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Migration from MMGeneration 0.x

In 1.x version of MMGeneration, we redesign distributed training and mixed precision training, while the usage of optimizer and data flow is based on MMEngine. This document will help users of the 0.x version to quickly migrate to the newest version.

New dependencies

MMGeneration 1.x depends on some new packages, you can prepare a new clean environment and install again according to the install tutorial. Or install the below packages manually.

  1. MMEngine: MMEngine is the core the OpenMMLab 2.0 architecture, and we splited many compentents unrelated to computer vision from MMCV to MMEngine.

  2. MMCV: The computer vision package of OpenMMLab. This is not a new dependency, but you need to upgrade it to above 2.0.0rc0 version.

  3. rich: A terminal formatting package, and we use it to beautify some outputs in the terminal.

1. Runner and schedule

In 0.x version, MMGeneration use total_iters fields to control the total training iteration and use DynamicIterBasedRunner to handle the training process. In 1.x version, we use Runner and Loops provided by MMEngine and use train_cfg.max_iters field to control the total training iteration and use train_cfg.val_interval to control the evaluation interval.

To evaluate and test the model correctly, we need to set specific loop in val_cfg and test_cfg.

Static Model in 0.x Version Static Model in 1.x Version 
total_iters = 1000000

runner = dict(
    type='DynamicIterBasedRunner',
    is_dynamic_ddp=False,
    pass_training_status=True)

train_cfg = dict(
    by_epoch=False,  # use iteration based training
    max_iters=1000000,  # max training iteration
    val_begin=1,
    val_interval=10000)  # evaluation interval
val_cfg = dict(type='GenValLoop')  # specific loop in validation
test_cfg = dict(type='GenTestLoop')  # specific loop in testing

2. Evaluation and testing setting

The evaluation field is splited to val_evaluator and test_evaluator. And it won’t support interval and save_best arguments. The interval is moved to train_cfg.val_interval, see the schedule settings and the save_best is moved to default_hooks.checkpoint.save_best.

0.x Version 1.x Version 
evaluation = dict(
    type='GenerativeEvalHook',
    interval=10000,
    metrics=[
        dict(
            type='FID',
            num_images=50000,
            bgr2rgb=True,
            inception_args=dict(type='StyleGAN')),
        dict(type='IS', num_images=50000)
    ],
    best_metric=['fid', 'is'],
    sample_kwargs=dict(sample_model='ema'))

val_evaluator = dict(
    type='GenEvaluator',
    metrics=[
        dict(
            type='FID',
            prefix='FID-Full-50k',
            fake_nums=50000,
            inception_style='StyleGAN',
            sample_model='orig')
        dict(
            type='IS',
            prefix='IS-50k',
            fake_nums=50000)])
# set best config
default_hooks = dict(
    checkpoint=dict(
        type='CheckpointHook',
        interval=10000,
        by_epoch=False,
        less_keys=['FID-Full-50k/fid'],
        greater_keys=['IS-50k/is'],
        save_optimizer=True,
        save_best=['FID-Full-50k/fid', 'IS-50k/is'],
        rule=['less', 'greater']))
test_evaluator = val_evaluator

3. Distributed Training setting

In 0.x version, MMGeneration uses DDPWrapper and DynamicRunner to train static and dynamic model (e.g., PGGAN and StyleGANv2) respectively. In 1.x version, we use MMSeparateDistributedDataParallel provided by MMEngine to implement distributed training.

The configuration differences are shown below:

Static Model in 0.x Version Static Model in 1.x Version 
# Use DDPWrapper
use_ddp_wrapper = True
find_unused_parameters = False

runner = dict(
    type='DynamicIterBasedRunner',
    is_dynamic_ddp=False)

model_wrapper_cfg = dict(
    type='MMSeparateDistributedDataParallel',
    broadcast_buffers=False,
    find_unused_parameters=False)
Dynamic Model in 0.x Version Dynamic Model in 1.x Version 
use_ddp_wrapper = False
find_unused_parameters = False

# Use DynamicRunner
runner = dict(
    type='DynamicIterBasedRunner',
    is_dynamic_ddp=True)

model_wrapper_cfg = dict(
    type='MMSeparateDistributedDataParallel',
    broadcast_buffers=False,
    find_unused_parameters=True) # set `find_unused_parameters` for dynamic models

4. Optimizer

In version 0.x, MMGeneration uses PyTorch’s native Optimizer, which only provides general parameter optimization. In version 1.x, we use OptimizerWrapper provided by MMEngine.

