MobileNet v1 - Efficient Convolutional Neural Networks

Abstract

MobileNet V1 is a deep learning architecture designed specifically for mobile and embedded devices with limited computational resources . The main contributions of MobileNet V1 are:

1. Depthwise Separable Convolutions: MobileNet V1 replaces standard convolutions with depthwise separable convolutions that split the convolution into a depthwise convolution and a pointwise convolution . This reduces the number of parameters and computations required, making the model more efficient.

Main Ideas

The MobileNet model is based on depthwise separable convolutions which is a form of factorized convolutions which factorize a standard convolution into a depthwise convolution and a $1\times1$ convolution called a pointwise convolution.

The standard convolution operation has the effect of filtering features based on the convolutional kernels and combining features in order to produce a new representation. The filtering and combination steps can be split into two steps via the use of factorized convolutions called depthwise separable convolutions for substantial reduction in computational cost.

Depthwise separable convolution are made up of two layers: depthwise convolutions and pointwise convolutions.

• We use depthwise convolutions to apply a single filter per each input channel (input depth).
• Pointwise convolution, a simple $1\times1$ convolution, is then used to create a linear combination of the output of the depthwise layer.

MobileNet uses $3\times3$ depthwise separable convolutions which uses between 8 to 9 times less computation than standard convolutions at only a small reduction in accuracy.

Authors found that it was important to put very little or no weight decay (L2 regularization) on the depthwise filters since their are so few parameters in them.

Architecture & Training Details

Regarding the data, contrary to training large models we use less regularization and data augmentation techniques because small models have less trouble with overfitting.

For the architecture, MobileNet uses $3\times3$ depthwise separable convolutions. All layers are followed by a batchnorm and ReLU nonlinearity with the exception of the final fully connected layer which has no nonlinearity and feeds into a softmax layer for classification. Downsampling is handled with strided convolution in the depthwise convolutions as well as in the first layer.

MobileNet Body Architecture. dw stands for depthwise convolution and s for stride.

Code

The basic Depthwise Separable Convolution block can be built as follows:

1. Given the input volume x the first depthwise convolution will have the same output channels as the input channels ones. We will create same convolutions, using $3\times3$ kernels with stride 1 and padding 1, applying downsampling when needed through stride 2. Finally, for the depthwise convolution, we will use the parameter groups that is equals to the number of input-output channels, i.e., if groups is equals to input channels, each input channel is convolved with its own set of filters.
2. Next we apply the pointwise convolution , which takes the previous volume and applies as many $1\times1$ kernels as output channels are required.

class Block(nn.Module):
    '''Depthwise conv + Pointwise conv'''
    def __init__(self, in_planes, out_planes, stride=1):
        super(Block, self).__init__()
        self.conv1 = nn.Conv2d(
            in_planes, in_planes,
            kernel_size=3, stride=stride, padding=1, bias=False,
            groups=in_planes
        )
        self.bn1 = nn.BatchNorm2d(in_planes)
        self.conv2 = nn.Conv2d(
            in_planes, out_planes,
            kernel_size=1, stride=1, padding=0, bias=False
        )
        self.bn2 = nn.BatchNorm2d(out_planes)

    def forward(self, x):
        out = F.relu(self.bn1(self.conv1(x)))
        out = F.relu(self.bn2(self.conv2(out)))
        return out

With the basic depthwise separable convolution building block, we can create MobileNet network as follows:

class MobileNet(nn.Module):
    # (128,2) means conv planes=128, conv stride=2, by default conv stride=1
    cfg = [
      64, (128,  2),
      128, (256, 2),
      256, (512, 2),
      512, 512, 512, 512, 512,
      (1024, 2),
      1024
    ]

    def __init__(self, num_classes=10):
        super(MobileNet, self).__init__()
        self.conv1 = nn.Conv2d(3, 32, kernel_size=3, stride=1, padding=1, bias=False)
        self.bn1 = nn.BatchNorm2d(32)
        self.layers = self._make_layers(in_planes=32)
        self.linear = nn.Linear(1024, num_classes)

    def _make_layers(self, in_planes):
        layers = []
        for x in self.cfg:
            out_planes = x if isinstance(x, int) else x[0]
            stride = 1 if isinstance(x, int) else x[1]
            layers.append(Block(in_planes, out_planes, stride))
            in_planes = out_planes
        return nn.Sequential(*layers)

    def forward(self, x):
        out = F.relu(self.bn1(self.conv1(x)))
        out = self.layers(out)
        out = F.avg_pool2d(out, 2)
        out = out.view(out.size(0), -1)
        out = self.linear(out)
        return out

Credits

• Paper - MobileNets: Efficient Convolutional Neural Networks for Mobile Vision Applications
• Depthwise separable convolution