【图像分类】2019-MoblieNetV3 ICCV

论文题目：Searching for MobileNetV3

论文地址：https://arxiv.org/abs/1905.02244

代码链接: https://github.com/xiaolai-sqlai/mobilenetv3

发表时间：2019年5月

引用：Howard A, Sandler M, Chu G, et al. Searching for mobilenetv3[C]//Proceedings of the IEEE/CVF international conference on computer vision. 2019: 1314-1324.

引用数：2468

1. 简介

相对重量级网络而言，轻量级网络的特点是参数少、计算量小、推理时间短。更适用于存储空间和功耗受限的场景，例如移动端嵌入式设备等边缘计算设备。因此轻量级网络受到了广泛的关注，其中MobileNet可谓是其中的佼佼者。MobileNetV3经过了V1和V2前两代的积累，性能和速度都表现优异，受到学术界和工业界的追捧，无疑是轻量级网络的“抗把子“。MobileNetV3 参数是由NAS（network architecture search）搜索获取的，又继承的V1和V2的一些实用成果，并引人SE通道注意力机制，可谓集大成者。本文以应用为主，结合代码剖析MobileNetV3的网络结构，不会对NAS以及其设计思想做过多解析。

特点

1. 论文推出两个版本：Large 和 Small，分别适用于不同的场景;
2. 使用NetAdapt算法获得卷积核和通道的最佳数量;
3. 继承V1的深度可分离卷积;
4. 继承V2的具有线性瓶颈的残差结构;
5. 引入SE通道注意力结构;
6. 使用了一种新的激活函数h-swish(x)代替Relu6，h的意思表示hard;
7. 使用了Relu6(x + 3)/6来近似SE模块中的sigmoid;
8. 修改了MobileNetV2后端输出head;

2. 网络

2.1 通道可分离卷积

通道分离卷积是MobileNet系列的主要特点，也是其发挥轻量级作用的主要因素。如下图，通道可分离卷积分为两个过程：1.channel方向通道可分离卷积；2.正常的1X1卷积输出指定的channel个数。

代码

# 首先利用1X1卷积进行通道压缩，可以进一步压缩模型大小
self.conv1 = nn.Conv2d(in_size, expand_size, kernel_size=1, stride=1, padding=0, bias=False)
self.bn1 = nn.BatchNorm2d(expand_size)
self.nolinear1 = nolinear # 激活函数，使用H-swish或H-relu
# 注意，通道可分离卷积使用分组卷积操作进行，这里分成和卷积核相同的channel组数实现。
self.conv2 = nn.Conv2d(expand_size, expand_size, kernel_size=kernel_size, stride=stride,
                               padding=kernel_size // 2, groups=expand_size, bias=False)
self.bn2 = nn.BatchNorm2d(expand_size)
self.nolinear2 = nolinear
# 利用1X1卷积输出指定通道个数的卷积，这一步一定要有，不然无法控制输出通道个数。也有聚合分离特征的作用
self.conv3 = nn.Conv2d(expand_size, out_size, kernel_size=1, stride=1, padding=0, bias=False)
self.bn3 = nn.BatchNorm2d(out_size)

2.2 SE模块

SE通道注意力机制，老生常谈的话题。这里不进行解析，直接给出代码。值得注意的是，这里利用1X1卷积实现的FC操作，本质上和FC是一样的。这里利用hsigmoid模拟sigmoid操作。

class SeModule(nn.Module):
    def __init__(self, in_size, reduction=4):
        super(SeModule, self).__init__()
        self.se = nn.Sequential(
            nn.AdaptiveAvgPool2d(1),
            nn.Conv2d(in_size, in_size // reduction, kernel_size=1, stride=1, padding=0, bias=False),
            nn.BatchNorm2d(in_size // reduction),
            nn.ReLU(inplace=True),
            nn.Conv2d(in_size // reduction, in_size, kernel_size=1, stride=1, padding=0, bias=False),
            nn.BatchNorm2d(in_size),
            hsigmoid())

    def forward(self, x):
        return x * self.se(x)

2.3 h-swish 和 h-sigmoid

利用近似操作模拟swish和relu，公式如下：
$$h-swish[x]=x\frac{ReLU6(x+3)}{6}$$

代码实现

class hswish(nn.Module):
    def forward(self, x):
        out = x * F.relu6(x + 3, inplace=True) / 6
        return out

class hsigmoid(nn.Module):
    def forward(self, x):
        out = F.relu6(x + 3, inplace=True) / 6
        return out

