1 Introduction
Deep Convolutional Neural Networks (CNNs) have achieved stunning success in various machine learning and pattern recognition tasks by learning highly semantic and discriminative representation of data. Albeit the power of CNNs, they are usually over-parameterized and of large parameter space, which makes it difficult for deployment of CNNs on mobile platforms or other platforms with limited storage. In the recently emerging architecture such as Residual Network
(He et al., 2016) and Densely Connected Network (Huang et al., 2017), most parameters concentrate on convolution filters, which are used to learn deformation invariant features in the input volume. The deep learning community has developed several compression methods of reducing the parameter space of filters, such as filter pruning
(Luo et al., 2017), weight sharing and quantization (Han et al., 2016) and low-rank and sparse representation of the filters (Ioannou et al., 2016; Yu et al., 2017).Weight sharing has been proved to be an effective way of reducing the parameter space of CNNs. The success of deep compression (Han et al., 2016) and filter pruning (Luo et al., 2017) suggest that there is considerable redundancy in the parameter space of filters of CNNs. Based on this observation, our goal of compression can be achieved by encouraging filters to share weights.
In this paper, we propose a novel representation of filters, termed Filter Summary (FS), which enforces weight sharing across filters so as to achieve model compression. FS is a D tensor from which filters are extracted as overlapping D blocks. Because of weight sharing across nearby filters that overlap each other, the parameter space of convolution layer with FS is much smaller than its counterpart in conventional CNNs. In contrast, the model compression literature broadly adopts a two-step approach: learning a large CNN first, then compressing the model by various model compression techniques such as pruning, quantization and coding (Han et al., 2016; Luo et al., 2017), or low-rank and sparse representation of filters (Ioannou et al., 2016; Yu et al., 2017). The weight sampling network (Jin et al., 2018) studies overlapping filters for compression of D CNNs, and our work is a generalized one accounting for weight sharing through overlapping in regular D CNNs. The idea of FS resembles that of epitome (Jojic et al., 2003)
, which is developed for learning a condensed version of Gaussian Mixture Models (GMMs). In epitome, the Gaussian means are represented by a two dimensional matrix wherein each window in this matrix contains parameters of the Gaussian means for a Gaussian component. The idea of using overlapping structure in generative model is adopted for representing filters of CNNs in our work.
CNNs where each convolution layer has a FS representing its filters are named FSNet. FSNet has a compact architecture compared to its conventional CNNs counterpart. Instead of a two-step process of compression, FSNet is trained from scratch without the need of training a large model beforehand, which could be space and energy consuming. In the following text, we use to indicate integers between and inclusively, and subscript indicates index of a element of a tensor.
2 Formulation
We propose Filter Summary Convolutional Neural Networks (FSNet) in this section. Each convolution layer of FSNet has a D tensor named Filter Summary (FS), where filters are D blocks residing in the FS in a weight sharing manner. FSNet and its baseline CNN have the same architecture except that each convolution layer of FSNet has a compact representation of filters, namely a FS, rather than a set of independent filters in the baseline. FS is designed to generate the same number of filters as that of the filters in the corresponding convolution layer of the baseline CNN. Figure 1 shows an example of FS and how filters are extracted from the FS. A more concrete example is given here to describe the compact architecture of FSNet. Suppose that a convolution layer of the baseline CNN model has filters of channel size and spatial size , the corresponding convolution layer in the FSNet has a FS of size . The filters of size
are extracted by striding along each spatial dimension by
, and striding along the channel dimension by . The ratio of the parameter size of the filters to that of the corresponding FS is , indicating that the parameter space of the FS is times smaller than that of the independent filters in the baseline CNN.Formally, let a FS generate filters of size where is the spatial size and is the channel size. Let the sampling strides along the two spatial dimensions and the channel dimension of FS are respectively. Then the dimension of the FS is , where is the spatial size and is the channel size. In this paper we set the channel size of FS to that of the filter, i.e. . This is based on our observation that weight sharing along channel dimension tends not to hurt the prediction performance of FSNet. Therefore, the ratio of the parameter size of independent filters to that of the corresponding FS is as follows:
(1) |
In a typical setting where and the spatial stride is smaller than the corresponding filter size, i.e. , , FS has a more compact size than that of the individual filters. Note that a large , namely the sampling number along the channel dimension, contributes to better compression ratio, and this is the case shown in our experimental results. Compared to (Yang et al., 2018), FSNet also compresses convolution layer using FS and we offer much more extensive experimental results in this paper. In the case of convolution, we set and indicating that all the filters are extracted along the channel dimension of FS. In addition, the channel size of FS can be larger than that of the filter so that a proper compression ratio for convolution layer is obtained.
Algorithm 1 describes the forward and backward process in a convolution layer of FSNet. We use the mapping which maps the indices of the elements of the extracted filters to the indices of the corresponding elements in the FS. Namely, for a filter and the corresponding FS , . The mapping is used to conveniently track the origin of the elements of the filters extracted from FS. It should be emphasized that the inference speed of FSNet is the same as that of conventional CNNs, since filters are accessed by correctly indexing into FS and convolution is performed in a normal way. The model compression is achieved at the cost of the backward operation when training FSNet, wherein the gradients of the elements of filters that share the same weight are averaged to obtain the gradient of the corresponding weight of FS. More details are referred to Algorithm 1. Figure 2 illustrates typical architecture of FSNet where each convolution has a FS.
(2) |
3 Experimental Results
We conduct experiments with CNNs for image classification and object detection tasks in this section, demonstrating the compression results of FSNet.
