# High-Performance Deep Learning: How to train smaller, faster, and better models – Part 4

With the right software, hardware, and techniques at your fingertips, your capability to effectively develop high-performing models now hinges on leveraging automation to expedite the experimental process and building with the most efficient model architectures for your data.

In the previous parts (Part 1, Part 2, Part 3), we discussed why efficiency is important for deep learning models to achieve high-performance models that are pareto-optimal, as well as the focus areas for efficiency in Deep Learning. We also covered two of the focus areas (Compression Techniques and Learning Techniques). Let us continue to spend more time covering the other focus areas. You are also welcome to go through our survey paper on efficiency in deep learning.

### Automation

Another stream of efforts has been around delegating some of the manual work in efficiency to automation. If we let automation help with network design and tuning, it would reduce human involvement and the bias that comes along with it. The trade-off, though, is increased computational costs that come along with it.

**Hyper-Parameter Optimization (HPO)**: One of the commonly used methods that fall under this category is Hyper-Parameter Optimization (HPO) [1]. We know that tuning hyper-parameters such as initial learning rate, weight decay, etc., are crucial for faster convergence [2]. There can also be parameters that decide the network architecture, such as the number of fully connected layers, the number of filters in a convolutional layer, etc.

While we can build intuition with experimentation, finding the best hyper-parameter values requires a manual search for the exact values that optimize the given objective function (typically the loss value on the validation set).

If the user has prior experience on the hyper-parameters to be tuned, a simple algorithm for automating HPO that can be used is Grid Search (also referred to as Parameter Sweep). In this case we search for all the distinct and valid combinations of the given hyper-parameters based on the valid range of each provided by the user. For example, if the possible values for learning rate (lr) are {0.01, 0.05}, and the possible values for the weight decay (decay) are {0.1, 0.2}, then there would be 4 possible combinations possible {lr=0.01, decay=0.1}, {lr=0.01, decay=0.2}, {lr=0.05, decay=0.1}, and {lr=0.05, decay=0.2}.

Each of the above combinations is a *trial,* and each trial can then be run in parallel. The optimal combination of the hyper-parameters is found once all the trials have been completed. Since this approach tries all possible combinations, the total number of trials grows very quickly, and hence it suffers from the curse of dimensionality [3].

Another approach is Random Search [4], where trials are sampled randomly from the search space which is constructed using the range of possible values provided by the user. Similar to Grid Search, each trial still runs independently in parallel. However, Random Search is easy to scale depending on the computational capacity available since the trials are independently and identically distributed (iid), and thus the likelihood of finding the optimal trial increases with the number of trials. This allows for pre-empting the search if the best trial so far is good enough. There are also methods like Successive Halving (SHA) [5] and HyperBand [6] that are similar to random search, but they allocate more resources to the trials which are performing well.

*Grid Search, Random Search, and Bayesian Optimization. Source*

Bayesian Optimization (BO)-based search [7] is a more involved method, where it keeps a separate model for predicting if a given trial is likely to improve on the optimal trial found so far. The model learns to predict this likelihood based on the performance of the past trials. BO improves over Random Search in that the search is guided rather than random. Thus, fewer trials are required to reach the optimum. Since the selection of trials depends on the results of the past trials, this method is sequential. However, it is possible to spawn multiple trials at the same time in parallel (based on the same estimates), which might lead to faster convergence at the cost of some wasted trials when compared to purely sequential BO.

In terms of practical usage, HPO is available for users in several software toolkits that incorporate the algorithms themselves as well as an easy-to-use interface (UI to specify the hyper-parameters and their ranges), including Vizier [8] (an internal Google tool, also available via Google Cloud for black-box tuning). Amazon offers Sagemaker [9], which is functionally similar and can also be accessed as an AWS service. NNI [10], Tune [11], and Advisor [12] are other open-source HPO software packages that can be used locally. These toolkits also provide an option for Early Stopping of trials that are not promising. Vizier uses the Median Stopping Rule, where a trial is terminated if its performance at a time step *t* is below the median performance of all trials run till that point in time.

**Neural Architecture Search (NAS)**: We can think of NAS as an extension of HPO where we are searching for parameters that change the network architecture itself. NAS can be considered to comprise of the following parts:

*Search Space:*These are the neural net operations that are allowed in the graph (Convolution (1×1, 3× 3, 5 × 5), Fully Connected, Pooling, etc.), and how they connect with each other. This is provided by the user.*Search Algorithm & State:*This is the algorithm that controls the architecture search itself. Typically the standard algorithms that apply in HPO (Grid Search, Random Search, Bayesian Optimization, Evolutionary Algorithms) can be used for NAS as well, along with Reinforcement Learning (RL) [13] and Gradient Descent [14].*Evaluation Strategy:*This defines what metric we use to evaluate the model’s*fitness*. It can simply be a conventional metric like validation loss, accuracy, etc. Or it can also be a compound metric, as in the case of MNasNet [15], which creates a single custom metric based on accuracy as well as model latency.

