Quantization¶

Warning

Quantization is in beta and subject to change.

Introduction to Quantization¶

Quantization refers to techniques for performing computations and storing tensors at lower bitwidths than floating point precision. A quantized model executes some or all of the operations on tensors with reduced precision rather than full precision (floating point) values. This allows for a more compact model representation and the use of high performance vectorized operations on many hardware platforms. PyTorch supports INT8 quantization compared to typical FP32 models allowing for a 4x reduction in the model size and a 4x reduction in memory bandwidth requirements. Hardware support for INT8 computations is typically 2 to 4 times faster compared to FP32 compute. Quantization is primarily a technique to speed up inference and only the forward pass is supported for quantized operators.

PyTorch supports multiple approaches to quantizing a deep learning model. In most cases the model is trained in FP32 and then the model is converted to INT8. In addition, PyTorch also supports quantization aware training, which models quantization errors in both the forward and backward passes using fake-quantization modules. Note that the entire computation is carried out in floating point. At the end of quantization aware training, PyTorch provides conversion functions to convert the trained model into lower precision.

At lower level, PyTorch provides a way to represent quantized tensors and perform operations with them. They can be used to directly construct models that perform all or part of the computation in lower precision. Higher-level APIs are provided that incorporate typical workflows of converting FP32 model to lower precision with minimal accuracy loss.

Quantization API Summary¶

PyTorch provides three different modes of quantization: Eager Mode Quantization, FX Graph Mode Quantization (maintenance) and PyTorch 2 Export Quantization.

Eager Mode Quantization is a beta feature. User needs to do fusion and specify where quantization and dequantization happens manually, also it only supports modules and not functionals.

FX Graph Mode Quantization is an automated quantization workflow in PyTorch, and currently it’s a prototype feature, it is in maintenance mode since we have PyTorch 2 Export Quantization. It improves upon Eager Mode Quantization by adding support for functionals and automating the quantization process, although people might need to refactor the model to make the model compatible with FX Graph Mode Quantization (symbolically traceable with torch.fx). Note that FX Graph Mode Quantization is not expected to work on arbitrary models since the model might not be symbolically traceable, we will integrate it into domain libraries like torchvision and users will be able to quantize models similar to the ones in supported domain libraries with FX Graph Mode Quantization. For arbitrary models we’ll provide general guidelines, but to actually make it work, users might need to be familiar with torch.fx, especially on how to make a model symbolically traceable.

PyTorch 2 Export Quantization is the new full graph mode quantization workflow, released as prototype feature in PyTorch 2.1. With PyTorch 2, we are moving to a better solution for full program capture (torch.export) since it can capture a higher percentage (88.8% on 14K models) of models compared to torch.fx.symbolic_trace (72.7% on 14K models), the program capture solution used by FX Graph Mode Quantization. torch.export still has limitations around some python constructs and requires user involvement to support dynamism in the exported model, but overall it is an improvement over the previous program capture solution. PyTorch 2 Export Quantization is built for models captured by torch.export, with flexibility and productivity of both modeling users and backend developers in mind. The main features are (1). Programmable API for configuring how a model is quantized that can scale to many more use cases (2). Simplified UX for modeling users and backend developers since they only need to interact with a single object (Quantizer) for expressing user’s intention about how to quantize a model and what the backend support. (3). Optional reference quantized model representation that can represent quantized computation with integer operations that maps closer to actual quantized computations that happens in hardware.

New users of quantization are encouraged to try out PyTorch 2 Export Quantization first, if it does not work well, user can try eager mode quantization.

The following table compares the differences between Eager Mode Quantization, FX Graph Mode Quantization and PyTorch 2 Export Quantization:

	Eager Mode Quantization	FX Graph Mode Quantization	PyTorch 2 Export Quantization
Release Status	beta	prototype (maintenance)	prototype
Operator Fusion	Manual	Automatic	Automatic
Quant/DeQuant Placement	Manual	Automatic	Automatic
Quantizing Modules	Supported	Supported	Supported
Quantizing Functionals/Torch Ops	Manual	Automatic	Supported
Support for Customization	Limited Support	Fully Supported	Fully Supported
Quantization Mode Support	Post Training Quantization: Static, Dynamic, Weight Only Quantization Aware Training: Static	Post Training Quantization: Static, Dynamic, Weight Only Quantization Aware Training: Static	Defined by Backend Specific Quantizer
Input/Output Model Type	`torch.nn.Module`	`torch.nn.Module` (May need some refactors to make the model compatible with FX Graph Mode Quantization)	`torch.fx.GraphModule` (captured by `torch.export`

There are three types of quantization supported:

dynamic quantization (weights quantized with activations read/stored in floating point and quantized for compute)
static quantization (weights quantized, activations quantized, calibration required post training)
static quantization aware training (weights quantized, activations quantized, quantization numerics modeled during training)

Please see our Introduction to Quantization on PyTorch blog post for a more comprehensive overview of the tradeoffs between these quantization types.

