摘要：CosineWarmup是一种非常实用的训练策略，本次教程将带领大家实现该训练策略。教程将从理论和代码实战两个方面进行。
本文分享自华为云社区《CosineWarmup理论介绍与代码实战》，作者： 李长安。
CosineWarmup是一种非常实用的训练策略，本次教程将带领大家实现该训练策略。教程将从理论和代码实战两个方面进行。
在代码实战部分，模型采用LeNet-5模型进行测试，数据采用Cifar10数据集作为基准数据，
Warmup最早出现于这篇文章中：Accurate, Large Minibatch SGD:Training ImageNet in 1 Hour，warmup类似于跑步中的热身，在刚刚开始训练的时候进行热身，使得网络逐渐熟悉数据的分布，随着训练的进行学习率慢慢变大，到了指定的轮数，再使用初始学习率进行训练。
consine learning rate则来自于这篇文章Bag of Tricks for Image Classification with Convolutional Neural Networks，通过余弦函数对学习率进行调整
一般情况下，只在前五个Epoch中使用Warmup，并且通常情况下，把warm up和consine learning rate一起使用会达到更好的效果。
Warmup是在ResNet论文中提到的一种学习率预热的方法，它在训练开始的时候先选择使用一个较小的学习率，训练了一些epoches或者steps(比如4个epoches,10000steps),再修改为预先设置的学习来进行训练。由于刚开始训练时,模型的权重(weights)是随机初始化的，此时若选择一个较大的学习率,可能带来模型的不稳定(振荡)，选择Warmup预热学习率的方式，可以使得开始训练的几个epoches或者一些steps内学习率较小,在预热的小学习率下，模型可以慢慢趋于稳定,等模型相对稳定后再选择预先设置的学习率进行训练,使得模型收敛速度变得更快，模型效果更佳。
当我们使用梯度下降算法来优化目标函数的时候，当越来越接近Loss值的全局最小值时，学习率应该变得更小来使得模型尽可能接近这一点，而余弦退火（Cosine annealing）可以通过余弦函数来降低学习率。余弦函数中随着x的增加余弦值首先缓慢下降，然后加速下降，再次缓慢下降。这种下降模式能和学习率配合，以一种十分有效的计算方式来产生很好的效果。
以单个周期余弦退火衰减为例，介绍带Warmup的余弦退火策略，如下图所示，学习率首先缓慢升高，达到设定的最高值之后，通过余弦函数进行衰减调整。但是通常面对大数据集的时候，学习率可能会多次重复上述调整策略。
from paddle.optimizer.lr import LinearWarmup from paddle.optimizer.lr import CosineAnnealingDecay class Cosine(CosineAnnealingDecay): """ Cosine learning rate decay lr = 0.05 * (math.cos(epoch * (math.pi / epochs)) + 1) Args: lr(float): initial learning rate step_each_epoch(int): steps each epoch epochs(int): total training epochs """ def __init__(self, lr, step_each_epoch, epochs, **kwargs): super(Cosine, self).__init__( learning_rate=lr, T_max=step_each_epoch * epochs, ) self.update_specified = False class CosineWarmup(LinearWarmup): """ Cosine learning rate decay with warmup [0, warmup_epoch): linear warmup [warmup_epoch, epochs): cosine decay Args: lr(float): initial learning rate step_each_epoch(int): steps each epoch epochs(int): total training epochs warmup_epoch(int): epoch num of warmup """ def __init__(self, lr, step_each_epoch, epochs, warmup_epoch=5, **kwargs): assert epochs > warmup_epoch, "total epoch({}) should be larger than warmup_epoch({}) in CosineWarmup.".format( epochs, warmup_epoch) warmup_step = warmup_epoch * step_each_epoch start_lr = 0.0 end_lr = lr lr_sch = Cosine(lr, step_each_epoch, epochs - warmup_epoch) super(CosineWarmup, self).__init__( learning_rate=lr_sch, warmup_steps=warmup_step, start_lr=start_lr, end_lr=end_lr) self.update_specified = False
import paddle import paddle.nn.functional as F from paddle.vision.transforms import ToTensor from paddle import fluid import paddle.nn as nn print(paddle.__version__) 2.0.2 transform = ToTensor() cifar10_train = paddle.vision.datasets.Cifar10(mode='train', transform=transform) cifar10_test = paddle.vision.datasets.Cifar10(mode='test', transform=transform) # 构建训练集数据加载器 train_loader = paddle.io.DataLoader(cifar10_train, batch_size=64, shuffle=True) # 构建测试集数据加载器 test_loader = paddle.io.DataLoader(cifar10_test, batch_size=64, shuffle=True) Cache file /home/aistudio/.cache/paddle/dataset/cifar/cifar-10-python.tar.gz not found, downloading https://dataset.bj.bcebos.com/cifar/cifar-10-python.tar.gz Begin to download Download finished class MyNet(paddle.nn.Layer): def __init__(self, num_classes=10): super(MyNet, self).