Runner类是MMCV的一个
核心组件，它是一个用来管理训练流程的引擎，并且支持用户用少量代码按照它提供的接口定制化修改训练流程。下面博主按照官方Doc的思路再结合自己的理解讲解一下它。
Runner的目的就是给用户提供统一的训练流程管理，并支持弹性、可配置的定制化修改（通过Hook实现），因此其主要特性如下：
EpochBasedRunner和IterBasedRunnner，同时也支持用户实现自定义Runner。 顾名思义，EpochBasedRunner就是以Epoch为基础迭代的Runner，下面我们实现一个简单的例子去演示它的工作流workflow控制原理。
class ToyRunner(nn.Module):
def __init__(self):
super().__init__()
def train(self, data_loader, **kwargs):
print(data_loader)
def val(self, data_loader, **kwargs):
print(data_loader)
def run(self):
# training epochs
max_epochs = 3
curr_epoch = 0
# denotes 2 epochs for training and 1 epoch for validation
workflow = [("train", 2), ("val", 1)]
data_loaders = ["dl_train", "dl_val"]
# the condition to stop training
while curr_epoch < max_epochs:
# workflow
for i, flow in enumerate(workflow):
mode, epochs = flow
epoch_func = getattr(self, mode)
for _ in range(epochs):
if mode == 'train' and curr_epoch >= max_epochs:
break
epoch_func(f'data_loader: {data_loaders[i]}, epoch={curr_epoch}')
if mode == 'train':
# validation doesn't affect curr_epoch
curr_epoch += 1
然后我们运行一下ToyRunner:
runner = ToyRunner()
runner.run()
"""
Output:
data_loader: dl_train, epoch=0
data_loader: dl_train, epoch=1
data_loader: dl_val, epoch=2
data_loader: dl_train, epoch=2
data_loader: dl_val, epoch=3
"""
上面代码逻辑十分简单，博主这里说一下有几点需要注意的：
workflow在这里代表训练2个epochs之后再验证1个epoch，其长度为2需要和data_loaders长度一致。可以理解为这里的workflow有train和val两个flow，因此就需要dl_train和dl_val这两个data_loader提供数据。max_epoch代表的是训练epoch，所以只有当mode为train的时候epoch才会增加接下来我们通过一个简单的例子，按照mmcv的规范去使用一下mmcv提供的Runner类。首先，我们定义一些构建数据集的工具函数：
import platform
import random
from functools import partial
import numpy as np
import torch
from mmcv.parallel import collate
from mmcv.runner import get_dist_info
from mmcv.utils import digit_version
from torch.utils.data import DataLoader, IterableDataset
if platform.system() != 'Windows':
# https://github.com/pytorch/pytorch/issues/973
import resource
rlimit = resource.getrlimit(resource.RLIMIT_NOFILE)
base_soft_limit = rlimit[0]
hard_limit = rlimit[1]
soft_limit = min(max(4096, base_soft_limit), hard_limit)
resource.setrlimit(resource.RLIMIT_NOFILE, (soft_limit, hard_limit))
def build_dataloader(dataset,
samples_per_gpu,
workers_per_gpu,
num_gpus=1,
dist=False,
shuffle=True,
seed=None,
drop_last=False,
pin_memory=True,
persistent_workers=True,
**kwargs):
"""Build PyTorch DataLoader.
In distributed training, each GPU/process has a dataloader.
In non-distributed training, there is only one dataloader for all GPUs.
Args:
dataset (Dataset): A PyTorch dataset.
samples_per_gpu (int): Number of training samples on each GPU, i.e.,
batch size of each GPU.
workers_per_gpu (int): How many subprocesses to use for data loading
for each GPU.
num_gpus (int): Number of GPUs. Only used in non-distributed training.
dist (bool): Distributed training/test or not. Default: True.
shuffle (bool): Whether to shuffle the data at every epoch.
Default: True.
seed (int | None): Seed to be used. Default: None.
drop_last (bool): Whether to drop the last incomplete batch in epoch.
Default: False
pin_memory (bool): Whether to use pin_memory in DataLoader.
Default: True
persistent_workers (bool): If True, the data loader will not shutdown
the worker processes after a dataset has been consumed once.