Operator coverage varies between dynamic and static quantization and is captured in the table below.

	Static Quantization	Dynamic Quantization
nn.Linear nn.Conv1d/2d/3d	Y Y	Y N
nn.LSTM nn.GRU	Y (through custom modules) N	Y Y
nn.RNNCell nn.GRUCell nn.LSTMCell	N N N	Y Y Y
nn.EmbeddingBag	Y (activations are in fp32)	Y
nn.Embedding	Y	Y
nn.MultiheadAttention	Y (through custom modules)	Not supported
Activations	Broadly supported	Un-changed, computations stay in fp32

Eager Mode Quantization¶

For a general introduction to the quantization flow, including different types of quantization, please take a look at General Quantization Flow.

Post Training Dynamic Quantization¶

This is the simplest to apply form of quantization where the weights are quantized ahead of time but the activations are dynamically quantized during inference. This is used for situations where the model execution time is dominated by loading weights from memory rather than computing the matrix multiplications. This is true for LSTM and Transformer type models with small batch size.

Diagram:

# original model
# all tensors and computations are in floating point
previous_layer_fp32 -- linear_fp32 -- activation_fp32 -- next_layer_fp32
                 /
linear_weight_fp32

# dynamically quantized model
# linear and LSTM weights are in int8
previous_layer_fp32 -- linear_int8_w_fp32_inp -- activation_fp32 -- next_layer_fp32
                     /
   linear_weight_int8

PTDQ API Example:

import torch

# define a floating point model
class M(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.fc = torch.nn.Linear(4, 4)

    def forward(self, x):
        x = self.fc(x)
        return x

# create a model instance
model_fp32 = M()
# create a quantized model instance
model_int8 = torch.ao.quantization.quantize_dynamic(
    model_fp32,  # the original model
    {torch.nn.Linear},  # a set of layers to dynamically quantize
    dtype=torch.qint8)  # the target dtype for quantized weights

# run the model
input_fp32 = torch.randn(4, 4, 4, 4)
res = model_int8(input_fp32)

To learn more about dynamic quantization please see our dynamic quantization tutorial.

Post Training Static Quantization¶

Post Training Static Quantization (PTQ static) quantizes the weights and activations of the model. It fuses activations into preceding layers where possible. It requires calibration with a representative dataset to determine optimal quantization parameters for activations. Post Training Static Quantization is typically used when both memory bandwidth and compute savings are important with CNNs being a typical use case.

We may need to modify the model before applying post training static quantization. Please see Model Preparation for Eager Mode Static Quantization.

Diagram:

# original model
# all tensors and computations are in floating point
previous_layer_fp32 -- linear_fp32 -- activation_fp32 -- next_layer_fp32
                    /
    linear_weight_fp32

# statically quantized model
# weights and activations are in int8
previous_layer_int8 -- linear_with_activation_int8 -- next_layer_int8
                    /
  linear_weight_int8

PTSQ API Example:

import torch

# define a floating point model where some layers could be statically quantized
class M(torch.nn.Module):
    def __init__(self):
        super().__init__()
        # QuantStub converts tensors from floating point to quantized
        self.quant = torch.ao.quantization.QuantStub()
        self.conv = torch.nn.Conv2d(1, 1, 1)
        self.relu = torch.nn.ReLU()
        # DeQuantStub converts tensors from quantized to floating point
        self.dequant = torch.ao.quantization.DeQuantStub()

    def forward(self, x):
        # manually specify where tensors will be converted from floating
        # point to quantized in the quantized model
        x = self.quant(x)
        x = self.conv(x)
        x = self.relu(x)
        # manually specify where tensors will be converted from quantized
        # to floating point in the quantized model
        x = self.dequant(x)
        return x

# create a model instance
model_fp32 = M()

# model must be set to eval mode for static quantization logic to work
model_fp32.eval()

# attach a global qconfig, which contains information about what kind
# of observers to attach. Use 'x86' for server inference and 'qnnpack'
# for mobile inference. Other quantization configurations such as selecting
# symmetric or asymmetric quantization and MinMax or L2Norm calibration techniques
# can be specified here.
# Note: the old 'fbgemm' is still available but 'x86' is the recommended default
# for server inference.
# model_fp32.qconfig = torch.ao.quantization.get_default_qconfig('fbgemm')
model_fp32.qconfig = torch.ao.quantization.get_default_qconfig('x86')

# Fuse the activations to preceding layers, where applicable.
# This needs to be done manually depending on the model architecture.
# Common fusions include `conv + relu` and `conv + batchnorm + relu`
model_fp32_fused = torch.ao.quantization.fuse_modules(model_fp32, [['conv', 'relu']])

