[ pytorch ] ——基本使用：(3) finetune冻结层操作 + 学习率超参数设置
1、冻结层不参与训练方法：
######### 模型定义 #########
class MyModel(nn.Module):
def __init__(self, feat_dim): # input the dim of output fea-map of Resnet:
super(MyModel, self).__init__()
BackBone = models.resnet50(pretrained=True)
add_block = []
add_block += [nn.Linear(2048, 512)]
add_block += [nn.LeakyReLU(inplace=True)]
add_block = nn.Sequential(*add_block)
add_block.apply(weights_init_xavier)
self.BackBone = BackBone
self.add_block = add_block
def forward(self, input): # input is 2048!
x = self.BackBone(input)
x = self.add_block(x)
return x
##############################
# 模型准备
model = MyModel()
# 优化、正则项、权重设置与冻结层
for param in model.parameters():
param.requires_grad = False
for param in model.add_block.parameters():
param.requires_grad = True
optimizer = optim.SGD(
filter(lambda p: p.requires_grad, model.parameters()), # 记住一定要加上filter()，不然会报错
lr=0.01,
weight_decay=1e-5, momentum=0.9, nesterov=True)
ignored_params = list(map(id, model.add_block.parameters()))
base_params = filter(lambda p: id(p) not in ignored_params, model.parameters())
2、各层采用不同学习率方法
######### 模型定义 #########
class MyModel(nn.Module):
def __init__(self, feat_dim): # input the dim of output fea-map of Resnet:
super(MyModel, self).__init__()
BackBone = models.resnet50(pretrained=True)
add_block = []
add_block += [nn.Linear(2048, 512)]
add_block += [nn.LeakyReLU(inplace=True)]
add_block = nn.Sequential(*add_block)
add_block.apply(weights_init_xavier)
self.BackBone = BackBone
self.add_block = add_block
def forward(self, input): # input is 2048!
x = self.BackBone(input)
x = self.add_block(x)
return x
##############################
# 模型准备
model = MyModel()
# 不同层学习率设置
ignored_params = list(map(id, model.add_block.parameters()))
base_params = filter(lambda p: id(p) not in ignored_params, model.parameters())
optimizer = optim.SGD(
[
{'params': base_params, 'lr': 0。01},
{'params': model.add_block.parameters(), 'lr': 0.1},
]
weight_decay=1e-5, momentum=0.9, nesterov=True)
3、调整学习率衰减。
方法一：使用torch.optim.lr_scheduler()函数：
####################
# model structure
#-------------------
model = Mymodel()
if use_gpu:
model = model.cuda()
####################
# loss
#-------------------
criterion = nn.CrossEntropyLoss()
####################
# optimizer
#-------------------
ignored_params = list(map(id, model.ViewModel.viewclassifier.parameters())) + list(map(id, model.Block.parameters()))
base_params = filter(lambda p: id(p) not in ignored_params, model.parameters())
optimizer_ft = optim.SGD([
{'params': base_params, 'lr': 0.01},
{'params': model.ViewModel.viewclassifier.parameters(), 'lr': 0.001},
{'params': model.Block.parameters(), 'lr': 0.01}
], weight_decay=1e-3, momentum=0.9, nesterov=True)
####################
#** Set lr_decay **
#-------------------
exp_lr_scheduler = torch.optim.lr_scheduler.StepLR(optimizer, step_size=60, gamma=0.1)
scheduler.step() # put it before model.train(True)
model.train(True) # Set model to training mode
....
方法二：使用optimizer.param_groups方法。（好处：能分别设定不同层的衰减率！）
####################
# model structure
#-------------------
model = Mymodel()
if use_gpu:
model = model.cuda()
####################
# loss
#-------------------
criterion = nn.CrossEntropyLoss()
####################
# optimizer
#-------------------
ignored_params = list(map(id, model.ViewModel.viewclassifier.parameters())) + list(map(id, model.Block.parameters()))
base_params = filter(lambda p: id(p) not in ignored_params, model.parameters())
optimizer_ft = optim.SGD([
{'params': base_params, 'lr': 0.01},
{'params': model.ViewModel.viewclassifier.parameters(), 'lr': 0.001},
{'params': model.Block.parameters(), 'lr': 0.03}],
weight_decay=1e-3, momentum=0.9, nesterov=True)
####################
#** Set lr_decay **
#-------------------
def adjust_lr(epoch):
step_size = 60
lr = args.lr * (0.1 ** (epoch // 30))
for g in optimizer.param_groups:
g['lr'] = lr * g.get('lr')
######################################
### optimizer.param_groups 类型与内容
[
{ 'params': base_params, 'lr': 0.01, 'momentum': 0.9, 'dampening': 0,
'weight_decay': 0.001, 'nesterov': True, 'initial_lr': 0.01 },
{ 'params': model.ViewModel.viewclassifier.parameters(), 'lr': 0.001,
'momentum': 0.9, 'dampening': 0, 'weight_decay': 0.001, 'nesterov': True,
'initial_lr': 0.001 },
{ 'params': model.Block.parameters(), 'lr': 0.03, 'momentum': 0.9,
'dampening': 0, 'weight_decay': 0.001, 'nesterov': True, 'initial_lr':
0.03 }
]
### optimizer.param_groups 类型与内容
######################################
for epoch in range(start_epoch, args.epochs):
adjust_lr(epoch) # 每epoch更新一次。
model.train(True) # Set model to training mode
....
补充知识：python中的字典方法.get()：