This allows to maintain the workers Dataset instances alive.
The argument also has effect in PyTorch>=1.7.0.
Default: True
kwargs: any keyword argument to be used to initialize DataLoader
Returns:
DataLoader: A PyTorch dataloader.
"""
rank, world_size = get_dist_info()
if dist and not isinstance(dataset, IterableDataset):
# not support dist for notebook
pass
elif dist:
sampler = None
shuffle = False
batch_size = samples_per_gpu
num_workers = workers_per_gpu
else:
sampler = None
batch_size = num_gpus * samples_per_gpu
num_workers = num_gpus * workers_per_gpu
init_fn = partial(
worker_init_fn, num_workers=num_workers, rank=rank,
seed=seed) if seed is not None else None
if digit_version(torch.__version__) >= digit_version('1.8.0'):
data_loader = DataLoader(
dataset,
batch_size=batch_size,
sampler=sampler,
num_workers=num_workers,
collate_fn=partial(collate, samples_per_gpu=samples_per_gpu),
pin_memory=pin_memory,
shuffle=shuffle,
worker_init_fn=init_fn,
drop_last=drop_last,
persistent_workers=persistent_workers,
**kwargs)
else:
data_loader = DataLoader(
dataset,
batch_size=batch_size,
sampler=sampler,
num_workers=num_workers,
collate_fn=partial(collate, samples_per_gpu=samples_per_gpu),
pin_memory=pin_memory,
shuffle=shuffle,
worker_init_fn=init_fn,
drop_last=drop_last,
**kwargs)
return data_loader
def worker_init_fn(worker_id, num_workers, rank, seed):
"""Worker init func for dataloader.
The seed of each worker equals to num_worker * rank + worker_id + user_seed
Args:
worker_id (int): Worker id.
num_workers (int): Number of workers.
rank (int): The rank of current process.
seed (int): The random seed to use.
"""
worker_seed = num_workers * rank + worker_id + seed
np.random.seed(worker_seed)
random.seed(worker_seed)
torch.manual_seed(worker_seed)
上述的工具函数来自MMSegmentation，我在这里为了运行演示方便做了一些删减。
接下来我们按照Runner的要求编写一个简单的模型：
from mmcv.runner import BaseModule
class ToyModel(BaseModule):
def __init__(self):
super().__init__()
self.backbone = nn.Linear(10, 2)
self.criterion = nn.CrossEntropyLoss()
def forward(self, x):
out = self.backbone(x)
return out
def train_step(self, data_batch, optimizer, **kwargs):
labels, imgs = data_batch
preds = self(imgs)
loss = self.criterion(preds, labels)
log_vars = dict(train_loss=loss.item())
num_samples = len(imgs)
outputs = dict(loss=loss,
preds=preds,
log_vars=log_vars,
num_samples=num_samples)
return outputs
def val_step(self, data_batch, optimizer, **kwargs):
labels, imgs = data_batch
preds = self(imgs)
loss = self.criterion(preds, labels)
log_vars = dict(val_loss=loss.item())
num_samples = len(imgs)
outputs = dict(log_vars=log_vars,
num_samples=num_samples)
return outputs
上述的模型代码有几点需要注意的：
mmcv.runner下的BaseModule，它主要有三个属性/方法：1）init_cfg用来控制模型初始化的配置；2）init_weights参数初始化的函数，记录着参数初始化的信息；3）_params_init_info用来追踪参数初始化信息的defaultdictinit_weights函数执行的时候生成，在所有参数初始化完成之后删除。train_step和val_step两个方法。train_step和val_step的返回结果都是一个字典，其中train_step返回的loss是为了后续在optimizer hook中进行反向传播，而log_vars和num_samples则是log hook输出需要的变量。接下来是一个只有一个类别的简单数据集类：
class ToyDataset(torch.utils.data.Dataset):
def __init__(self, data) -> None:
super().__init__()
self.data = data
def __getitem__(self, idx):
return 0, self.data[idx]