# Prepare the model for static quantization. This inserts observers in
# the model that will observe activation tensors during calibration.
model_fp32_prepared = torch.ao.quantization.prepare(model_fp32_fused)

# calibrate the prepared model to determine quantization parameters for activations
# in a real world setting, the calibration would be done with a representative dataset
input_fp32 = torch.randn(4, 1, 4, 4)
model_fp32_prepared(input_fp32)

# Convert the observed model to a quantized model. This does several things:
# quantizes the weights, computes and stores the scale and bias value to be
# used with each activation tensor, and replaces key operators with quantized
# implementations.
model_int8 = torch.ao.quantization.convert(model_fp32_prepared)

# run the model, relevant calculations will happen in int8
res = model_int8(input_fp32)

To learn more about static quantization, please see the static quantization tutorial.

Quantization Aware Training for Static Quantization¶

Quantization Aware Training (QAT) models the effects of quantization during training allowing for higher accuracy compared to other quantization methods. We can do QAT for static, dynamic or weight only quantization. During training, all calculations are done in floating point, with fake_quant modules modeling the effects of quantization by clamping and rounding to simulate the effects of INT8. After model conversion, weights and activations are quantized, and activations are fused into the preceding layer where possible. It is commonly used with CNNs and yields a higher accuracy compared to static quantization.

We may need to modify the model before applying post training static quantization. Please see Model Preparation for Eager Mode Static Quantization.

Diagram:

# original model
# all tensors and computations are in floating point
previous_layer_fp32 -- linear_fp32 -- activation_fp32 -- next_layer_fp32
                      /
    linear_weight_fp32

# model with fake_quants for modeling quantization numerics during training
previous_layer_fp32 -- fq -- linear_fp32 -- activation_fp32 -- fq -- next_layer_fp32
                           /
   linear_weight_fp32 -- fq

# quantized model
# weights and activations are in int8
previous_layer_int8 -- linear_with_activation_int8 -- next_layer_int8
                     /
   linear_weight_int8

QAT API Example:

import torch

# define a floating point model where some layers could benefit from QAT
class M(torch.nn.Module):
    def __init__(self):
        super().__init__()
        # QuantStub converts tensors from floating point to quantized
        self.quant = torch.ao.quantization.QuantStub()
        self.conv = torch.nn.Conv2d(1, 1, 1)
        self.bn = torch.nn.BatchNorm2d(1)
        self.relu = torch.nn.ReLU()
        # DeQuantStub converts tensors from quantized to floating point
        self.dequant = torch.ao.quantization.DeQuantStub()

    def forward(self, x):
        x = self.quant(x)
        x = self.conv(x)
        x = self.bn(x)
        x = self.relu(x)
        x = self.dequant(x)
        return x

# create a model instance
model_fp32 = M()

# model must be set to eval for fusion to work
model_fp32.eval()

# attach a global qconfig, which contains information about what kind
# of observers to attach. Use 'x86' for server inference and 'qnnpack'
# for mobile inference. Other quantization configurations such as selecting
# symmetric or asymmetric quantization and MinMax or L2Norm calibration techniques
# can be specified here.
# Note: the old 'fbgemm' is still available but 'x86' is the recommended default
# for server inference.
# model_fp32.qconfig = torch.ao.quantization.get_default_qconfig('fbgemm')
model_fp32.qconfig = torch.ao.quantization.get_default_qat_qconfig('x86')

# fuse the activations to preceding layers, where applicable
# this needs to be done manually depending on the model architecture
model_fp32_fused = torch.ao.quantization.fuse_modules(model_fp32,
    [['conv', 'bn', 'relu']])

# Prepare the model for QAT. This inserts observers and fake_quants in
# the model needs to be set to train for QAT logic to work
# the model that will observe weight and activation tensors during calibration.
model_fp32_prepared = torch.ao.quantization.prepare_qat(model_fp32_fused.train())

# run the training loop (not shown)
training_loop(model_fp32_prepared)

# Convert the observed model to a quantized model. This does several things:
# quantizes the weights, computes and stores the scale and bias value to be
# used with each activation tensor, fuses modules where appropriate,
# and replaces key operators with quantized implementations.
model_fp32_prepared.eval()
model_int8 = torch.ao.quantization.convert(model_fp32_prepared)

# run the model, relevant calculations will happen in int8
res = model_int8(input_fp32)

To learn more about quantization aware training, please see the QAT tutorial.

Model Preparation for Eager Mode Static Quantization¶

It is necessary to currently make some modifications to the model definition prior to Eager mode quantization. This is because currently quantization works on a module by module basis. Specifically, for all quantization techniques, the user needs to:

Convert any operations that require output requantization (and thus have additional parameters) from functionals to module form (for example, using torch.nn.ReLU instead of torch.nn.functional.relu).
Specify which parts of the model need to be quantized either by assigning .qconfig attributes on submodules or by specifying qconfig_mapping. For example, setting model.conv1.qconfig = None means that the model.conv layer will not be quantized, and setting model.linear1.qconfig = custom_qconfig means that the quantization settings for model.linear1 will be using custom_qconfig instead of the global qconfig.