2.4 bneck

核心模块，也是网络的基本模块。主要实现了通道可分离卷积+SE通道注意力机制+残差连接。结构图如下：

代码实现

def forward(self, x):
    out = self.nolinear1(self.bn1(self.conv1(x))) # 降维
    out = self.nolinear2(self.bn2(self.conv2(out))) # 通道可分离卷积
    out = self.bn3(self.conv3(out)) # 1X1卷积聚合特征
    if self.se != None: 
        out = self.se(out) # 通道注意力机制
    out = out + self.shortcut(x) if self.stride == 1 else out # 残差连接
    return out

2.5 总体架构

移除之前的瓶颈层连接，进一步降低网络参数。可以有效降低11%的推理耗时，而性能几乎没有损失。修改结构如下：

上图为MobileNetV3的网络结构图，large和small的整体结构一致，区别就是基本单元bneck的个数以及内部参数上，主要是通道数目。

上表为具体的参数设置，其中bneck是网络的基本结构。SE代表是否使用通道注意力机制。NL代表激活函数的类型，包括HS(h-swish),RE(ReLU)。NBN 代表没有BN操作。 s 是stride的意思，网络使用卷积stride操作进行降采样，没有使用pooling操作。

3. 代码

'''MobileNetV3 in PyTorch. See the paper "Inverted Residuals and Linear Bottlenecks: Mobile Networks for Classification, Detection and Segmentation" for more details. '''
import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.nn import init



class hswish(nn.Module):
    def forward(self, x):
        out = x * F.relu6(x + 3, inplace=True) / 6
        return out


class hsigmoid(nn.Module):
    def forward(self, x):
        out = F.relu6(x + 3, inplace=True) / 6
        return out


class SeModule(nn.Module):
    def __init__(self, in_size, reduction=4):
        super(SeModule, self).__init__()
        self.se = nn.Sequential(
            nn.AdaptiveAvgPool2d(1),
            nn.Conv2d(in_size, in_size // reduction, kernel_size=1, stride=1, padding=0, bias=False),
            nn.BatchNorm2d(in_size // reduction),
            nn.ReLU(inplace=True),
            nn.Conv2d(in_size // reduction, in_size, kernel_size=1, stride=1, padding=0, bias=False),
            nn.BatchNorm2d(in_size),
            hsigmoid()
        )

    def forward(self, x):
        return x * self.se(x)


class Block(nn.Module):
    '''expand + depthwise + pointwise'''
    def __init__(self, kernel_size, in_size, expand_size, out_size, nolinear, semodule, stride):
        super(Block, self).__init__()
        self.stride = stride
        self.se = semodule

        self.conv1 = nn.Conv2d(in_size, expand_size, kernel_size=1, stride=1, padding=0, bias=False)
        self.bn1 = nn.BatchNorm2d(expand_size)
        self.nolinear1 = nolinear
        self.conv2 = nn.Conv2d(expand_size, expand_size, kernel_size=kernel_size, stride=stride, padding=kernel_size//2, groups=expand_size, bias=False)
        self.bn2 = nn.BatchNorm2d(expand_size)
        self.nolinear2 = nolinear
        self.conv3 = nn.Conv2d(expand_size, out_size, kernel_size=1, stride=1, padding=0, bias=False)
        self.bn3 = nn.BatchNorm2d(out_size)

        self.shortcut = nn.Sequential()
        if stride == 1 and in_size != out_size:
            self.shortcut = nn.Sequential(
                nn.Conv2d(in_size, out_size, kernel_size=1, stride=1, padding=0, bias=False),
                nn.BatchNorm2d(out_size),
            )

    def forward(self, x):
        out = self.nolinear1(self.bn1(self.conv1(x)))
        out = self.nolinear2(self.bn2(self.conv2(out)))
        out = self.bn3(self.conv3(out))
        if self.se != None:
            out = self.se(out)
        out = out + self.shortcut(x) if self.stride==1 else out
        return out


class MobileNetV3_Large(nn.Module):
    def __init__(self, num_classes=1000):
        super(MobileNetV3_Large, self).__init__()
        self.conv1 = nn.Conv2d(3, 16, kernel_size=3, stride=2, padding=1, bias=False)
        self.bn1 = nn.BatchNorm2d(16)
        self.hs1 = hswish()

        self.bneck = nn.Sequential(
            Block(3, 16, 16, 16, nn.ReLU(inplace=True), None, 1),
            Block(3, 16, 64, 24, nn.ReLU(inplace=True), None, 2),
            Block(3, 24, 72, 24, nn.ReLU(inplace=True), None, 1),
            Block(5, 24, 72, 40, nn.ReLU(inplace=True), SeModule(40), 2),
            Block(5, 40, 120, 40, nn.ReLU(inplace=True), SeModule(40), 1),
            Block(5, 40, 120, 40, nn.ReLU(inplace=True), SeModule(40), 1),
            Block(3, 40, 240, 80, hswish(), None, 2),
            Block(3, 80, 200, 80, hswish(), None, 1),
            Block(3, 80, 184, 80, hswish(), None, 1),
            Block(3, 80, 184, 80, hswish(), None, 1),
            Block(3, 80, 480, 112, hswish(), SeModule(112), 1),
            Block(3, 112, 672, 112, hswish(), SeModule(112), 1),
            Block(5, 112, 672, 160, hswish(), SeModule(160), 1),
            Block(5, 160, 672, 160, hswish(), SeModule(160), 2),
            Block(5, 160, 960, 160, hswish(), SeModule(160), 1),
        )