def __len__(self):
return len(self.data)
我们写一段脚本去运行mmcv的Runner并查看效果，下面是初始化参数的定义代码：
from mmcv import ConfigDict
from mmcv.runner import build_optimizer
# initialize
model = ToyModel() # model
cfg = ConfigDict(data=dict(samples_per_gpu=2, workers_per_gpu=1),
workflow=[('train', 1), ('val', 1)],
optimizer=dict(type='SGD',
lr=0.1,
momentum=0.9,
weight_decay=0.0001),
lr_config=dict(policy='step', step=[100, 150]),
log_config=dict(interval=1,
hooks=[
dict(type='TextLoggerHook',
by_epoch=True),
]),
runner=dict(type='EpochBasedRunner', max_epochs=3)) # config
optimizer = build_optimizer(model, cfg.optimizer) # optimizer
ds_train = ToyDataset(torch.rand(5, 10)) # training loader
ds_val = ToyDataset(torch.rand(3, 10)) # vallidation loader
datasets = [ds_train, ds_val] # dataset
# data_loaders
data_loaders = [
build_dataloader(ds, cfg.data.samples_per_gpu, cfg.data.workers_per_gpu)
for ds in datasets
]
然后我们build runner并注册好参数就可以一键训练模型了：
from mmcv.runner import build_runner
from mmcv.utils import get_logger
# get logger
logger = get_logger(name='toyproj', log_file='logger.log')
# initialize runner
runner = build_runner(cfg.runner,
default_args=dict(model=model,
batch_processor=None,
optimizer=optimizer,
work_dir='./',
logger=logger))
# register default hooks necessary for training
runner.register_training_hooks(
# configs of learning rate, it is typically set as:
lr_config=cfg.lr_config,
# configuration of logs
log_config=cfg.log_config)
# start running
runner.run(data_loaders, cfg.workflow)
'''
Output:
2022-11-23 11:55:50,539 - toyproj - INFO - workflow: [('train', 1), ('val', 1)], max: 3 epochs
2022-11-23 11:55:55,541 - toyproj - INFO - Epoch [1][1/3] lr: 1.000e-01, eta: 0:00:39, time: 4.998, data_time: 4.995, memory: 0, train_loss: 0.6041
2022-11-23 11:55:55,548 - toyproj - INFO - Epoch [1][2/3] lr: 1.000e-01, eta: 0:00:17, time: 0.009, data_time: 0.009, memory: 0, train_loss: 0.5542
2022-11-23 11:55:55,552 - toyproj - INFO - Epoch [1][3/3] lr: 1.000e-01, eta: 0:00:10, time: 0.003, data_time: 0.003, memory: 0, train_loss: 0.5444
2022-11-23 11:55:57,674 - toyproj - INFO - Epoch(val) [1][2] val_loss: 0.3617
2022-11-23 11:55:59,746 - toyproj - INFO - Epoch [2][1/3] lr: 1.000e-01, eta: 0:00:08, time: 2.067, data_time: 2.066, memory: 0, train_loss: 0.5542
2022-11-23 11:55:59,752 - toyproj - INFO - Epoch [2][2/3] lr: 1.000e-01, eta: 0:00:05, time: 0.007, data_time: 0.007, memory: 0, train_loss: 0.6041
2022-11-23 11:55:59,756 - toyproj - INFO - Epoch [2][3/3] lr: 1.000e-01, eta: 0:00:03, time: 0.004, data_time: 0.004, memory: 0, train_loss: 0.5444
2022-11-23 11:56:01,918 - toyproj - INFO - Epoch(val) [2][2] val_loss: 0.3617
2022-11-23 11:56:03,991 - toyproj - INFO - Epoch [3][1/3] lr: 1.000e-01, eta: 0:00:02, time: 2.068, data_time: 2.067, memory: 0, train_loss: 0.6041
2022-11-23 11:56:03,998 - toyproj - INFO - Epoch [3][2/3] lr: 1.000e-01, eta: 0:00:01, time: 0.008, data_time: 0.007, memory: 0, train_loss: 0.5897
2022-11-23 11:56:04,002 - toyproj - INFO - Epoch [3][3/3] lr: 1.000e-01, eta: 0:00:00, time: 0.004, data_time: 0.004, memory: 0, train_loss: 0.4735
2022-11-23 11:56:06,181 - toyproj - INFO - Epoch(val) [3][2] val_loss: 0.3617
'''