For static quantization techniques which quantize activations, the user needs to do the following in addition:

Specify where activations are quantized and de-quantized. This is done using QuantStub and DeQuantStub modules.
Use FloatFunctional to wrap tensor operations that require special handling for quantization into modules. Examples are operations like add and cat which require special handling to determine output quantization parameters.
Fuse modules: combine operations/modules into a single module to obtain higher accuracy and performance. This is done using the fuse_modules() API, which takes in lists of modules to be fused. We currently support the following fusions: [Conv, Relu], [Conv, BatchNorm], [Conv, BatchNorm, Relu], [Linear, Relu]

(Prototype - maintenance mode) FX Graph Mode Quantization¶

There are multiple quantization types in post training quantization (weight only, dynamic and static) and the configuration is done through qconfig_mapping (an argument of the prepare_fx function).

FXPTQ API Example:

import torch
from torch.ao.quantization import (
  get_default_qconfig_mapping,
  get_default_qat_qconfig_mapping,
  QConfigMapping,
)
import torch.ao.quantization.quantize_fx as quantize_fx
import copy

model_fp = UserModel()

#
# post training dynamic/weight_only quantization
#

# we need to deepcopy if we still want to keep model_fp unchanged after quantization since quantization apis change the input model
model_to_quantize = copy.deepcopy(model_fp)
model_to_quantize.eval()
qconfig_mapping = QConfigMapping().set_global(torch.ao.quantization.default_dynamic_qconfig)
# a tuple of one or more example inputs are needed to trace the model
example_inputs = (input_fp32)
# prepare
model_prepared = quantize_fx.prepare_fx(model_to_quantize, qconfig_mapping, example_inputs)
# no calibration needed when we only have dynamic/weight_only quantization
# quantize
model_quantized = quantize_fx.convert_fx(model_prepared)

#
# post training static quantization
#

model_to_quantize = copy.deepcopy(model_fp)
qconfig_mapping = get_default_qconfig_mapping("qnnpack")
model_to_quantize.eval()
# prepare
model_prepared = quantize_fx.prepare_fx(model_to_quantize, qconfig_mapping, example_inputs)
# calibrate (not shown)
# quantize
model_quantized = quantize_fx.convert_fx(model_prepared)

#
# quantization aware training for static quantization
#

model_to_quantize = copy.deepcopy(model_fp)
qconfig_mapping = get_default_qat_qconfig_mapping("qnnpack")
model_to_quantize.train()
# prepare
model_prepared = quantize_fx.prepare_qat_fx(model_to_quantize, qconfig_mapping, example_inputs)
# training loop (not shown)
# quantize
model_quantized = quantize_fx.convert_fx(model_prepared)

#
# fusion
#
model_to_quantize = copy.deepcopy(model_fp)
model_fused = quantize_fx.fuse_fx(model_to_quantize)

Please follow the tutorials below to learn more about FX Graph Mode Quantization:

(Prototype) PyTorch 2 Export Quantization¶

API Example:

import torch
from torch.ao.quantization.quantize_pt2e import prepare_pt2e
from torch._export import capture_pre_autograd_graph
from torch.ao.quantization.quantizer import (
    XNNPACKQuantizer,
    get_symmetric_quantization_config,
)

class M(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.linear = torch.nn.Linear(5, 10)

   def forward(self, x):
       return self.linear(x)

# initialize a floating point model
float_model = M().eval()

# define calibration function
def calibrate(model, data_loader):
    model.eval()
    with torch.no_grad():
        for image, target in data_loader:
            model(image)

# Step 1. program capture
# NOTE: this API will be updated to torch.export API in the future, but the captured
# result should mostly stay the same
m = capture_pre_autograd_graph(m, *example_inputs)
# we get a model with aten ops

# Step 2. quantization
# backend developer will write their own Quantizer and expose methods to allow
# users to express how they
# want the model to be quantized
quantizer = XNNPACKQuantizer().set_global(get_symmetric_quantization_config())
# or prepare_qat_pt2e for Quantization Aware Training
m = prepare_pt2e(m, quantizer)

# run calibration
# calibrate(m, sample_inference_data)
m = convert_pt2e(m)

# Step 3. lowering
# lower to target backend

Please follow these tutorials to get started on PyTorch 2 Export Quantization:

Modeling Users:

Backend Developers (please check out all Modeling Users docs as well):

How to Write a Quantizer for PyTorch 2 Export Quantization

Quantization Stack¶

Quantization is the process to convert a floating point model to a quantized model. So at high level the quantization stack can be split into two parts: 1). The building blocks or abstractions for a quantized model 2). The building blocks or abstractions for the quantization flow that converts a floating point model to a quantized model

Quantized Model¶

Quantized Tensor¶

In order to do quantization in PyTorch, we need to be able to represent quantized data in Tensors. A Quantized Tensor allows for storing quantized data (represented as int8/uint8/int32) along with quantization parameters like scale and zero_point. Quantized Tensors allow for many useful operations making quantized arithmetic easy, in addition to allowing for serialization of data in a quantized format.