We demonstrate the performance of FSNet in this subsection by comparative results between FSNet and its baseline CNN for classification task on the CIFAR- dataset (Krizhevsky, 2009). Using ResNet (He et al., 2016) or DenseNet (Huang et al., 2017) as baseline CNNs, we design FSNet by replacing all the convolution layers of ResNet or DenseNet by convolution layers with FS. We train FSNet and its baseline CNN, and show the test accuracy and the parameter number of all the models in Table 2. The convolution layers of ResNet and DenseNet have filters of spatial size of either or . We design the size of FS according to the number of filters in the corresponding convolution layer of the baseline CNN. Throughout this section, we set the spatial strides for convolution layers. According to the formula of compression ratio (1), a large indicates a large compression ratio, while also risks more prediction performance loss. In this experiment, we set according to Table 1. We do not specifically tune
, and one can choose other settings of these hyperparameters as long as their product matches the number of filters in the baseline CNN. All the
convolution layers are compressed by times.It can be observed in Table 2 that FSNet with a compact parameter space achieves accuracy comparable with that of different baselines including ResNet-, ResNet-, ResNet-, ResNet- and DenseNet-. DenseNet- denotes DenseNet with a growth rate of and layers. The baseline CNNs are trained with the initial learning rate of , and it is divided by when half and
of the total epoches are finished. We use the idea of cyclical learning rates
(Smith, 2015) for training FSNet. cycles are used for training FSNet, and each cycle uses the same schedule of learning rate as that of the baseline. A new cycle starts with the initial learning rate of after the previous cycle ends. The training loss and training error for the first cycle of FSNet is shown in Figure 3, and the test loss and test error are shown in Figure 3. We can see that the patterns of training and test of FSNet are similar to that of its baseline, ResNet-.Furthermore, FSNet with weight quantization, or FSNet-WQ, boosts the compression ratio without sacrificing performance. One-time weight quantization is performed for each convolution layer of trained FSNet. levels are evenly set between the maximum and minimum values of all the element of the FS in a convolution layer, and then each element of the FS is set to its nearest level. In this way, a quantized FS uses a byte to store each of its element, together with the original maximum and minimum values. The number of effective parameters of FSNet-WQ is computed by considering an element of a quantized FS as parameter since the storage required for a byte is of that for a floating number.
3.1 FSNet for Classification
FSNet-WQ achieves more than compression ratio for all the four types of ResNet, and it has less than accuracy drop for ResNet and DenseNet. It is interesting to observe that FSNet-WQ even enjoys slight better accuracy than FSNet for ResNet- and ResNet-. In addition, FSNet-WQ achieves exactly the same accuracy as the baseline with compression ratio for DenseNet-. We argue that weight quantization imposes regularization on the filter summary which may improve its prediction performance.
In order to evaluate FSNet on large-scale dataset, Table 3 shows its performance on ILSVRC- dataset (Russakovsky et al., 2015) using ResNet- as baseline. The accuracy is reported on the standard k validation set. We train ResNet- and the corresponding FSNet for epoches. The initial learning rate is , and it is divided by at epoch respectively.
#Filters | 12 | 16 | 32 | 64 | 128 | 256 | 512 |
2 | 4 | 2 | 4 | 4 | 4 | 8 |
Model | Before Compression | FSNet | Compression Ratio | FSNet-WQ | Compression Ratio | ||||
# Param | Accuracy | # Param | Accuracy | # Param | Accuracy | ||||
ResNet | ResNet- | M | M | M | |||||
ResNet- | M | M | M | ||||||
ResNet- | M | M | M | ||||||
ResNet- | M | M | M | ||||||
DenseNet | DenseNet- | M | M | M |
ModelPerformance | # Params | Top- | Top- |
---|---|---|---|
ResNet- | M | ||
FSNet | M | ||
FSNet-WQ | M |
Performance of FSNet on ImageNet
3.2 FSNet for Object Detection
We evaluate the performance of FSNet for object detection in this subsection. The baseline neural network is the Single Shot MultiBox Detector (SSD) (Liu et al., 2016)
. The baseline is adjusted by adding batch normalization
(Ioffe & Szegedy, 2015) layers so that it can be trained from scratch. Both SSD and FSNet are trained on the VOC training datasets, and the mean average precision (mAP) is reported on the VOC test dataset shown in Table 4. We employ two versions of FSNet with different compression ratios by adjusting the sampling number along the channel dimension, denoted by FSNet- and FSNet- respectively. Again, weight quantization either slightly improves mAP (for FSNet-), or only slightly hurts it (for FSNet-). Compared to Tiny SSD (Wong et al., 2018), FSNet--WQ enjoys smaller parameter space while its mAP is much better. Note that while the reported number of parameters of Tiny SSD is M, its number of effective parameters is only half of this number. i.e. M, as the parameters are stored in half precision floating-point. In addition, the model size of FSNet--WQ is MB, around smaller than that of Tiny SSD, MB.Model | # Params | mAP |
---|---|---|
SSD | M | |
Tiny SSD (Wong et al., 2018) | M | |
FSNet-1 | M | |
FSNet-1-WQ | M | |
FSNet-2 | M | |
FSNet-2-WQ | M |
4 Conclusion
We present a novel method for compression of CNNs through learning weight sharing by Filter Summary (FS). Each convolution layer of the proposed FSNet learns a FS from which the convolution filters are extracted from, and nearby filters share weights naturally. By virtue of the weight sharing scheme, FSNet enjoys much smaller parameter space than its baseline while maintaining competitive predication performance. The compression ratio is further improved by one-time weight quantization. Experimental results demonstrate the effectiveness of FSNet in tasks of image classification and object detection.
Acknowledgments
We would like to express our gratitude to Boyang Li and Pradyumna Tambwekar for their efforts in training the baseline SSD on the VOC dataset.
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