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Title: Apprentice: Using Knowledge Distillation Techniques To Improve Low-Precision Network Accuracy

Paper content:
Deep learning networks have achieved state-of-the-art accuracies on computer vision workloads like image classification and object detection.
The performant systems, however, typically involve big models with numerous parameters.
Once trained, a challenging aspect for such top performing models is deployment on resource constrained inference systems -- the models (often deep networks or wide networks or both) are compute and memory intensive.
Low precision numerics and model compression using knowledge distillation are popular techniques to lower both the compute requirements and memory footprint of these deployed models.
In this paper, we study the combination of these two techniques and show that the performance of low precision networks can be significantly improved by using knowledge distillation techniques.
We call our approach Apprentice and show state-of-the-art accuracies using ternary precision and 4-bit precision for many variants of ResNet architecture on ImageNet dataset.
We study three schemes in which one can apply knowledge distillation techniques to various stages of the train-and-deploy pipeline.
Background: Today's high performing deep neural networks (DNNs) for computer vision applications comprise of multiple layers and involve numerous parameters.
These networks have O(Giga-FLOPS) compute requirements and generate models which are O(Mega-Bytes) in storage BID4 .
Further, the memory and compute requirements during training and inference are quite different BID23 .
Training is performed on big datasets with large batch-sizes where memory footprint of activations dominates the model memory footprint.
On the other hand, batchsize during inference is typically small and the model's memory footprint dominates the runtime memory requirements.Because of complexity in compute, memory and storage requirements, training phase of the networks is performed on CPU and/or GPU clusters in a distributed computing environment.
Once trained, a challenging aspect is deployment of trained models on resource constrained inference systems such as portable devices or sensor networks, and for applications in which real-time predictions are required.
Performing inference on edge-devices comes with severe constraints on memory, compute and power.
Additionally, ensemble based methods, which one can potentially use to get improved accuracy predictions, become prohibitive in resource constrained systems.Quantization using low-precision numerics BID37 BID45 BID21 BID24 BID10 BID46 BID26 BID35 BID23 and model compression BID3 BID16 BID27 have emerged as popular solutions for resource constrained deployment scenarios.
With quantization, a low-precision version of network model is generated and deployed on the device.
Operating in lower precision mode reduces compute as well as data movement and storage requirements.
However, majority of existing works in low-precision DNNs sacrifice accuracy over baseline full-precision networks.
With model compression, a smallerIn the second scheme, we start with a full-precision trained network and transfer knowledge from this trained network continuously to train a low-precision network from scratch.
We find that the low-precision network converges faster (albeit to similar accuracies as the first scheme) when a trained complex network guides its training.In the third scheme, we start with a trained full-precision large network and an apprentice network that has been initialised with full-precision weights.
The apprentice network's precision is lowered and is fine-tuned using knowledge distillation techniques.
We find that the low-precision network's accuracy marginally improves and surpasses the accuracy obtained via the first scheme.
This scheme then sets the new state-of-the-art accuracies for the ResNet models at ternary and 4-bit precision.Overall, the contributions of this paper are the techniques to obtain low-precision DNNs using knowledge distillation technique.
Each of our scheme produces a low-precision model that surpasses the accuracy of the equivalent low-precision model published to date.
One of our schemes also helps a low-precision model converge faster.
We envision these accurate low-precision models to simplify the inference deployment process on resource constrained systems and even otherwise on cloud-based deployment systems.
In scheme-A, we use a teacher network that is always as large or larger in number of parameters than the student network.
We experimented with a ternary ResNet-34 student network which was paired with a full-precision ResNet-18.
The ternary model for ResNet-34 is about 8.5x smaller in size compared to the full-precision ResNet-18 model.
The final trained accuracy of the ResNet-34 ternary model with this setup is 2.7% worse than that obtained by pairing the ternary ResNet-34 network with a ResNet-50 teacher network.
This suggests that the distillation scheme works only when the teacher network is higher in accuracy than the student network (and not necessarily bigger in capacity).
Further, the benefit from using a larger teacher network saturates at some point.
This can be seen by picking up a precision point, say "32A, 2W" and looking at the error rates along the row in TAB2 , 2 and 3.One concern, we had in the early stages of our investigation, with joint training of a low-precision small network and a high precision large network was the influence of the small network's accuracy on the accuracy of the large network.