PyTorch supports both per tensor and per channel symmetric and asymmetric quantization. Per tensor means that all the values within the tensor are quantized the same way with the same quantization parameters. Per channel means that for each dimension, typically the channel dimension of a tensor, the values in the tensor are quantized with different quantization parameters. This allows for less error in converting tensors to quantized values since outlier values would only impact the channel it was in, instead of the entire Tensor.

The mapping is performed by converting the floating point tensors using

$_images/math-quantizer-equation.png$

Note that, we ensure that zero in floating point is represented with no error after quantization, thereby ensuring that operations like padding do not cause additional quantization error.

Here are a few key attributes for quantized Tensor:

QScheme (torch.qscheme): a enum that specifies the way we quantize the Tensor
- torch.per_tensor_affine
- torch.per_tensor_symmetric
- torch.per_channel_affine
- torch.per_channel_symmetric
dtype (torch.dtype): data type of the quantized Tensor
- torch.quint8
- torch.qint8
- torch.qint32
- torch.float16
quantization parameters (varies based on QScheme): parameters for the chosen way of quantization
- torch.per_tensor_affine would have quantization parameters of
  - scale (float)
  - zero_point (int)
- torch.per_channel_affine would have quantization parameters of
  - per_channel_scales (list of float)
  - per_channel_zero_points (list of int)
  - axis (int)

Quantize and Dequantize¶

The input and output of a model are floating point Tensors, but activations in the quantized model are quantized, so we need operators to convert between floating point and quantized Tensors.

Quantize (float -> quantized)
- torch.quantize_per_tensor(x, scale, zero_point, dtype)
- torch.quantize_per_channel(x, scales, zero_points, axis, dtype)
- torch.quantize_per_tensor_dynamic(x, dtype, reduce_range)
- to(torch.float16)
Dequantize (quantized -> float)
- quantized_tensor.dequantize() - calling dequantize on a torch.float16 Tensor will convert the Tensor back to torch.float
- torch.dequantize(x)

Quantized Operators/Modules¶

Quantized Operator are the operators that takes quantized Tensor as inputs, and outputs a quantized Tensor.
Quantized Modules are PyTorch Modules that performs quantized operations. They are typically defined for weighted operations like linear and conv.

Quantized Engine¶

When a quantized model is executed, the qengine (torch.backends.quantized.engine) specifies which backend is to be used for execution. It is important to ensure that the qengine is compatible with the quantized model in terms of value range of quantized activation and weights.

Quantization Flow¶

Observer and FakeQuantize¶

Observer are PyTorch Modules used to:
- collect tensor statistics like min value and max value of the Tensor passing through the observer
- and calculate quantization parameters based on the collected tensor statistics
FakeQuantize are PyTorch Modules used to:
- simulate quantization (performing quantize/dequantize) for a Tensor in the network
- it can calculate quantization parameters based on the collected statistics from observer, or it can learn the quantization parameters as well

QConfig¶

QConfig is a namedtuple of Observer or FakeQuantize Module class that can are configurable with qscheme, dtype etc. it is used to configure how an operator should be observed
- Quantization configuration for an operator/module
  - different types of Observer/FakeQuantize
  - dtype
  - qscheme
  - quant_min/quant_max: can be used to simulate lower precision Tensors
- Currently supports configuration for activation and weight
- We insert input/weight/output observer based on the qconfig that is configured for a given operator or module

General Quantization Flow¶

In general, the flow is the following

prepare
- insert Observer/FakeQuantize modules based on user specified qconfig
calibrate/train (depending on post training quantization or quantization aware training)
- allow Observers to collect statistics or FakeQuantize modules to learn the quantization parameters
convert
- convert a calibrated/trained model to a quantized model

There are different modes of quantization, they can be classified in two ways:

In terms of where we apply the quantization flow, we have:

Post Training Quantization (apply quantization after training, quantization parameters are calculated based on sample calibration data)
Quantization Aware Training (simulate quantization during training so that the quantization parameters can be learned together with the model using training data)

And in terms of how we quantize the operators, we can have:

Weight Only Quantization (only weight is statically quantized)
Dynamic Quantization (weight is statically quantized, activation is dynamically quantized)
Static Quantization (both weight and activations are statically quantized)

We can mix different ways of quantizing operators in the same quantization flow. For example, we can have post training quantization that has both statically and dynamically quantized operators.

