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Title: Hardware-aware One-Shot Neural Architecture Search in Coordinate Ascent Framework

Paper content:
Designing accurate and efficient convolutional neural architectures for vast amount of hardware is challenging because hardware designs are complex and diverse.
This paper addresses the hardware diversity challenge in Neural Architecture Search (NAS).
Unlike previous approaches that apply search algorithms on a small, human-designed search space without considering hardware diversity, we propose HURRICANE that explores the automatic hardware-aware search over a much larger search space and a multistep search scheme in coordinate ascent framework, to generate tailored models for different types of hardware.
Extensive experiments on  ImageNet show that our algorithm consistently achieves a much lower inference latency with a similar or better accuracy than state-of-the-art NAS methods on three types of hardware.
Remarkably, HURRICANE achieves a 76.63% top-1 accuracy on ImageNet with a inference latency of only 16.5 ms for DSP, which is a 3.4% higher accuracy and a 6.35x inference speedup than FBNet-iPhoneX.
For VPU, HURRICANE achieves a 0.53% higher top-1 accuracy than Proxyless-mobile with a 1.49x speedup.
Even for well-studied mobile CPU, HURRICANE achieves a 1.63% higher top-1  accuracy than FBNet-iPhoneX with a comparable inference latency.
HURRICANE also reduces the training time by 54.7% on average compared to SinglePath-Oneshot.
Neural Architecture Search (NAS) is a powerful mechanism to automatically generate efficient Convolutional Neural Networks (CNNs) without requiring huge manual efforts of human experts to design good CNN models (Zoph & Le, 2016; Guo et al., 2019; Bender et al., 2017) .
However, most existing NAS methods focus on searching for a single DNN model of high accuracy but pay less attention on the performance of executing the model on hardware, e.g., inference latency or energy cost.
Recent NAS methods (Guo et al., 2019; Cai et al., 2019; Stamoulis et al., 2018b; Wu et al., 2019 ) start to consider model-inference performance but they use FLOPs 1 to estimate inference latency or only consider the same type of hardware, e.g., smartphones from different manufacturers but all ARM-based.
However, the emerging massive smart devices are equipped with very diverse processors, such as CPU, GPU, DSP, FPGA, and various AI accelerators that have fundamentally different hardware designs.
Such a big hardware diversity makes FLOPs an improper metric to predict model-inference performance and calls for new trade-offs and designs for NAS to generate efficient models for diverse hardware.
To demonstrate it, we conduct an experiment to measure the performance of a set of widely used neural network operators (a.k.a. operations) on three types of mobile processors: Hexagon TM 685 DSP, Snapdragon 845 ARM CPU, and Movidius TM Myriad TM X Vision Processing Unit (VPU).
Figure 1 shows the results and we make the following key observations.
First, from Figure 1
(a), we can see that even the operators have similar FLOPs, the same operator may have very different inference latency on different processors.
For example, the latency of operator SEP 5 is nearly 12× higher than that of operator Choice 3 on the ARM CPU, but the difference on the VPU is less than 4×.
Therefore, FLOPs is not the right metric to decide the inference latency on different hardware.
Second, the relative effectiveness of different operators on different processors is also different.
For example, operator SEP 3 has the smallest latency on the DSP, but operator Choice 3 has the 1 In this paper, the definition of F LOP s follows , i.e., the number of multiply-adds.
smallest latency on the VPU.
Thus, different processors should choose different operators for the best trade-off between model accuracy and inference latency.
Furthermore, as shown in Figure 1
(b), the computational complexity and latency of the same operator are also affected by the execution context, such as input feature map shapes, number of channels, etc.
Such a context is determined by which layer the operator is placed on.
As a result, even on the same hardware, optimal operators may change at different layers of the network.
In addition, we observe that the existing NAS methods tends to handle all the layers equally.
For instance, the uniform sampling in one-shot NAS (Brock et al., 2018; Guo et al., 2019) will give the same sampling opportunities to every layer.
However, not all the layers are the same: different layers may have different impacts on inference latency and model accuracy.
Indeed, some previous works (D.Zeiler & Fergus, 2014; Girish et al., 2019) have revealed different behaviors between the earlier layers (close to data input) and the latter layers (close to classification output) in CNN models.
The earlier layers extract low-level features from inputs (e.g., edges and colors), are computation intensive and demands more data to converge, while the latter layers capture high-level class-specific features but are less computation intensive.
From these findings, we argue that exploring more architecture selections in the latter layers may help find better architectures with the limited sampling budget, and limiting the latency in the earlier layers is critical to search for low-latency models.
To this end, it is desirable to explore how to leverage this layer diversity for better architecture sampling in NAS.
Motivated by these observations, we argue that there is no one-size-fits-all model for different hardware, and thus propose and develop a novel hardware-aware method, called HURRICANE (Hardware aware one-shot neUral aRchitecture seaRch In Coordinate AsceNt framEwork), to tackle the challenge of hardware diversity in NAS.
Different from the existing hardware-aware NAS methods that use a small set of operators (e.g., 6 or 9) manually selected for a specific hardware platform, HURRICANE is initialized with a large-size (32 in our implementation) candidate operators set to cover the diversity of hardware platforms.
However, doing so increases the search space by many orders of magnitude and thus leads to unacceptable search and training cost and may even cause non-convergence problem.
To reduce the cost, we propose hardware-aware search space reduction at both operator level and layer level.
In the operator-level search space reduction, a toolkit is developed to automatically score every layer's candidate operators on target hardware platforms, and choose a sub-set of them with low latency for further utilization.
In the layer-level search space reduction, we split the layers into two groups, the earlier group and the latter group according to their locations in the network.
Based on a coordinate ascent framework (Wright, 2015) (Appendix A), we propose a multistep search scheme, which searches the complete architecture by a sequence of simpler searching of sub-networks.
In each iteration (step), we alternatively fix one group of layers and optimize the other group of layers to maximize the validation accuracy by a one-shot NAS 2 .
The searching of sub-networks is much easier to complete because of the much smaller size of search space, and the better architectures are reached by a sequence of iterations.
This layer-level search space reduction is inspired by the layer diversity mentioned above.
We choose most latencyeffective operators for earlier layers and allocate more sampling opportunities to latter layers.
As a result, we are able to search for models with both low latency and high accuracy.
We evaluate the effectiveness of our proposed approach on ImageNet 2012 dataset and a small OUIAdience-Age dataset with the above three mobile hardware platforms (DSP/CPU/VPU).
Under all the three platforms, HURRICANE consistently achieves the same level (or better) accuracy with much lower inference latency than state-of-the-art hardware-aware NAS methods.
Remarkably, HURRICANE reduces the inference latency by 6.35× on DSP compared to FBNet-iPhoneX and 1.49× On VPU compared to Proxyless-mobile, respectively.
Compared to Singlepath-Oneshot, on average HURRICANE reduces the training time by 54.7% on ImageNet.
In this paper, we propose HURRICANE to address the challenge of hardware diversity in NAS.
By exploring hardware-aware search space and a multistep search scheme based on coordinate ascent framework, our solution achieves better accuracy and much lower latency on three hardware platforms than state-of-the-art hardware-aware NAS.
And the searching cost (searching time) is also significantly reduced.
For future work, we plan to support more diverse hardware and speed up more NAS methods.

assistant: text: We propose HURRICANE to address the challenge of hardware diversity in one-shot neural architecture search