Compared to PyTorch’s Optimizer, OptimizerWrapper supports the following features:

  • OptimizerWrapper.update_params implement zero_grad, backward and step in a single function.

  • Support gradient accumulation automatically.

  • Provide a context manager named OptimizerWrapper.optim_context to warp the forward process. optim_context can automatically call torch.no_sync according to current number of updating iteration. In AMP (auto mixed precision) training, autocast is called in optim_context as well.

For GAN models, generator and discriminator use different optimizer and training schedule. To ensure that the GAN model’s function signature of train_step is consistent with other models, we use OptimWrapperDict, inherited from OptimizerWrapper, to wrap the optimizer of the generator and discriminator. To automate this process MMGeneration implement GenOptimWrapperContructor. And you should specify this constructor in your config is you want to train GAN model.

The config for the 0.x and 1.x versions are shown below:

0.x Version 1.x Version 
optimizer = dict(
    generator=dict(type='Adam', lr=0.0001, betas=(0.0, 0.999), eps=1e-6),
    discriminator=dict(type='Adam', lr=0.0004, betas=(0.0, 0.999), eps=1e-6))

optim_wrapper = dict(
    # Use constructor implemented by MMGeneration
    constructor='GenOptimWrapperConstructor',
    generator=dict(optimizer=dict(type='Adam', lr=0.0002, betas=(0.0, 0.999), eps=1e-6)),
    discriminator=dict(
        optimizer=dict(type='Adam', lr=0.0004, betas=(0.0, 0.999), eps=1e-6)))

Note that, in the 1.x, MMGeneration uses OptimWrapper to realize gradient accumulation. This make the config of discriminator_steps (training trick for updating the generator once after multiple updates of the discriminator) and gradient accumulation different between 0.x and 1.x version.

  • In 0.x version, we use disc_steps, gen_steps and batch_accumulation_steps in configs. disc_steps and batch_accumulation_steps are counted by the number of calls of train_step (is also the number of data reads from the dataloader). Therefore the number of consecutive updates of the discriminator is disc_steps // batch_accumulation_steps. And for generators, gen_steps is the number of times the generator actually updates continuously.

  • In 1.x version, we use discriminator_steps, generator_steps and accumulative_counts in configs. discriminator_steps and generator_steps are the number of consecutive updates to itself before updating other modules.

Take config of BigGAN-128 as example.

0.x Version 1.x Version 
model = dict(
    type='BasiccGAN',
    generator=dict(
        type='BigGANGenerator',
        output_scale=128,
        noise_size=120,
        num_classes=1000,
        base_channels=96,
        shared_dim=128,
        with_shared_embedding=True,
        sn_eps=1e-6,
        init_type='ortho',
        act_cfg=dict(type='ReLU', inplace=True),
        split_noise=True,
        auto_sync_bn=False),
    discriminator=dict(
        type='BigGANDiscriminator',
        input_scale=128,
        num_classes=1000,
        base_channels=96,
        sn_eps=1e-6,
        init_type='ortho',
        act_cfg=dict(type='ReLU', inplace=True),
        with_spectral_norm=True),
    gan_loss=dict(type='GANLoss', gan_type='hinge'))