Quantization Support Matrix¶

Quantization Mode Support¶

	Quantization Mode		Dataset Requirement	Works Best For	Accuracy	Notes
Post Training Quantization	Dynamic/Weight Only Quantization	activation dynamically quantized (fp16, int8) or not quantized, weight statically quantized (fp16, int8, in4)	None	LSTM, MLP, Embedding, Transformer	good	Easy to use, close to static quantization when performance is compute or memory bound due to weights
Post Training Quantization	Static Quantization	activation and weights statically quantized (int8)	calibration dataset	CNN	good	Provides best perf, may have big impact on accuracy, good for hardwares that only support int8 computation
Quantization Aware Training	Dynamic Quantization	activation and weight are fake quantized	fine-tuning dataset	MLP, Embedding	best	Limited support for now
Quantization Aware Training	Static Quantization	activation and weight are fake quantized	fine-tuning dataset	CNN, MLP, Embedding	best	Typically used when static quantization leads to bad accuracy, and used to close the accuracy gap

Please see our Introduction to Quantization on Pytorch blog post for a more comprehensive overview of the tradeoffs between these quantization types.

Quantization Flow Support¶

PyTorch provides two modes of quantization: Eager Mode Quantization and FX Graph Mode Quantization.

Eager Mode Quantization is a beta feature. User needs to do fusion and specify where quantization and dequantization happens manually, also it only supports modules and not functionals.

FX Graph Mode Quantization is an automated quantization framework in PyTorch, and currently it’s a prototype feature. It improves upon Eager Mode Quantization by adding support for functionals and automating the quantization process, although people might need to refactor the model to make the model compatible with FX Graph Mode Quantization (symbolically traceable with torch.fx). Note that FX Graph Mode Quantization is not expected to work on arbitrary models since the model might not be symbolically traceable, we will integrate it into domain libraries like torchvision and users will be able to quantize models similar to the ones in supported domain libraries with FX Graph Mode Quantization. For arbitrary models we’ll provide general guidelines, but to actually make it work, users might need to be familiar with torch.fx, especially on how to make a model symbolically traceable.

New users of quantization are encouraged to try out FX Graph Mode Quantization first, if it does not work, user may try to follow the guideline of using FX Graph Mode Quantization or fall back to eager mode quantization.

The following table compares the differences between Eager Mode Quantization and FX Graph Mode Quantization:

	Eager Mode Quantization	FX Graph Mode Quantization
Release Status	beta	prototype
Operator Fusion	Manual	Automatic
Quant/DeQuant Placement	Manual	Automatic
Quantizing Modules	Supported	Supported
Quantizing Functionals/Torch Ops	Manual	Automatic
Support for Customization	Limited Support	Fully Supported
Quantization Mode Support	Post Training Quantization: Static, Dynamic, Weight Only Quantization Aware Training: Static	Post Training Quantization: Static, Dynamic, Weight Only Quantization Aware Training: Static
Input/Output Model Type	`torch.nn.Module`	`torch.nn.Module` (May need some refactors to make the model compatible with FX Graph Mode Quantization)

Backend/Hardware Support¶

Hardware	Kernel Library	Eager Mode Quantization	FX Graph Mode Quantization	Quantization Mode Support
server CPU	fbgemm/onednn	Supported		All Supported
mobile CPU	qnnpack/xnnpack	Supported		All Supported
server GPU	TensorRT (early prototype)	Not support this it requires a graph	Supported	Static Quantization

Today, PyTorch supports the following backends for running quantized operators efficiently:

x86 CPUs with AVX2 support or higher (without AVX2 some operations have inefficient implementations), via x86 optimized by fbgemm and onednn (see the details at RFC)
ARM CPUs (typically found in mobile/embedded devices), via qnnpack
(early prototype) support for NVidia GPU via TensorRT through fx2trt (to be open sourced)

Note for native CPU backends¶

We expose both x86 and qnnpack with the same native pytorch quantized operators, so we need additional flag to distinguish between them. The corresponding implementation of x86 and qnnpack is chosen automatically based on the PyTorch build mode, though users have the option to override this by setting torch.backends.quantization.engine to x86 or qnnpack.

