Quantization of Deep Neural Networks

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Most pretrained neural networks and neural networks trained using Deep Learning Toolbox™ use single-precision floating point data types. As such, even small trained neural networks require a considerable amount of memory and hardware that can perform floating-point arithmetic. These restrictions can inhibit deployment of deep learning capabilities to low-power microcontrollers and FPGAs.

You can use the Deep Learning Toolbox Model Compression Library to quantize a network to use 8-bit scaled integer data types. Converting to integer data types reduces the memory of parameters in your network and facilitates deploying your network to hardware that performs fixed-point arithmetic.

The Deep Learning Toolbox Model Compression Library support package also facilitates structural compression techniques like projection and pruning of deep neural networks to reduce their memory requirements. Structural compression reduces the memory of a neural network by reducing the total number of parameters. For more information about other model compression techniques, see Reduce Memory Footprint of Deep Neural Networks.

Benefits of Quantization

When deploying deep learning models to hardware, especially those with data type and memory constraints, quantization improves network performance by reducing peak activation energy of the network by up to a factor of four. Network accuracy is largely maintained despite the conversion to lower-precision data types.

To learn more about the scaling, precision, and ranges of the 8-bit scaled integer data types used in the Deep Learning Toolbox Model Compression Library support package, see Data Types and Scaling for Quantization of Deep Neural Networks.

Quantization Workflow Overview

The pre-deployment quantization workflow is similar for all intended deployment hardware: create and prepare a quantization object, calibrate, quantize, and validate. You can complete these steps at the command line or in the Deep Network Quantizer app. Both options follow the same steps, but the app provides additional visualization of the data types.

Before starting the quantization workflow, consider using the structural compression techniques of pruning and projection. For more information on compression techniques in the Deep Learning Toolbox Model Compression Library, see Reduce Memory Footprint of Deep Neural Networks.

For information about maximum possible memory reduction and layer support for pruning, projection, and quantization for your network, analyze your network for compression in the Deep Network Designer app.

Create and Prepare Quantization Object

To start the quantization workflow, create a dlquantizer object with your deep neural network. Select the execution environment that corresponds to your deployment hardware.

• For microcontrollers or CPUs, such as the ARM^® Cortex^®-M or ARM Cortex-A, select "MATLAB".

• For NVIDIA^® GPUs, select "GPU".

• For Intel^® or Xilinx^® FPGA boards, select "FPGA".

Prepare your network before calibration using the prepareNetwork function. Network preparation modifies your neural network to improve performance and avoid error conditions in the quantization workflow.

In the Deep Network Quantizer app, create a dlquantizer object by clicking New and selecting Quantize a Network. To prepare the network, select Prepare network for quantization.

Calibrate

To collect the dynamic ranges of the learnable parameters of your network, calibrate the dlquantizer object using the calibrate function. Calibration helps determine data types that will cover the range, avoid overflow, and allow underflow since scaled 8-bit integer data types have limited precision and range when compared to single-precision floating point data types. For the best quantization results, the calibration data must be representative of inputs to the network.

In the Deep Network Quantizer app, select your calibration data, then click Calibrate. When the calibration is complete, the app displays a table of the minimum and maximum values of the learnable parameters of the network. To the right of the table, the app displays histograms of the dynamic ranges of the parameters. To learn more about the histograms, see Data Types and Scaling for Quantization of Deep Neural Networks.

Quantize

Quantize the dlquantizer object using the quantize function. This action produces a simulatable quantized network.

In the Deep Network Quantizer app, click Quantize. When quantization is complete, the histograms of the dynamic ranges update to show the data that can be represented with quantized representation. Data that cannot be represented is indicated with a gray region.

Validate

To determine the accuracy of your quantized network, validate the dlquantizer object using the validate function. For an example on how to validate a quantized network in the command line, see Write a Custom Metric Function for Quantized Network Validation.

In the Deep Network Quantizer app, select a validation scheme and click Validate. For an example on how to validate a quantized network in the app, see Write a Custom Metric Function in the Deep Network Quantizer App.

