DefaultQuantization Algorithm

Overview

DefaultQuantization algorithm is designed to perform a fast and in many cases accurate 8-bits quantization of NNs.

pot/docs/images/default_quantization_pipeline.png

The algorithm consists of three methods that are sequentially applied to a model:

  • ActivationChannelAlignment - Used as a preliminary step before quantization and allows you to align ranges of output activations of Convolutional layers in order to reduce the quantization error.

  • MinMaxQuantization - This is a vanilla quantization method that automatically inserts FakeQuantize operations into the model graph based on the specified target hardware and initializes them using statistics collected on the calibration dataset.

  • FastBiasCorrection - Adjusts biases of Convolutional and Fully-Connected layers based on the quantization error of the layer in order to make the overall error unbiased.

This algorithm uses a two-stage statistic collection procedure, where the model is being inferred over the calibration subset, so the wall-time of quantization basically depends on the size of the subset.

Parameters

The algorithm accepts all the parameters introduced by three algorithms that it relies on. These parameters should be described in the corresponding section in the configuration file (see example below):

"compression": {
    "algorithms": [
        {
            "name": "DefaultQuantization", // the name of optimization algorithm
            "params": {
                ...
            }
        }
    ]
}

DefaultQuantization algorithm’ parameters can be roughly divided into two groups: mandatory and optional.

Mandatory parameters

  • "preset" - preset which controls the quantization mode (symmetric and asymmetric). It can take two values:

    • "performance" (default) - stands for symmetric quantization of weights and activations. This is the most performant across all the HW.

    • "mixed" - symmetric quantization of weights and asymmetric quantization of activations. This mode can be useful for quantization of NN which has both negative and positive input values in quantizing operations, e.g. non-ReLU based CNN.


  • "stat_subset_size" - size of subset to calculate activations statistics used for quantization. The whole dataset is used if no parameter specified. We recommend using not less than 300 samples.

Optional parameters

All other options can be considered as an advanced mode and require deep knowledge of the quantization process. Below is an overall description of all possible parameters:

  • "model type" - An optional parameter, needed for additional patterns in the model, default value is None (supported only “Transformer” now)

  • "ignored" - NN subgraphs which should be excluded from the optimization process

    • "scope" - list of particular nodes to exclude

    • "operations" - list of operation types to exclude (expressed in OpenVINO IR notation). This list consists of the following tuples:

      • "type" - type of ignored operation

      • "attributes" - if attributes are defined they will be considered during the ignorance. They are defined by a dictionary of "<NAME>": "<VALUE>" pairs.

  • "weights" - this section manually defines quantization scheme for weights and the way to estimate the quantization range for that. It worth noting that changing the quantization scheme may lead to inability to infer such mode on the existing HW.

    • "bits" - bit-width, default is 8

    • "mode" - quantization mode (symmetric or asymmetric)

    • "level_low" - minimum level in the integer range in which we quantize to, default is 0 for unsigned range, -2^(bit-1) - for signed

    • "level_high" - maximum level in the integer range in which we quantize to, default is 2^bits-1 for unsigned range, 2^(bit-1)-1 - for signed

    • "granularity" - quantization scale granularity and can take the following two values:

      • "pertensor" (default) - per-tensor quantization with one scale factor and zero-point

      • "perchannel" - per-channel quantization with per-channel scale factor and zero-point

    • "range_estimator" - this section describes parameters of range estimator that is used in MinMaxQuantization method to get the quantization ranges and filter outliers based on the collected statistics. These are the parameters that user can vary to get better accuracy results:

      • "max" - parameters to estimate top border of quantizing floating-point range:

        • "type" - type of the estimator:

          • "max" (default) - estimates the maximum in the quantizing set of value

          • "quantile" - estimates the quantile in the quantizing set of value

        • "outlier_prob" - outlier probability used in the “quantile” estimator

      • "min" - parameters to estimate bottom border of quantizing floating-point range:

        • "type" - type of the estimator:

          • "min" (default) - estimates the minimum in the quantizing set of value

          • "quantile" - estimates the quantile in the quantizing set of value

        • "outlier_prob" - outlier probability used in the “quantile” estimator

  • "activations" - this section manually defines quantization scheme for activations and the way to estimate the quantization range for that. Again, changing the quantization scheme may lead to inability to infer such mode on the existing HW.

    • "bits" - bit-width, default is 8

    • "mode" - quantization mode (symmetric or asymmetric)

    • "level_low" - minimum level in the integer range in which we quantize to, default is 0 for unsigned range, -2^(bit-1) - for signed

    • "level_high" - maximum level in the integer range in which we quantize to, default is 2^bits-1 for unsigned range, 2^(bit-1)-1 - for signed

    • "granularity" - quantization scale granularity and can take the following two values:

      • "pertensor" (default) - per-tensor quantization with one scale factor and zero-point

      • "perchannel" - per-channel quantization with per-channel scale factor and zero-point

    • "range_estimator" - this section describes parameters of range estimator that is used in MinMaxQuantization method to get the quantization ranges and filter outliers based on the collected statistics. These are the parameters that user can vary to get better accuracy results:

      • "preset" - preset that defines the same estimator both for top and bottom borders of quantizing floating-point range. Possible value is "quantile".