When preparing a quantized model, it is necessary to ensure that qconfig and the engine used for quantized computations match the backend on which the model will be executed. The qconfig controls the type of observers used during the quantization passes. The qengine controls whether x86 or qnnpack specific packing function is used when packing weights for linear and convolution functions and modules. For example:

Default settings for x86:

# set the qconfig for PTQ
# Note: the old 'fbgemm' is still available but 'x86' is the recommended default on x86 CPUs
qconfig = torch.ao.quantization.get_default_qconfig('x86')
# or, set the qconfig for QAT
qconfig = torch.ao.quantization.get_default_qat_qconfig('x86')
# set the qengine to control weight packing
torch.backends.quantized.engine = 'x86'

Default settings for qnnpack:

# set the qconfig for PTQ
qconfig = torch.ao.quantization.get_default_qconfig('qnnpack')
# or, set the qconfig for QAT
qconfig = torch.ao.quantization.get_default_qat_qconfig('qnnpack')
# set the qengine to control weight packing
torch.backends.quantized.engine = 'qnnpack'

Operator Support¶

Operator coverage varies between dynamic and static quantization and is captured in the table below. Note that for FX Graph Mode Quantization, the corresponding functionals are also supported.

	Static Quantization	Dynamic Quantization
nn.Linear nn.Conv1d/2d/3d	Y Y	Y N
nn.LSTM nn.GRU	N N	Y Y
nn.RNNCell nn.GRUCell nn.LSTMCell	N N N	Y Y Y
nn.EmbeddingBag	Y (activations are in fp32)	Y
nn.Embedding	Y	Y
nn.MultiheadAttention	Not Supported	Not supported
Activations	Broadly supported	Un-changed, computations stay in fp32

Note: this will be updated with some information generated from native backend_config_dict soon.

Quantization API Reference¶

The Quantization API Reference contains documentation of quantization APIs, such as quantization passes, quantized tensor operations, and supported quantized modules and functions.

Quantization Backend Configuration¶

The Quantization Backend Configuration contains documentation on how to configure the quantization workflows for various backends.

Quantization Accuracy Debugging¶

The Quantization Accuracy Debugging contains documentation on how to debug quantization accuracy.

Quantization Customizations¶

While default implementations of observers to select the scale factor and bias based on observed tensor data are provided, developers can provide their own quantization functions. Quantization can be applied selectively to different parts of the model or configured differently for different parts of the model.

We also provide support for per channel quantization for conv1d(), conv2d(), conv3d() and linear().

Quantization workflows work by adding (e.g. adding observers as .observer submodule) or replacing (e.g. converting nn.Conv2d to nn.quantized.Conv2d) submodules in the model’s module hierarchy. It means that the model stays a regular nn.Module-based instance throughout the process and thus can work with the rest of PyTorch APIs.

Quantization Custom Module API¶

Both Eager mode and FX graph mode quantization APIs provide a hook for the user to specify module quantized in a custom way, with user defined logic for observation and quantization. The user needs to specify:

The Python type of the source fp32 module (existing in the model)
The Python type of the observed module (provided by user). This module needs to define a from_float function which defines how the observed module is created from the original fp32 module.
The Python type of the quantized module (provided by user). This module needs to define a from_observed function which defines how the quantized module is created from the observed module.
A configuration describing (1), (2), (3) above, passed to the quantization APIs.

The framework will then do the following:

during the prepare module swaps, it will convert every module of type specified in (1) to the type specified in (2), using the from_float function of the class in (2).
during the convert module swaps, it will convert every module of type specified in (2) to the type specified in (3), using the from_observed function of the class in (3).

Currently, there is a requirement that ObservedCustomModule will have a single Tensor output, and an observer will be added by the framework (not by the user) on that output. The observer will be stored under the activation_post_process key as an attribute of the custom module instance. Relaxing these restrictions may be done at a future time.

Custom API Example:

import torch
import torch.ao.nn.quantized as nnq
from torch.ao.quantization import QConfigMapping
import torch.ao.quantization.quantize_fx

# original fp32 module to replace
class CustomModule(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.linear = torch.nn.Linear(3, 3)

    def forward(self, x):
        return self.linear(x)

# custom observed module, provided by user
class ObservedCustomModule(torch.nn.Module):
    def __init__(self, linear):
        super().__init__()
        self.linear = linear

    def forward(self, x):
        return self.linear(x)

    @classmethod
    def from_float(cls, float_module):
        assert hasattr(float_module, 'qconfig')
        observed = cls(float_module.linear)
        observed.qconfig = float_module.qconfig
        return observed

# custom quantized module, provided by user
class StaticQuantCustomModule(torch.nn.Module):
    def __init__(self, linear):
        super().__init__()
        self.linear = linear

    def forward(self, x):
        return self.linear(x)

    @classmethod
    def from_observed(cls, observed_module):
        assert hasattr(observed_module, 'qconfig')
        assert hasattr(observed_module, 'activation_post_process')
        observed_module.linear.activation_post_process = \
            observed_module.activation_post_process
        quantized = cls(nnq.Linear.from_float(observed_module.linear))
        return quantized

