pytorch调整模型学习率

1.定义衰减函数

[Source code]

定义learning rate衰减方式：
$$L{R}_{epoch}=L{R}_0\times 0.8^{\frac{epoch+1}{10}}$$

import matplotlib.pyplot as plt
from torch import nn
import math
from torch import optim

class Net(nn.Module):
    def __init__(self):
        super(Net, self).__init__()
        self.net = nn.Linear(10,10)
    def forward(self, input):
        out = self.net(input)
        return out

learning_rate = 1e-3
L2_DECAY = 5e-4
MOMENTUM = 0.9

def step_decay(epoch, learning_rate):
    """ Schedule step decay of learning rate with epochs. """
    initial_learning_rate = learning_rate
    drop = 0.8
    epochs_drop = 10.0
    learning_rate = initial_learning_rate * math.pow(drop, math.floor((1 + epoch) / epochs_drop))

    return learning_rate

model = Net()
LR = 0.01
lr_list = []
for epoch in range(100):
    current_lr = step_decay(epoch, learning_rate)
    optimizer = torch.optim.SGD(model.parameters(), lr=current_lr)
    lr_list.append(current_lr)
plt.plot(range(100), lr_list, color = 'r')
plt.show()

绘出的lr更新曲线如下所示:

2. 手动修改optinmizer中的lr

[source code]

import matplotlib.pyplot as plt
from torch import nn
import torch
 
class Net(nn.Module):
    def __init__(self):
        super(Net, self).__init__()
        self.net = nn.Linear(10,10)
    def forward(self, input):
        out = self.net(input)
        return out
 
model = Net()
LR = 0.01
optimizer = torch.optim.Adam(model.parameters(), lr = LR)
lr_list = []
for epoch in range(100):
    if epoch % 5 == 0:
        for p in optimizer.param_groups:
            p['lr'] *= 0.9 # 学习率调整为原来的0.9
    lr_list.append(optimizer.state_dict()['param_groups'][0]['lr'])
plt.plot(range(100), lr_list, color = 'r')
plt.show()

学习率每个epoch变为原来的0.9，衰减曲线如下图所示：

Pytorch中给了很多学习率衰减的函数，我们不需要自己定义也可以动态调整学习率。

3. lr_scheduler.LambdaLR

如何在pytorch中调整lr具体请参考Pytorch文档How to adjust learning rate

CLASS torch.optim.lr_scheduler.LambdaLR(optimizer, lr_lambda, last_epoch=- 1, verbose=False)

import numpy as np
import torch
from torch import nn
import matplotlib.pyplot as plt

class Net(nn.Module):
    def __init__(self):
        super(Net, self).__init__()
        self.net = nn.Linear(10,10)
    def forward(self, input):
        out = self.net(input)
        return out
 
model = Net()
lr_list = []
LR = 0.01
optimizer = torch.optim.Adam(model.parameters(), lr = LR)
lambda1 = lambda epoch: 0.95 ** epoch
scheduler = torch.optim.lr_scheduler.LambdaLR(optimizer,lr_lambda = lambda1)
for epoch in range(100):
    optimizer.zero_grad() # clear previous gradients
    optimizer.step() # performs updates using calculated gradients
    scheduler.step() # update lr
    lr_list.append(optimizer.state_dict()['param_groups'][0]['lr'])
plt.plot(range(100),lr_list,color = 'r')
plt.show()

学习率衰减如图所示：

4. lr_scheduler.StepLR

CLASS torch.optim.lr_scheduler.StepLR(optimizer, step_size, gamma=0.1, last_epoch=-1, verbose=False)

• step_size: lr的衰减步长
• gamma: lr衰减的乘法因子

每隔step_size个epoch，学习率变为原来的gamma，例如，step_size设置为30，初始学习率设置为0.05，学习率变化如下所示：

>>> # Assuming optimizer uses lr = 0.05 for all groups
>>> # lr = 0.05 if epoch < 30
>>> # lr = 0.005 if 30 <= epoch < 60
>>> # lr = 0.0005 if 60 <= epoch < 90
>>> # ...
>>> scheduler = StepLR(optimizer, step_size=30, gamma=0.1)
>>> for epoch in range(100):
>>>     train(...)
>>>     validate(...)
>>>     scheduler.step()

import numpy as np
import torch
from torch import nn
import matplotlib.pyplot as plt

class Net(nn.Module):
    def __init__(self):
        super(Net, self).__init__()
        self.net = nn.Linear(10,10)
    def forward(self, input):
        out = self.net(input)
        return out
 
model = Net()
lr_list = []
LR = 0.01
optimizer = torch.optim.Adam(model.parameters(), lr = LR)
lambda1 = lambda epoch: 0.95 ** epoch
scheduler = torch.optim.lr_scheduler.StepLR(optimizer,step_size=5,gamma = 0.8)
for epoch in range(100):
    optimizer.zero_grad() # clear previous gradients
    optimizer.step() # performs updates using calculated gradients
    scheduler.step() # update lr
    lr_list.append(optimizer.state_dict()['param_groups'][0]['lr'])
plt.plot(range(100),lr_list,color = 'r')
plt.show()

学习率衰减曲线如下所示：

5. lr_scheduler.ExponentialLR

CLASStorch.optim.lr_scheduler.ExponentialLR(optimizer, gamma, last_epoch=- 1, verbose=False)

• gamma: 学习率衰减乘数因子

import numpy as np
import torch
from torch import nn
import matplotlib.pyplot as plt

class Net(nn.Module):
    def __init__(self):
        super(Net, self).__init__()
        self.net = nn.Linear(10,10)
    def forward(self, input):
        out = self.net(input)
        return out
 
model = Net()
lr_list = []
LR = 0.01
optimizer = torch.optim.Adam(model.parameters(), lr = LR)
lambda1 = lambda epoch: 0.95 ** epoch
scheduler = torch.optim.lr_scheduler.ExponentialLR(optimizer,gamma = 0.8)
for epoch in range(100):
    optimizer.zero_grad() # clear previous gradients
    optimizer.step() # performs updates using calculated gradients
    scheduler.step() # update lr
    lr_list.append(optimizer.state_dict()['param_groups'][0]['lr'])
plt.plot(range(100),lr_list,color = 'r')
plt.show()

学习率曲线如下图所示:

注意，如果相打印每一个epoch的当前学习率，官网中说的方法是调用print_Lr()，但实际上pytorch中并没有该方法，我们需要使用get_last_lr()方法才能打印每一个epoch的学习率。

更多学习率调整方法的介绍可以参考文档.

References:

[1] https://github.com/agrija9/Deep-Unsupervised-Domain-Adaptation/tree/master/DDC
[2] Pytorch学习率lr衰减(decay)(scheduler)