        self.conv2 = nn.Conv2d(160, 960, kernel_size=1, stride=1, padding=0, bias=False)
        self.bn2 = nn.BatchNorm2d(960)
        self.hs2 = hswish()
        self.linear3 = nn.Linear(960, 1280)
        self.bn3 = nn.BatchNorm1d(1280)
        self.hs3 = hswish()
        self.linear4 = nn.Linear(1280, num_classes)
        self.init_params()

    def init_params(self):
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                init.kaiming_normal_(m.weight, mode='fan_out')
                if m.bias is not None:
                    init.constant_(m.bias, 0)
            elif isinstance(m, nn.BatchNorm2d):
                init.constant_(m.weight, 1)
                init.constant_(m.bias, 0)
            elif isinstance(m, nn.Linear):
                init.normal_(m.weight, std=0.001)
                if m.bias is not None:
                    init.constant_(m.bias, 0)

    def forward(self, x):
        out = self.hs1(self.bn1(self.conv1(x)))
        out = self.bneck(out)
        out = self.hs2(self.bn2(self.conv2(out)))
        out = F.avg_pool2d(out, 7)
        out = out.view(out.size(0), -1)
        out = self.hs3(self.bn3(self.linear3(out)))
        out = self.linear4(out)
        return out



class MobileNetV3_Small(nn.Module):
    def __init__(self, num_classes=1000):
        super(MobileNetV3_Small, self).__init__()
        self.conv1 = nn.Conv2d(3, 16, kernel_size=3, stride=2, padding=1, bias=False)
        self.bn1 = nn.BatchNorm2d(16)
        self.hs1 = hswish()

        self.bneck = nn.Sequential(
            Block(3, 16, 16, 16, nn.ReLU(inplace=True), SeModule(16), 2),
            Block(3, 16, 72, 24, nn.ReLU(inplace=True), None, 2),
            Block(3, 24, 88, 24, nn.ReLU(inplace=True), None, 1),
            Block(5, 24, 96, 40, hswish(), SeModule(40), 2),
            Block(5, 40, 240, 40, hswish(), SeModule(40), 1),
            Block(5, 40, 240, 40, hswish(), SeModule(40), 1),
            Block(5, 40, 120, 48, hswish(), SeModule(48), 1),
            Block(5, 48, 144, 48, hswish(), SeModule(48), 1),
            Block(5, 48, 288, 96, hswish(), SeModule(96), 2),
            Block(5, 96, 576, 96, hswish(), SeModule(96), 1),
            Block(5, 96, 576, 96, hswish(), SeModule(96), 1),
        )


        self.conv2 = nn.Conv2d(96, 576, kernel_size=1, stride=1, padding=0, bias=False)
        self.bn2 = nn.BatchNorm2d(576)
        self.hs2 = hswish()
        self.linear3 = nn.Linear(576, 1280)
        self.bn3 = nn.BatchNorm1d(1280)
        self.hs3 = hswish()
        self.linear4 = nn.Linear(1280, num_classes)
        self.init_params()

    def init_params(self):
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                init.kaiming_normal_(m.weight, mode='fan_out')
                if m.bias is not None:
                    init.constant_(m.bias, 0)
            elif isinstance(m, nn.BatchNorm2d):
                init.constant_(m.weight, 1)
                init.constant_(m.bias, 0)
            elif isinstance(m, nn.Linear):
                init.normal_(m.weight, std=0.001)
                if m.bias is not None:
                    init.constant_(m.bias, 0)

    def forward(self, x):
        out = self.hs1(self.bn1(self.conv1(x)))
        out = self.bneck(out)
        out = self.hs2(self.bn2(self.conv2(out)))
        out = F.avg_pool2d(out, 7)
        out = out.view(out.size(0), -1)
        out = self.hs3(self.bn3(self.linear3(out)))
        out = self.linear4(out)
        return out



def test():
    net = MobileNetV3_Small()
    x = torch.randn(2,3,224,224)
    y = net(x)
    print(y.size())

# test()

参考资料

轻量化网络：MobileNet v3解析_zhw864680355的博客-CSDN博客_mobilenetv3

轻量级骨架首选：MobileNetV3完全解析 - 知乎 (zhihu.com)