      • "max" - parameters to estimate top border of quantizing floating-point range:

        • "aggregator" - type of the function used to aggregate statistics obtained with estimator over the calibration dataset to get a value of the top border:

          • "mean" (default) - aggregates mean value

          • "max" - aggregates max value

          • "min" - aggregates min value

          • "median" - aggregates median value

          • "mean_no_outliers" - aggregates mean value after removal of extreme quantiles

          • "median_no_outliers" - aggregates median value after removal of extreme quantiles

          • "hl_estimator" - Hodges-Lehmann filter based aggregator

        • "type" - type of the estimator:

          • "max" (default) - estimates the maximum in the quantizing set of value

          • "quantile" - estimates the quantile in the quantizing set of value

        • "outlier_prob" - outlier probability used in the “quantile” estimator

      • "min" - parameters to estimate bottom border of quantizing floating-point range:

        • "type" - type of the estimator:

          • "max" (default) - estimates the maximum in the quantizing set of value

          • "quantile" - estimates the quantile in the quantizing set of value

        • "outlier_prob" - outlier probability used in the “quantile” estimator

  • "use_layerwise_tuning" - enables layer-wise fine-tuning of model parameters (biases, Convolution/MatMul weights and FakeQuantize scales) by minimizing the mean squared error between original and quantized layer outputs. Enabling this option may increase compressed model accuracy, but will result in increased execution time and memory consumption.

    Below is a fragment of the configuration file that shows overall structure of parameters for this algorithm.

"compression": {
    "model_type": "None",   //  An optional parameter, needed for additional patterns in the model,
                                default value is None (supported only "Transformer" now)
    "algorithms": [
        "name": "DefaultQuantization", // optimization algorithm name
        "params": {
                /* Preset is a collection of optimization algorithm parameters that will specify to the algorithm
                to improve which metric the algorithm needs to concentrate. Each optimization algorithm supports
                [performance, mixed, accuracy] presets which control the quantization mode (symmetric, mixed(weights symmetric and activations asymmetric), and fully asymmetric respectively)*/
                "preset": "mixed",
                "stat_subset_size": 300, // Size of subset to calculate activations statistics that can be used
                                         // For quantization parameters calculation.
                "ignored": {
                    "scope": [
                        "<NODE_NAME>" // List of nodes that are excluded from optimization
                    ],
                    "operations": [ // List of types that are excluded from optimization
                        {
                            "type": "<NODE_TYPE>", // Type of ignored operation
                            "attributes": { // If attributes are defined they will be considered during the ignorance
                                "<NAME>": "<VALUE>" // Lists of values to filter by
                            }
                        }
                    ]
                },
                /* Manually specified quantization parameters */
                /* Quantization parameters for weights */
                "weights": {  // Weights quantization parameters used by MinMaxAlgorithm
                    "bits": 8, // Bit-width, default is 8
                    "mode": "symmetric", // Quantization mode, default is "symmetric"
                    "level_low": 0,      // Minimum level in the integer range in which we quantize to, default is 0 for unsigned range, -2^(bit-1) - for signed
                    "level_high": 255,   // Maximum level in the integer range in which we quantize to, default is 2^bits-1 for unsigned range, 2^(bit-1)-1 - for signed
                    "granularity": "perchannel", // Quantization scale granularity: ["pertensor" (default), "perchannel"]
                    "range_estimator": {         // Range estimator that is used to get the quantization ranges and filter outliers based on the statistics
                        "max": {                 // Parameters to estimate top quantization border
                            "type": "quantile",    // Estimator type: ["max" (default), "quantile"]
                            "outlier_prob": 0.0001 // Outlier probability used in the "quantile" estimator
                        },
                        "min": {                   // Parameters to estimate bottom quantization border (used only in asymmetric mode)
                            "type": "quantile",    // Estimator type: ["max" (default), "quantile"]
                            "outlier_prob": 0.0001 // Outlier probability used in the "quantile" estimator
                        }

                    }
                },
                /* Quantization parameters for activations */
                "activations": {
                    "bits": 8, // Number of quantization bits
                    "mode": "symmetric", // Quantization mode
                    "granularity": "pertensor", // Granularity: one scale for output tensor
                    "range_estimator": {           // Range estimator that is used to get the quantization ranges and filter outliers based on the statistics
                        "preset": "quantile",
                        /* OR */
                        /* minimum of quantization range */
                        /* maximum of quantization range */
                        "max": {                   // Parameters to estimate top quantization border
                            "aggregator": "mean",  // Batch aggregation type: ["mean" (default), "max", "min", "median", "mean_no_outliers", "median_no_outliers", "hl_estimator"]
                            "type": "quantile",    // Estimator type: ["max" (default), "quantile"]
                            "outlier_prob": 0.0001 // Outlier probability used in the "quantile" estimator
                        },
                        "min": {                   // Parameters to estimate top quantization border
                            "aggregator": "mean",  // Batch aggregation type: ["mean" (default), "max", "min", "median", "mean_no_outliers", "median_no_outliers", "hl_estimator"]
                            "type": "quantile",    // Estimator type [min, max, abs_max, quantile, abs_quantile]
                            "outlier_prob": 0.0001 // Outlier probability used in the "quantile" estimator
                        }
                    }
                }
                "use_layerwise_tuning": false // An optional parameter, enables layer-wise fine-tuning, false by default
            }
        ]
    }