# continuous update discriminator for `disc_steps // batch_accumulation_steps = 8 // 8 = 1` times
# continuous update generator for `gen_steps = 1` times
# generators and discriminators perform `batch_accumulation_steps = 8` times gradient accumulations before each update
train_cfg = dict(
    disc_steps=8, gen_steps=1, batch_accumulation_steps=8, use_ema=True)

model = dict(
    type='BigGAN',
    num_classes=1000,
    data_preprocessor=dict(type='GANDataPreprocessor'),
    generator=dict(
        type='BigGANGenerator',
        output_scale=128,
        noise_size=120,
        num_classes=1000,
        base_channels=96,
        shared_dim=128,
        with_shared_embedding=True,
        sn_eps=1e-6,
        init_type='ortho',
        act_cfg=dict(type='ReLU', inplace=True),
        split_noise=True,
        auto_sync_bn=False),
    discriminator=dict(
        type='BigGANDiscriminator',
        input_scale=128,
        num_classes=1000,
        base_channels=96,
        sn_eps=1e-6,
        init_type='ortho',
        act_cfg=dict(type='ReLU', inplace=True),
        with_spectral_norm=True),
    # continuous update discriminator for `discriminator_steps = 1` times
    # continuous update generator for `generator_steps = 1` times
    generator_steps=1,
    discriminator_steps=1)

optim_wrapper = dict(
    constructor='GenOptimWrapperConstructor',
    generator=dict(
        # generator perform `accumulative_counts = 8` times gradient accumulations before each update
        accumulative_counts=8,
        optimizer=dict(type='Adam', lr=0.0001, betas=(0.0, 0.999), eps=1e-6)),
    discriminator=dict(
        # discriminator perform `accumulative_counts = 8` times gradient accumulations before each update
        accumulative_counts=8,
        optimizer=dict(type='Adam', lr=0.0004, betas=(0.0, 0.999), eps=1e-6)))

5. Learining rate schedule

In 0.x version, MMGeneration use lr_config field to define the learning reate scheduler. In 1.x version, we use param_scheduler to replace it.

0.x Version 1.x Version 
lr_config = dict(
    policy='Linear',
    by_epoch=False,
    target_lr=0,
    start=135000,
    interval=1350)

param_scheduler = dict(
    type='LinearLrInterval',
    interval=1350,
    by_epoch=False,
    start_factor=0.0002,
    end_factor=0,
    begin=135000,
    end=270000)

6. Visualization setting

In 0.x, MMGeneration use MMGenVisualizationHook and VisualizeUnconditionalSamples to visualization generating results in training process. In 1.x version, we unify the function of those hooks into GenVisualizationHook. Additionally, follow the design of MMEngine, we implement GenVisualizer and a group of VisBackend to draw and save the visualization results.

0.x version 1.x Version 
custom_hooks = [
    dict(
        type='VisualizeUnconditionalSamples',
        output_dir='training_samples',
        interval=1000)
]

custom_hooks = [
    dict(
        type='GenVisualizationHook',
        interval=5000,
        fixed_input=True,
        vis_kwargs_list=dict(type='GAN', name='fake_img'))
]
vis_backends = [dict(type='GenVisBackend')]
visualizer = dict(type='GenVisualizer', vis_backends=vis_backends)

To learn more about the visualization function, please refers to this tutorial.

7. AMP (auto mixed precision) training

In 0.x, MMGeneration do not support AMP training for the entire forward process. Instead, users must use auto_fp16 decorator to warp the specific submodule and convert the parameter of submodule to fp16. This allows for fine-grained control of the model parameters, but is more cumbersome to use. In addition, users need to handle operations such as scaling of the loss function during the training process by themselves.

In 1.x version, MMGeneration use AmpOptimWrapper provided by MMEngine. In AmpOptimWrapper.update_params, gradient scaling and GradScaler updating is automatically performed. And in optim_context context manager, auto_cast is applied to the entire forward process.

