前言
深度学习剪枝(Pruning)是一种用于减少神经网络模型大小、减少计算量和提高推理效率的技术，通过去除神经网络中的冗余连接(权重)或节点(神经元)，从而实现模型的稀疏化。
深度学习剪枝(Pruning)具有以下几个好处：1. 模型压缩和存储节省；2. 计算资源节省；3. 加速推理速度；4. 防止过拟合。
“假剪枝”(Fake Pruning)是一种剪枝算法的称呼，它在剪枝过程中并不真正删除权重或节点，而是通过一些技巧将它们置零或禁用，以模拟剪枝的效果，不少优秀的论文就采用了"假剪枝"策略，尽管可以在一定程度上提高模型的推理速度，但假剪枝算法没有真正减少模型的大小，博主将通过讲解一个小案例，简洁易懂的说明一种对"假剪枝"卷积层进行真正的剪枝的的方法。
卷积层剪枝
可以先将最后的完整代码拷贝到自己的py文件中，然后按照博主的思路学习如何将置零卷积核进行真实剪枝：
- 初始化卷积层，并查看卷积层权重
# 示例使用一个具有3个输入通道和5个输出通道的卷积层 conv = nn.Conv2d(3, 5, 3) print("原始卷积层权重：") print(conv.weight.data) print(conv.weight.size()) print("原始卷积层偏置：") print(conv.bias.data) print(conv.bias.size()) - 通过随机函数让部分卷积核权重置为0，模拟完成了假剪枝。
# remove_zero_kernels方法内的代码 weight = conv_layer.weight.data # 卷积核个数 num_kernels = weight.size(0) # 随机对部分卷积置0 pruned = torch.ones(num_kernels, 1, 1, 1) # 选择随着置0的卷积序号 random_int = random.randint(1, num_kernels-1) for i in range(random_int): pruned[i, 0, 0, 0] = 0 conv_layer.weight.data = weight * pruned weight = conv_layer.weight.data bias = conv_layer.bias.data - 保存未被剪枝的卷积核的权重和偏置
# 计算每个卷积核的L2范数,目的是为了检查卷积核的所有位置是不是都置0了 norms = torch.norm(weight.view(num_kernels, -1), dim=1) zero_kernel_indices = torch.nonzero(norms==0).squeeze() print(zero_kernel_indices) # 移除L2范数为零的卷积核 new_weight = torch.stack([weight[i, :, :, :] for i in range(num_kernels) if i not in zero_kernel_indices]) new_bias = torch.stack([bias[i] for i in range(num_kernels) if i not in zero_kernel_indices]) - 构建新的卷积层，用来替换此前的卷积层，完成置零卷积核的真实剪枝
# 构建新的卷积层 if zero_kernel_indices.numel() > 0: # 输入channel in_channels = weight.size(1) # 输出channel out_channels = new_weight.size(0) # 卷积核大小 kernel_size = weight.size(2) # 步长 stride = conv_layer.stride padding = conv_layer.padding dilation = conv_layer.dilation groups = conv_layer.groups new_conv_layer = nn.Conv2d(in_channels, out_channels, kernel_size, stride, padding, dilation, groups) new_conv_layer.weight.data = new_weight new_conv_layer.bias.data = new_bias else: new_conv_layer = conv_layer
完整代码
import torch
import torch.nn as nn
import random
def remove_zero_kernels(conv_layer):
# 卷积核权重
weight = conv_layer.weight.data
# 卷积核个数
num_kernels = weight.size(0)
# 随机对部分卷积置0
pruned = torch.ones(num_kernels, 1, 1, 1)
# 选择随着置0的卷积序号
random_int = random.randint(1, num_kernels-1)
for i in range(random_int):
pruned[i, 0, 0, 0] = 0
conv_layer.weight.data = weight * pruned
weight = conv_layer.weight.data
bias = conv_layer.bias.data
# 计算每个卷积核的L2范数，目的是为了检查卷积核的所有位置是不是都置0了
norms = torch.norm(weight.view(num_kernels, -1), dim=1)
zero_kernel_indices = torch.nonzero(norms==0).squeeze()
print(zero_kernel_indices)
# 移除L2范数为零的卷积核
new_weight = torch.stack([weight[i, :, :, :] for i in range(num_kernels) if i not in zero_kernel_indices])
new_bias = torch.stack([bias[i] for i in range(num_kernels) if i not in zero_kernel_indices])
# 构建新的卷积层
if zero_kernel_indices.numel() > 0:
# 输入channel
in_channels = weight.size(1)
# 输出channel
out_channels = new_weight.size(0)
# 卷积核大小
kernel_size = weight.size(2)
# 步长
stride = conv_layer.stride
padding = conv_layer.padding
dilation = conv_layer.dilation
groups = conv_layer.groups
new_conv_layer = nn.Conv2d(in_channels, out_channels, kernel_size, stride, padding, dilation, groups)
new_conv_layer.weight.data = new_weight
new_conv_layer.bias.data = new_bias
else:
new_conv_layer = conv_layer
return new_conv_layer
# 示例使用一个具有3个输入通道和5个输出通道的卷积层
conv = nn.Conv2d(3, 5, 3)
# print("原始卷积层权重：")
# print(conv.weight.data)
# print(conv.weight.size())
# print("原始卷积层偏置：")
# print(conv.bias.data)
# print(conv.bias.size())
# 将置零的卷积核移除
new_conv = remove_zero_kernels(conv)
# print("原始卷积层权重：")
# print(new_conv.weight.data)
# print(new_conv.weight.size())
# print("原始卷积层偏置：")
# print(new_conv.bias.data)
# print(new_conv.bias.size())
总结
博主的思路就是用卷积层中保留的(未被剪枝)权重初始化一个新的卷积层，这样就将假剪枝的置零卷积核真实的除去，有没有研究这方面的读者可以给博主分享其他的方法，共同进步。