Quantization for Microcontroller Deployment

For deploying quantized networks to a microcontroller or CPU, the generated code is generic C/C++, which allows flexibility in hardware choices. However, ARM Cortex-M or ARM Cortex-A are the recommended hardware.

To generate C code for a quantized network, use one of these approaches.

• Generate C code in MATLAB^® with the codegen (MATLAB Coder) function from MATLAB Coder™. For an example of this workflow, see Generate C Code from Quantized Networks in MATLAB. For more details about support for neural networks for this workflow, see Networks and Layers Supported for Code Generation (MATLAB Coder).

• Generate C code in Simulink^® from Deep Learning Network blocks with Embedded Coder^® or Simulink Coder. For an example of this workflow, see Generate C Code for Quantized Networks with Simulink Network Blocks. For more details about support for neural networks for this workflow, see Networks and Layers Supported for Code Generation (MATLAB Coder).

• Generate C code in Simulink from Deep Learning Layers blocks with Embedded Coder or Simulink Coder. For an example of this workflow, see Export Quantized Networks to Simulink and Generate Code. For more details about support for neural networks for this workflow, see List of Deep Learning Layer Blocks and Subsystems.

Quantization for GPU Deployment

For deploying quantized networks to a GPU, the Deep Learning Toolbox Model Compression Library supports NVIDIA GPUs. For more information on supported hardware, see GPU Coder Supported Hardware (GPU Coder).

To deploy a quantized network to a GPU:

• Follow the pre-deployment workflow steps in Quantization Workflow Overview, with the execution environment set to "GPU" when you create the dlquantizer object.

• Provide the calibration results file from the calibrate function to the codegen (MATLAB Coder) command.

For more information on generating code for GPU deployment, see Generate INT8 Code for Deep Learning Networks (GPU Coder).

For an example of quantization for GPU deployment, see Quantize Semantic Segmentation Network and Generate CUDA Code.

Quantization for FPGA Deployment

For deploying quantized networks to an FPGA board, the Deep Learning Toolbox Model Compression Library supports Intel and Xilinx FPGA boards. For more information on supported hardware, see Supported Networks, Boards, and Tools (Deep Learning HDL Toolbox).

To deploy a quantized network to an FPGA board:

• Follow the pre-deployment workflow steps in Quantization Workflow Overview, with the execution environment set to "FPGA" when you create the dlquantizer object.

• Provide the calibrated dlquantizer object to the dlhdl.Workflow (Deep Learning HDL Toolbox) class.

For more information on generating code for FPGA deployment, see Code Generation and Deployment (Deep Learning HDL Toolbox).

For an example of quantization for FPGA deployment, see Classify Images on FPGA by Using Quantized GoogLeNet Network (Deep Learning HDL Toolbox).

Other Considerations for Quantization

Prerequisites

To learn about the products required to quantize and deploy a deep learning network, see Quantization Workflow System Requirements.

For information on the layers and networks supported for quantization, see Supported Layers for Quantization.

To learn how to prepare your data set for the quantization workflow, see Prepare Data for Quantizing Networks.

Additional Tools

These tools can help you throughout the quantization workflow.

• Analyze for compression in the Deep Network Designer app for information about maximum possible memory reduction and layer support for projection, pruning, and quantization of your network.

• Estimate network metrics for neural network layers with the estimateNetworkMetrics function.

• Equalize layer parameters with the equalizeLayers function. Note that the prepareNetwork function equalizes layer parameters using the same method as equalizeLayers.

Additional Compression Methods

Quantization is one of three compression methods in the Deep Learning Toolbox Model Compression Library. For greater overall compression of your model, consider pruning and projecting in addition to quantization. To learn more about model compression techniques, see Reduce Memory Footprint of Deep Neural Networks. For an example that combines pruning, projection, and quantization, see Train and Compress AI Model for Road Damage Detection.

CPU Execution Environment

The "CPU" execution environment is available for the quantization workflow. You can use the CPU execution environment to generate code dependent on the ARM Compute Library, but the execution environment has limited supported layers and supported methods. The "MATLAB" execution environment is recommended for use to deploy to CPU devices.