Specifically, the difference between the 0.x and 1.x is as follows:

0.x version 1.x Version 
# config
runner = dict(fp16_loss_scaler=dict(init_scale=512))

# code
import torch.nn as nn
from mmgen.models.builder import build_model
from mmgen.core.runners.fp16_utils import auto_fp16


class DemoModule(nn.Module):
    def __init__(self, cfg):
        self.net = build_model(cfg)

    @auto_fp16
    def forward(self, x):
        return self.net(x)

class DemoModel(nn.Module):

    def __init__(self, cfg):
        super().__init__(self)
        self.demo_network = DemoModule(cfg)

    def train_step(self,
                   data_batch,
                   optimizer,
                   ddp_reducer=None,
                   loss_scaler=None,
                   use_apex_amp=False,
                   running_status=None):
        # get data from data_batch
        inputs = data_batch['img']
        output = self.demo_network(inputs)

        optimizer.zero_grad()
        loss, log_vars = self.get_loss(data_dict_)

        if ddp_reducer is not None:
            ddp_reducer.prepare_for_backward(_find_tensors(loss_disc))

        if loss_scaler:
            # add support for fp16
            loss_scaler.scale(loss_disc).backward()
        elif use_apex_amp:
            from apex import amp
            with amp.scale_loss(loss_disc, optimizer,
                    loss_id=0) as scaled_loss_disc:
                scaled_loss_disc.backward()
        else:
            loss_disc.backward()

        if loss_scaler:
            loss_scaler.unscale_(optimizer)
            loss_scaler.step(optimizer)
        else:
            optimizer.step()

# config
optim_wrapper = dict(
    constructor='GenOptimWrapperConstructor',
    generator=dict(
        accumulative_counts=8,
        optimizer=dict(type='Adam', lr=0.0001, betas=(0.0, 0.999), eps=1e-06),
        type='AmpOptimWrapper',  # use amp wrapper
        loss_scale='dynamic'),
    discriminator=dict(
        accumulative_counts=8,
        optimizer=dict(type='Adam', lr=0.0004, betas=(0.0, 0.999), eps=1e-06),
        type='AmpOptimWrapper',  # use amp wrapper
        loss_scale='dynamic'))

# code
import torch.nn as nn
from mmgen.registry import MODULES
from mmengine.model import BaseModel


class DemoModule(nn.Module):
    def __init__(self, cfg):
        self.net = MODULES.build(cfg)

    def forward(self, x):
        return self.net(x)

class DemoModel(BaseModel):
    def __init__(self, cfg):
        super().__init__(self)
        self.demo_network = DemoModule(cfg)

    def train_step(self, data, optim_wrapper):
        # get data from data_batch
        data = self.data_preprocessor(data, True)
        inputs = data['inputs']

        with optim_wrapper.optim_context(self.discriminator):
            output = self.demo_network(inputs)
        loss_dict = self.get_loss(output)
        # use parse_loss provide by `BaseModel`
        loss, log_vars = self.parse_loss(loss_dict)
        optimizer_wrapper.update_params(loss)

        return log_vars

To avoid user modifications to the configuration file, MMGeneration provides the --amp option in train.py, which allows the user to start AMP training without modifying the configuration file. Users can start AMP training by following command:

bash tools/dist_train.sh CONFIG GPUS --amp

# for slurm users
bash tools/slurm_train.sh PARTITION JOB_NAME CONFIG WORK_DIR --amp

8. Data settings

MMGeneration redesign data flow and data transforms pipelien based on MMCV 2.x and MMEngine.

Changes in data:

  1. The original data field is splited to train_dataloader, val_dataloader and test_dataloader. This allows us to configure them in fine-grained. For example, you can specify different sampler and batch size during training and test.

  2. The samples_per_gpu is renamed to batch_size.

  3. The workers_per_gpu is renamed to num_workers.

0.x Version 1.x Version 
data = dict(
    samples_per_gpu=None,
    workers_per_gpu=4,
    train=dict(...),
    val=dict(...),
    test=dict(...))

# `batch_size` and `data_root` need to be set.
train_dataloader = dict(
    batch_size=4,
    num_workers=8,
    persistent_workers=True,
    sampler=dict(type='InfiniteSampler', shuffle=True),
    dataset=dict(...))

val_dataloader = dict(
    batch_size=4,
    num_workers=8,
    dataset=dict(...),
    sampler=dict(type='DefaultSampler', shuffle=False),
    persistent_workers=True)

Changes in pipeline

  1. Normalization, color space transforms are no longer performed in transforms pipelines, but converted to data_preprocessor.

