/cifar100_bs16.py).

```python
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

```bash
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
```

## 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](../get_started/15_minutes.md) as an example:

```python
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:

```python
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:

```bash
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`:

```python
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.

```python
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.
```