Source: Deep Learning on Medium
1 — Pruning Neural Networks
Contrary to what you may think, pruning has nothing to do with cutting trees! In machine learning, model pruning consists of removing unimportant weights in order to get a smaller and faster network.
Model pruning was first introduced in 1989 by Yann Le Cun in his paper “Optimal Brain Damage”. The idea is to take a fully trained network and prune weights whose deletion will cause the least increase in the objective function. The contribution of each parameter can be approximated using the Hessian matrix. Once unimportant weights have been removed, the smaller network can be trained once again, and the process can be repeated several times until the network has both a satisfying size and a reasonable performance.
Since then, plenty of pruning technique variations have been developed. In 2015, Han et al, in “Learning both Weights and Connections for Efficient Neural Networks”, introduced a three-step method that consists of training a neural network, then pruning connections whose weight is lower than a chosen threshold, and finally retraining the sparse network to learn the final weights for the remaining connections.
You may wonder: How is the pruning threshold determined? Good question! Both convolution and fully connected layers can be pruned; however, experience has shown that convolution layers are more sensitive to pruning than fully connected layers. Thus, the threshold is chosen according to the sensitivity of each layer, as shown in the figure below, which is taken from the research paper from Han et al.
According to the research paper, 173 hours were needed to retrain the pruned AlexNet on an NVIDIA Titan X GPU. But retraining time isn’t a key concern, because the end goal is to make the smaller model run quickly on a resource-constrained device.
On ImageNet, the method reduced the number of parameters of AlexNet by a factor of 9x (from 61 million parameters to 6.7 million) and of VGG-16 by a factor of 13x (from 138 million parameters to 10.3 million). After pruning, the storage requirements of AlexNet and VGGNet are much lower, and all weights can be stored on-chip rather than on off-chip DRAM (which takes significantly more energy to access).
2 — Deep Compression
Neural networks are both computationally intensive and memory intensive, making them difficult to deploy on embedded systems with limited hardware resources. To address this limitation, the Deep Compression paper from Han et al, introduced a 3-stage pipeline (shown below): pruning, trained quantization, and Huffman coding, which work together to reduce the storage requirements of neural networks by 35 to 49 times, without affecting their accuracy.
The method first prunes the network by learning only the important connections. Next, the method quantizes the weights to enforce weight sharing. Finally, the method uses Huffman coding. After the first two steps, the authors retrain the network to fine-tune the remaining connections and the quantized centroids. Pruning reduces the number of connections by 9 to 13 times. Quantization then reduces the number of bits that represent each connection from 32 to 5.
On ImageNet, the method reduced the storage required by AlexNet by 35 times (from 240 MB to 6.9 MB) without accuracy loss. The method also reduced the size of the VGG-16 pre-trained model by 49 times (from 552 MB to 11.3 MB), also without accuracy loss.
Finally, this deep compression algorithm facilitates the use of complex neural networks in mobile applications, where the application size and download bandwidth are constrained. When benchmarked on CPU, GPU, and mobile GPU, the compressed network has a 3 to 4 times layer-wise speedup and 3 to 7 times better energy efficiency.
3 — Data Quantization
In recent years, convolutional neural network-based methods have achieved great success in a large number of applications and have been among the most widely used architectures in computer vision. However, CNN-based methods are computational-intensive and resource-consuming, and thus are hard to be integrated into embedded systems such as smartphones, smart glasses, and robots. Field Programmable Gate Array (FGPA) is a promising platform for accelerating CNNs, but the limited bandwidth and on-chip memory size limit the performance of the FPGA accelerator for CNNs.
In the paper Going Deeper with Embedded FPGA Platform for CNN from researchers at Tsinghua University, a CNN accelerator design on embedded FPGA for ImageNet large-scale image classification was proposed. The authors empirically showed that inside the architecture of current state-of-the-art CNN models, the convolutional layers are computational-centric, and the fully-connected layers are memory-centric. As such, they proposed a dynamic-precision data quantization method (shown below) to help improve the bandwidth and resource utilization.
In this data quantization flow, the fractional length between any two fixed-point numbers is dynamic for different layers and feature map sets, while static in one layer to minimize the truncation error of each layer.
- The weight quantization phase aims to find the optimal fractional length for weights in one layer. In this phase, the dynamic ranges of weights in each layer are analyzed first. After that, the fractional length is initialized to avoid data overflow.
- The data quantization phase aims to find the optimal fractional length for a set of feature maps between two layers. In this phase, the intermediate data of the fixed-point CNN model and the floating-point CNN model are compared layer-by-layer using a greedy algorithm to reduce the accuracy loss.
Their results (after further analysis of different strategies with different neural network architectures) show that dynamic-precision quantization is much more favorable compared with static-precision quantization. With dynamic-precision quantization, they can use much shorter representations of operations while still achieving comparable accuracy.