*Neural Architecture Search: The controller is responsible for generating candidate models based on the search space and the feedback received from the model evaluation.*

The search algorithm with the search space and state can be viewed as a ‘controller’ which generates sample candidate networks. The evaluation stage trains and evaluates the generated candidates for fitness. This fitness value is then passed as feedback to the search algorithm, which will use it for generating better candidates.

Zoph et al.’s paper from 2016 [13] demonstrated that end-to-end neural network architectures could be generated using Reinforcement Learning. In this case, the controller is itself a Recurrent Neural Network (RNN), which generates the architectural hyper-parameters of a feed-forward network one layer at a time, such as the number of filters, stride, filter size, etc. Training the controller itself is expensive, though (taking 22,400 GPU hours [16]), since the entire candidate network has to be trained from scratch for a single gradient update to happen. In a follow-up paper [16], the authors refine the search space to search for *cells*: A ‘Normal Cell’ that takes in an input, processes it, and returns an output of the same spatial dimensions. And a ‘Reduction Cell’ processes its input and returns an output whose spatial dimensions are scaled down by a factor of 2. Each cell is a combination of 𝐵 blocks. The controller’s RNN generates one block at a time, where it picks outputs of two blocks in the past, the respective operations to apply on them, and how to combine them into a single output. The Normal and Reduction cells are stacked in alternating fashion (𝑁 Normal cells followed by 1 Reduction cell, where 𝑁 is tunable) to construct an end-to-end network for CIFAR-10 and ImageNet.

Learning these cells individually rather than learning the entire network seems to improve the search time by 7× when compared to the end-to-end network search in [13] while beating the state-of-the-art in CIFAR-10 at that time.

*A Normal and Reduction Cell. Source: [16]*

Other approaches such as evolutionary techniques [17], differentiable architecture search [14], progressive search [18], parameter sharing [19], etc. try to reduce the cost of architecture search (in some cases reducing the compute cost to a couple of GPU days instead of thousands of GPU days). These are covered in detail in [20].

When evaluating candidate networks, it is also possible to focus on not just quality but also footprint metrics like model size, latency, etc. Architecture Search can help with multi-objective searches that optimize for both. As an example, MNasNet [15] incorporates the model’s latency on a target mobile device into the objective function directly, as follows:

*Multi-Object reward function in MNasNet.*

*Generating candidate models in MNASNet, which also optimize for latency on mobile devices. Source: [15]*

where 𝑚 is the candidate model, 𝐴𝐶𝐶 is the accuracy metric, and 𝐿𝐴𝑇 is the latency of the given model on the desired device. 𝑇 is the target latency. 𝑤 is recommended to be −0.07.

Overall, automation plays a critical role in model efficiency. HPO is now a natural step in training models and can extract significant quality improvements while minimizing human involvement. HPO is also available in both independent software libraries, as well as through Cloud services. Similarly, recent advances in Neural Architecture Search (NAS) also make it feasible to construct architectures in a learned manner while having constraints on both quality and footprint. Assuming several hundred GPU hours worth of compute required for the NAS run to finish, and an approx cost of $3 GPU/hour on leading cloud computing services, this makes using NAS methods financially feasible and not similar in cost to manual experimentation with model architecture when optimizing for multiple objectives.

### Efficient Architectures (Models & Layers)

Another common theme is to redesign efficient layers and models that are better than the baseline and can be used for a specific task or as a black box in general. In this section, we lay out examples of such efficient layers and models to illustrate this idea.

**Vision:** One of the classic examples of efficient layers in the Vision domain is the use of convolutional layers, which improved over Fully Connected (FC) layers in Vision models. FC layers suffer from two primary issues:

(1) FC layers ignore the spatial information of the input pixels. Intuitively, it is hard to build an understanding of the given input by looking at individual pixel values in isolation. They also ignore the spatial locality in nearby regions.

(2) Using FC layers also leads to an explosion in the number of parameters when working with even moderately sized inputs. A 100 × 100 RGB image with 3 channels would lead to each neuron in the first layer having 3 × 10^{4} connections. This makes the network susceptible to overfitting also.

Convolutional Layers avoid this by learning filters, where each filter is a 3D weight matrix of a fixed size (3x3, 5x5, etc.), with the third dimension being the same as the number of channels in the input. Each filter is convolved over the input to generate a *feature map* for that given filter. Each filter can learn to detect features like edges (horizontal, vertical, diagonal, etc.), leading to higher values in the feature maps where that feature is present. Collectively, the feature maps from a single convolutional layer can extract meaningful information from the image. Convolutional layers stacked on top will then use the feature maps generated by the previous layer as the input, progressively learning more complex features (each pixel in the feature maps is generated from a progressively larger fraction of the image as one starts stacking the layers, increasing the *receptive field *for the filter in the next layer).