  2. Data is packed to GenDataSample by PackGenInputs in the last step of transforms pipeline. To know more about datasample please refers to this tutorial.

Take config for FFHQ-Flip dataset as example:

0.x Version 1.x Version 
dataset_type = 'UnconditionalImageDataset'

train_pipeline = [
    dict(
        type='LoadImageFromFile',
        key='real_img',
        io_backend='disk',
    ),
    dict(type='Flip', keys=['real_img'], direction='horizontal'),
    dict(
        type='Normalize',
        keys=['real_img'],
        mean=[127.5] * 3,
        std=[127.5] * 3,
        to_rgb=False),
    dict(type='ImageToTensor', keys=['real_img']),
    dict(type='Collect', keys=['real_img'], meta_keys=['real_img_path'])
]

val_pipeline = [
    dict(
        type='LoadImageFromFile',
        key='real_img',
        io_backend='disk',
    ),
    dict(
        type='Normalize',
        keys=['real_img'],
        mean=[127.5] * 3,
        std=[127.5] * 3,
        to_rgb=True),
    dict(type='ImageToTensor', keys=['real_img']),
    dict(type='Collect', keys=['real_img'], meta_keys=['real_img_path'])
]

dataset_type = 'UnconditionalImageDataset'

train_pipeline = [
    dict(type='LoadImageFromFile', key='img'),
    dict(type='Flip', keys=['img'], direction='horizontal'),
    dict(type='PackGenInputs', keys=['img'], meta_keys=['img_path'])
]

val_pipeline = [
    dict(type='LoadImageFromFile', key='img'),
    dict(type='PackGenInputs', keys=['img'], meta_keys=['img_path'])
]
data_preprocessor = dict(
    mean=[127.5, 127.5, 127.5], std=[127.5, 127.5, 127.5], bgr_to_rgb=False)

9. Runtime setting

Changes in checkpoint_config and log_config:

The checkpoint_config are moved to default_hooks.checkpoint and the log_config are moved to default_hooks.logger. And we move many hooks settings from the script code to the default_hooks field in the runtime configuration.

default_hooks = dict(
    # record time of every iteration.
    timer=dict(type='GenIterTimerHook'),
    # print log every 100 iterations.
    logger=dict(type='LoggerHook', interval=100, log_metric_by_epoch=False),
    # save checkpoint per 10000 iterations
    checkpoint=dict(
        type='CheckpointHook',
        interval=10000,
        by_epoch=False,
        less_keys=['FID-Full-50k/fid'],
        greater_keys=['IS-50k/is'],
        save_optimizer=True))

In addition, we splited the original logger to logger and visualizer. The logger is used to record information and the visualizer is used to show the logger in different backends, like terminal, TensorBoard and Wandb.

Original 
log_config = dict(
    interval=100,
    hooks=[
        dict(type='TextLoggerHook'),
    ])
New 
default_hooks = dict(
    ...
    logger=dict(type='LoggerHook', interval=100),
)

Changes in load_from and resume_from:

  • The resume_from is removed. And we use resume and load_from to replace it.

    • If resume=True and load_from is not None, resume training from the checkpoint in load_from.

    • If resume=True and load_from is None, try to resume from the latest checkpoint in the work directory.

    • If resume=False and load_from is not None, only load the checkpoint, not resume training.

    • If resume=False and load_from is None, do not load nor resume.

Changes in dist_params: The dist_params field is a sub field of env_cfg now. And there are some new configurations in the env_cfg.

env_cfg = dict(
    # whether to enable cudnn benchmark
    cudnn_benchmark=False,
    # set multi process parameters
    mp_cfg=dict(mp_start_method='fork', opencv_num_threads=0),
    # set distributed parameters
    dist_cfg=dict(backend='nccl'))

Changes in workflow: workflow related functionalities are removed.

New field default_scope: The start point to search module for all registries. The default_scope in MMGeneration is mmgen. See the registry tutorial for more details.

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