Tutorial 6: Customize Runtime Settings

Customize optimization settings

Customize optimizer supported by Pytorch

We already support to use all the optimizers implemented by PyTorch, and the only modification is to change the optimizer field of config files. For example, if you want to use ADAM (note that the performance could drop a lot), the modification could be as the following.

optimizer = dict(type='Adam', lr=0.0003, weight_decay=0.0001)

To modify the learning rate of the model, the users only need to modify the lr in the config of optimizer. The users can directly set arguments following the API doc of PyTorch.

Customize self-implemented optimizer

1. Define a new optimizer

A customized optimizer could be defined as following.

Assume you want to add a optimizer named MyOptimizer, which has arguments a, b, and c. You need to create a new directory named mmseg/core/optimizer. And then implement the new optimizer in a file, e.g., in mmseg/core/optimizer/my_optimizer.py:

from .registry import OPTIMIZERS
from torch.optim import Optimizer


@OPTIMIZERS.register_module()
class MyOptimizer(Optimizer):

    def __init__(self, a, b, c)

2. Add the optimizer to registry

To find the above module defined above, this module should be imported into the main namespace at first. There are two options to achieve it.

Modify mmseg/core/optimizer/__init__.py to import it.

The newly defined module should be imported in mmseg/core/optimizer/__init__.py so that the registry will find the new module and add it:

from .my_optimizer import MyOptimizer

Use custom_imports in the config to manually import it

custom_imports = dict(imports=['mmseg.core.optimizer.my_optimizer'], allow_failed_imports=False)

The module mmseg.core.optimizer.my_optimizer will be imported at the beginning of the program and the class MyOptimizer is then automatically registered. Note that only the package containing the class MyOptimizer should be imported. mmseg.core.optimizer.my_optimizer.MyOptimizer cannot be imported directly.

Actually users can use a totally different file directory structure using this importing method, as long as the module root can be located in PYTHONPATH.

3. Specify the optimizer in the config file

Then you can use MyOptimizer in optimizer field of config files. In the configs, the optimizers are defined by the field optimizer like the following:

optimizer = dict(type='SGD', lr=0.02, momentum=0.9, weight_decay=0.0001)

To use your own optimizer, the field can be changed to

optimizer = dict(type='MyOptimizer', a=a_value, b=b_value, c=c_value)

Customize optimizer constructor

Some models may have some parameter-specific settings for optimization, e.g. weight decay for BatchNorm layers. The users can do those fine-grained parameter tuning through customizing optimizer constructor.

from mmcv.utils import build_from_cfg

from mmcv.runner.optimizer import OPTIMIZER_BUILDERS, OPTIMIZERS
from mmseg.utils import get_root_logger
from .my_optimizer import MyOptimizer


@OPTIMIZER_BUILDERS.register_module()
class MyOptimizerConstructor(object):

    def __init__(self, optimizer_cfg, paramwise_cfg=None):

    def __call__(self, model):

        return my_optimizer

The default optimizer constructor is implemented here, which could also serve as a template for new optimizer constructor.

Additional settings

Tricks not implemented by the optimizer should be implemented through optimizer constructor (e.g., set parameter-wise learning rates) or hooks. We list some common settings that could stabilize the training or accelerate the training. Feel free to create PR, issue for more settings.

Use gradient clip to stabilize training: Some models need gradient clip to clip the gradients to stabilize the training process. An example is as below:
```
optimizer_config = dict(
    _delete_=True, grad_clip=dict(max_norm=35, norm_type=2))
```
If your config inherits the base config which already sets the optimizer_config, you might need _delete_=True to overide the unnecessary settings. See the config documenetation for more details.
Use momentum schedule to accelerate model convergence: We support momentum scheduler to modify model's momentum according to learning rate, which could make the model converge in a faster way. Momentum scheduler is usually used with LR scheduler, for example, the following config is used in 3D detection to accelerate convergence. For more details, please refer to the implementation of CyclicLrUpdater and CyclicMomentumUpdater.
```
lr_config = dict(
    policy='cyclic',
    target_ratio=(10, 1e-4),
    cyclic_times=1,
    step_ratio_up=0.4,
)
momentum_config = dict(
    policy='cyclic',
    target_ratio=(0.85 / 0.95, 1),
    cyclic_times=1,
    step_ratio_up=0.4,
)
```

Customize training schedules

By default we use step learning rate with 40k/80k schedule, this calls PolyLrUpdaterHook in MMCV. We support many other learning rate schedule here, such as CosineAnnealing and Poly schedule. Here are some examples

Step schedule:

lr_config = dict(policy='step', step=[9, 10])

ConsineAnnealing schedule:

lr_config = dict(
    policy='CosineAnnealing',
    warmup='linear',
    warmup_iters=1000,
    warmup_ratio=1.0 / 10,
    min_lr_ratio=1e-5)

Customize workflow

Workflow is a list of (phase, epochs) to specify the running order and epochs. By default it is set to be

workflow = [('train', 1)]

which means running 1 epoch for training. Sometimes user may want to check some metrics (e.g. loss, accuracy) about the model on the validate set. In such case, we can set the workflow as

[('train', 1), ('val', 1)]

so that 1 epoch for training and 1 epoch for validation will be run iteratively.

Note:

The parameters of model will not be updated during val epoch.
Keyword total_epochs in the config only controls the number of training epochs and will not affect the validation workflow.
Workflows [('train', 1), ('val', 1)] and [('train', 1)] will not change the behavior of EvalHook because EvalHook is called by after_train_epoch and validation workflow only affect hooks that are called through after_val_epoch. Therefore, the only difference between [('train', 1), ('val', 1)] and [('train', 1)] is that the runner will calculate losses on validation set after each training epoch.

Customize hooks

Use hooks implemented in MMCV

If the hook is already implemented in MMCV, you can directly modify the config to use the hook as below

custom_hooks = [
    dict(type='MyHook', a=a_value, b=b_value, priority='NORMAL')
]

Modify default runtime hooks

There are some common hooks that are not registerd through custom_hooks, they are

log_config
checkpoint_config
evaluation
lr_config
optimizer_config
momentum_config

In those hooks, only the logger hook has the VERY_LOW priority, others' priority are NORMAL. The above-mentioned tutorials already covers how to modify optimizer_config, momentum_config, and lr_config. Here we reveals how what we can do with log_config, checkpoint_config, and evaluation.

Checkpoint config

The MMCV runner will use checkpoint_config to initialize CheckpointHook.

checkpoint_config = dict(interval=1)

The users could set max_keep_ckpts to only save only small number of checkpoints or decide whether to store state dict of optimizer by save_optimizer. More details of the arguments are here

Log config

The log_config wraps multiple logger hooks and enables to set intervals. Now MMCV supports WandbLoggerHook, MlflowLoggerHook, and TensorboardLoggerHook. The detail usages can be found in the doc.

log_config = dict(
    interval=50,
    hooks=[
        dict(type='TextLoggerHook'),
        dict(type='TensorboardLoggerHook')
    ])

Evaluation config

The config of evaluation will be used to initialize the EvalHook. Except the key interval, other arguments such as metric will be passed to the dataset.evaluate()

evaluation = dict(interval=1, metric='mIoU')