*Illustration of a convolution operation on a 2D input (blue), with a 2x2 filter (green). Source.*

*3D visualization of the convolution operation. Source*

The core idea behind the efficiency of Convolutional Layers is that the same filter is used everywhere in the image, regardless of where the filter is applied, hence, enforcing spatial invariance while sharing the parameters. Going back to the example of a 100×100 RGB image with 3 channels, a 5 × 5 filter would imply a total of 75 (5 × 5 × 3) parameters. Each layer can learn multiple unique filters and still be within a very reasonable parameter budget. This also has a regularizing effect, wherein a dramatically reduced number of parameters allow for easier optimization and better generalization.

**Depth-Separable Convolutional Layers**: In the convolution operation, each filter is used to convolve over the two spatial dimensions and the third channel dimension. As a result, the size of each filter is s_{x} x s_{y} x input_channels, where s_{x} and s_{y} are typically equal. This is done for each filter, resulting in the convolution operation happening both spatially in the *x* and *y* dimensions and depthwise in the z dimension.

Depth-separable convolution breaks this into two steps:

- Doing a point-wise convolution with 1 x 1 filters, such that the resulting feature map now has a depth of output_channels.
- Doing a spatial convolution with s
_{x}x s_{y}filters in the x and y dimensions.

These two operations stacked together (without any intermediate non-linear activation) results in an output of the same shape as a regular convolution, with much fewer parameters (1 x 1 x input_channels x output_channels + s_{x} x s_{y} x output_channels, v/s s_{x} x s_{y} x input_channels x output_channels for the regular convolution). Similarly, there is an order of magnitude less computation since the point-wise convolution is much cheaper for convolving with each input channel depth-wise (more calculations here & here). The Xception model architecture [21] demonstrated using depth-wise separable convolutions in the Inception architecture, allowing convergence sooner in terms of steps and a higher accuracy on the ImageNet dataset while keeping the number of parameters the same.

The MobileNet model architecture [22], which was designed for mobile and embedded devices, also uses depth-wise separable layers instead of regular convolutional layers. This helps them reduce the number of parameters as well as the number of multiply-add operations by 7-10x and allows deployment on Mobile for Computer Vision tasks. Users can expect a latency between 10-100 ms, depending on the model. MobileNet also provides a knob via the depth multiplier for scaling the network to allow the user to trade-off between accuracy and latency.

**Attention Mechanism**: On the Natural Language front, we have seen rapid progress too. For sequence-to-sequence models, a persistent issue was that of information-bottleneck. These models typically have an encoder layer, which encodes an input sequence, and a decoder sequence that generates another sequence in response. An example of such a task can be machine translation, where the input sequence is a sentence in the source language and the output sequence is the sentence in the target language.

The way this was traditionally done was with RNNs in both encoders and decoders. However, the first decoder layer could only see the hidden state of the final encoder step. This is a ‘bottleneck’ because the first decoder step would have to extract all information from the final hidden state.

*Information bottleneck for the decoder for a machine translation task from English to Hindi.*

The Attention mechanism was introduced in Bahdanau et al. [23] to allow the decoder to be able to see all the encoder states. It is a way to highlight the relevant parts of an input sequence and compress the input sequence to a *context vector*, based on the similarity of the sequence with another vector (known as the query vector). In the case of sequence-to-sequence tasks like machine translation, this allows tailoring the input to the decoder based on all the encoder states (represented as keys and values) and the previous hidden state of the decoder (query vector). The context vector is a weighted sum of the encoder states based on the previous hidden state of the decoder. Since Attention creates a weighted sum of the encoder states, the weights can also be used for visualizing the behavior of the network.

*Use of attention in the decoder. Source*

**Transformer & Friends**: The Transformer architecture [24] was proposed in 2017, which introduced using attention for both the decoder and the encoder. In the encoder, they use self-attention where the keys, values, and query vectors are all derived from the previous encoder layers. Transformer networks were two orders of magnitude cheaper to train than the comparable alternatives at the time of the paper being authored.

*Transformer architecture. Source*

Another core idea is that self-attention allows parallelizing the process of deriving relationships between the tokens in the input sequences. RNNs inherently force the process to occur one step at a time. For example, in an RNN, the context of a token might not be fully understood until the entire sequence has been processed. With attention, all tokens are processed together, and pairwise relationships can be learned. This makes it easier to leverage optimized training devices like GPUs and TPUs.

As introduced in Part 3, the BERT model architecture [25] beat the state-of-the-art in several NLU benchmarks. BERT is a stack of Transformer encoder layers that are pre-trained using a bi-directional masked language model training objective. It can also be used as a general-purpose encoder which can then be used for other tasks. Other similar models like the GPT family [26] have also been used for solving many NLU tasks.

### References

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