__init__() self.conv1 = paddle.nn.Conv2D(in_channels=3, out_channels=32, kernel_size=(3, 3), stride=1, padding = 1) # self.pool1 = paddle.nn.MaxPool2D(kernel_size=2, stride=2) self.conv2 = paddle.nn.Conv2D(in_channels=32, out_channels=64, kernel_size=(3,3), stride=2, padding = 0) # self.pool2 = paddle.nn.MaxPool2D(kernel_size=2, stride=2) self.conv3 = paddle.nn.Conv2D(in_channels=64, out_channels=64, kernel_size=(3,3), stride=2, padding = 0) # self.DropBlock = DropBlock(block_size=5, keep_prob=0.9, name='le') self.conv4 = paddle.nn.Conv2D(in_channels=64, out_channels=64, kernel_size=(3,3), stride=2, padding = 1) self.flatten = paddle.nn.Flatten() self.linear1 = paddle.nn.Linear(in_features=1024, out_features=64) self.linear2 = paddle.nn.Linear(in_features=64, out_features=num_classes) def forward(self, x): x = self.conv1(x) x = F.relu(x) # x = self.pool1(x) # print(x.shape) x = self.conv2(x) x = F.relu(x) # x = self.pool2(x) # print(x.shape) x = self.conv3(x) x = F.relu(x) # print(x.shape) # x = self.DropBlock(x) x = self.conv4(x) x = F.relu(x) # print(x.shape) x = self.flatten(x) x = self.linear1(x) x = F.relu(x) x = self.linear2(x) return x # 可视化模型 cnn2 = MyNet() model2 = paddle.Model(cnn2) model2.summary((64, 3, 32, 32)) --------------------------------------------------------------------------- Layer (type) Input Shape Output Shape Param # =========================================================================== Conv2D-1 [[64, 3, 32, 32]] [64, 32, 32, 32] 896 Conv2D-2 [[64, 32, 32, 32]] [64, 64, 15, 15] 18,496 Conv2D-3 [[64, 64, 15, 15]] [64, 64, 7, 7] 36,928 Conv2D-4 [[64, 64, 7, 7]] [64, 64, 4, 4] 36,928 Flatten-1 [[64, 64, 4, 4]] [64, 1024] 0 Linear-1 [[64, 1024]] [64, 64] 65,600 Linear-2 [[64, 64]] [64, 10] 650 =========================================================================== Total params: 159,498 Trainable params: 159,498 Non-trainable params: 0 --------------------------------------------------------------------------- Input size (MB): 0.75 Forward/backward pass size (MB): 25.60 Params size (MB): 0.61 Estimated Total Size (MB): 26.96 --------------------------------------------------------------------------- {'total_params': 159498, 'trainable_params': 159498} # 配置模型 from paddle.metric import Accuracy scheduler = CosineWarmup( lr=0.5, step_each_epoch=100, epochs=8, warmup_steps=20, start_lr=0, end_lr=0.5, verbose=True) optim = paddle.optimizer.SGD(learning_rate=scheduler, parameters=model2.parameters()) model2.prepare( optim, paddle.nn.CrossEntropyLoss(), Accuracy() ) # 模型训练与评估 model2.fit(train_loader, test_loader, epochs=10, verbose=1, ) The loss value printed in the log is the current step, and the metric is the average value of previous step. Epoch 1/3 /opt/conda/envs/python35-paddle120-env/lib/python3.7/site-packages/paddle/fluid/layers/utils.py:77: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working return (isinstance(seq, collections.Sequence) and step 782/782 [==============================] - loss: 1.9828 - acc: 0.2280 - 106ms/step Eval begin... The loss value printed in the log is the current batch, and the metric is the average value of previous step. step 157/157 [==============================] - loss: 1.5398 - acc: 0.3646 - 35ms/step Eval samples: 10000 Epoch 2/3 step 782/782 [==============================] - loss: 1.7682 - acc: 0.3633 - 106ms/step Eval begin... The loss value printed in the log is the current batch, and the metric is the average value of previous step. step 157/157 [==============================] - loss: 1.7934 - acc: 0.3867 - 34ms/step Eval samples: 10000 Epoch 3/3 step 782/782 [==============================] - loss: 1.3394 - acc: 0.4226 - 105ms/step Eval begin... The loss value printed in the log is the current batch, and the metric is the average value of previous step. step 157/157 [==============================] - loss: 1.4539 - acc: 0.3438 - 35ms/step Eval samples: 10000
之前一直提到这个CosineWarmup，但是一直没有实现过，这次也算是填了一个很早之前就挖的坑。同样，这里也不再设置对比实验，因为这个东西确实很管用。小模型和小数据集可能不太能够体现该训练策略的有效性。大家如果有兴趣可以使用更大的模型、更大的数据集测试一下。