When using the joint cost function, the smaller network's probability scores are matched with the predictions from the teacher network.
The joint cost is added as a term to the total loss function (equation 1).
This led us to posit that the larger network's learning capability will be affected by the inherent impairment in the smaller low-precision network.
Further, since the smaller student network learns form the larger teacher network, a vicious cycle might form where the student network's accuracy will further drop because the teacher network's learning capability is being impeded.
However, in practice, we did not see this phenomenon occurring -in each case where the teacher network was jointly trained with a student network, the accuracy of the teacher network was always within 0.1% to 0.2% of the accuracy of the teacher network without it jointly supervising a student network.
This could be because of our choice of α, β and γ values.In Section 4, we mentioned about temperature, τ , for Softmax function and hyper-parameters α = 1, β = 0.5 and γ = 0.5.
Since, we train directly on the logits of the teacher network, we did not have to experiment with the appropriate value of τ .
τ is required when training on the soft targets produced by the teacher network.
Although we did not do extensive studies experimenting with training on soft targets as opposed to logits, we find that τ = 1 gives us best results when training on soft targets.
BID16 mention that when the student network is significantly smaller than the teacher network, small values of τ are more effective than large values.
For few of the low-precision configurations, we experimented with α = β = γ = 1, and, α = 0.9, β = 1 and γ = 0.1 or 0.3.
Each of these configurations, yielded a lower performance model compared to our original choice for these parameters.For the third term in equation 1, we experimented with a mean-squared error loss function and also a loss function with logits from both the student and the teacher network (i.e. H(z T , z A )).
We did not find any improvement in accuracy compared to our original choice of the cost function formulation.
A thorough investigation of the behavior of the networks with other values of hyper-parameters and different loss functions is an agenda for our future work.Overall, we find the distillation process to be quite effective in getting us high accuracy low-precision models.
All our low-precision models surpass previously reported low-precision accuracy figures.
For example, TTQ scheme achieves 33.4% Top-1 error rate for ResNet-18 with 2-bits weight.
Our best ResNet-18 model, using scheme-A, with 2-bits weight achieves ∼31.5% error rate, improving the model accuracy by ∼2% over TTQ.
Similarly, the scheme in BID22 achieves 29.2% Top-1 error with 2-bits weight and 8-bits activation.
The best performing Apprentice network at this precision achieves 27.2% Top-1 error.
For Scheme-B and Scheme-C, which we describe next, Scheme-A serves as the new baseline.
As mentioned earlier, low-precision is a form of model compression.
There are many works which target network sparsification and pruning techniques to compress a model.
With ternary precision models, the model size reduces by a factor of 2/32 compared to full-precision models.
With Apprentice, we show how one can get a performant model with ternary precision.
Many works targeting network pruning and sparsification target a full-precision model to implement their scheme.To be comparable in model size to ternary networks, a full-precision model needs to be sparsified by 93.75%.
Further, to be effective, a sparse model needs to store a key for every non-zero value denoting the position of the value in the weight tensor.
This adds storage overhead and a sparse model needs to be about 95% sparse to be at-par in memory size as a 2-bit model.
Note that ternary precision also has inherent sparsity (zero is a term in the ternary symbol dictionary) -we find our ternary models to be about 50% sparse.
In work by and BID12 , sparsification of full-precision networks is proposed but the sparsity achieved is less than 93.75%.
Further, the network accuracy using techniques in both these works lead to larger degradation in accuracy compared to our ternary models.
Overall, we believe, our ternary precision models to be state-of-the-art not only in accuracy (we better the accuracy compared to prior ternary precision models) but also when one considers the size of the model at the accuracy level achieved by low-precision or sparse networks.
While low-precision networks have system-level benefits, the drawback of such models is degraded accuracy when compared to full-precision models.
We present three schemes based on knowledge distillation concept to improve the accuracy of low-precision networks and close the gap between the accuracy of these models and full-precision models.
Each of the three schemes improve the accuracy of the low-precision network configuration compared to prior proposals.
We motivate the need for a smaller model size in low batch, real-time and resource constrained inference deployment systems.
We envision the low-precision models produced by our schemes to simplify the inference deployment process on resource constrained systems and on cloud-based deployment systems where low latency is a critical requirement.

assistant: text: We show that knowledge transfer techniques can improve the accuracy of low precision networks and set new state-of-the-art accuracy for ternary and 4-bits precision.