#
# example API call (Eager mode quantization)
#

m = torch.nn.Sequential(CustomModule()).eval()
prepare_custom_config_dict = {
    "float_to_observed_custom_module_class": {
        CustomModule: ObservedCustomModule
    }
}
convert_custom_config_dict = {
    "observed_to_quantized_custom_module_class": {
        ObservedCustomModule: StaticQuantCustomModule
    }
}
m.qconfig = torch.ao.quantization.default_qconfig
mp = torch.ao.quantization.prepare(
    m, prepare_custom_config_dict=prepare_custom_config_dict)
# calibration (not shown)
mq = torch.ao.quantization.convert(
    mp, convert_custom_config_dict=convert_custom_config_dict)
#
# example API call (FX graph mode quantization)
#
m = torch.nn.Sequential(CustomModule()).eval()
qconfig_mapping = QConfigMapping().set_global(torch.ao.quantization.default_qconfig)
prepare_custom_config_dict = {
    "float_to_observed_custom_module_class": {
        "static": {
            CustomModule: ObservedCustomModule,
        }
    }
}
convert_custom_config_dict = {
    "observed_to_quantized_custom_module_class": {
        "static": {
            ObservedCustomModule: StaticQuantCustomModule,
        }
    }
}
mp = torch.ao.quantization.quantize_fx.prepare_fx(
    m, qconfig_mapping, torch.randn(3,3), prepare_custom_config=prepare_custom_config_dict)
# calibration (not shown)
mq = torch.ao.quantization.quantize_fx.convert_fx(
    mp, convert_custom_config=convert_custom_config_dict)

Best Practices¶

1. If you are using the x86 backend, we need to use 7 bits instead of 8 bits. Make sure you reduce the range for the quant\_min, quant\_max, e.g. if dtype is torch.quint8, make sure to set a custom quant_min to be 0 and quant_max to be 127 (255 / 2) if dtype is torch.qint8, make sure to set a custom quant_min to be -64 (-128 / 2) and quant_max to be 63 (127 / 2), we already set this correctly if you call the torch.ao.quantization.get_default_qconfig(backend) or torch.ao.quantization.get_default_qat_qconfig(backend) function to get the default qconfig for x86 or qnnpack backend

2. If onednn backend is selected, 8 bits for activation will be used in the default qconfig mapping torch.ao.quantization.get_default_qconfig_mapping('onednn') and default qconfig torch.ao.quantization.get_default_qconfig('onednn'). It is recommended to be used on CPUs with Vector Neural Network Instruction (VNNI) support. Otherwise, setting reduce_range to True of the activation’s observer to get better accuracy on CPUs without VNNI support.

Frequently Asked Questions¶

How can I do quantized inference on GPU?:

We don’t have official GPU support yet, but this is an area of active development, you can find more information here
Where can I get ONNX support for my quantized model?

If you get errors exporting the model (using APIs under torch.onnx), you may open an issue in the PyTorch repository. Prefix the issue title with [ONNX] and tag the issue as module: onnx.

If you encounter issues with ONNX Runtime, open an issue at GitHub - microsoft/onnxruntime.
How can I use quantization with LSTM’s?:

LSTM is supported through our custom module api in both eager mode and fx graph mode quantization. Examples can be found at Eager Mode: pytorch/test_quantized_op.py TestQuantizedOps.test_custom_module_lstm FX Graph Mode: pytorch/test_quantize_fx.py TestQuantizeFx.test_static_lstm

Common Errors¶

Passing a non-quantized Tensor into a quantized kernel¶

If you see an error similar to:

RuntimeError: Could not run 'quantized::some_operator' with arguments from the 'CPU' backend...

This means that you are trying to pass a non-quantized Tensor to a quantized kernel. A common workaround is to use torch.ao.quantization.QuantStub to quantize the tensor. This needs to be done manually in Eager mode quantization. An e2e example:

class M(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.quant = torch.ao.quantization.QuantStub()
        self.conv = torch.nn.Conv2d(1, 1, 1)

    def forward(self, x):
        # during the convert step, this will be replaced with a
        # `quantize_per_tensor` call
        x = self.quant(x)
        x = self.conv(x)
        return x

Passing a quantized Tensor into a non-quantized kernel¶

If you see an error similar to:

RuntimeError: Could not run 'aten::thnn_conv2d_forward' with arguments from the 'QuantizedCPU' backend.

This means that you are trying to pass a quantized Tensor to a non-quantized kernel. A common workaround is to use torch.ao.quantization.DeQuantStub to dequantize the tensor. This needs to be done manually in Eager mode quantization. An e2e example:

class M(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.quant = torch.ao.quantization.QuantStub()
        self.conv1 = torch.nn.Conv2d(1, 1, 1)
        # this module will not be quantized (see `qconfig = None` logic below)
        self.conv2 = torch.nn.Conv2d(1, 1, 1)
        self.dequant = torch.ao.quantization.DeQuantStub()

    def forward(self, x):
        # during the convert step, this will be replaced with a
        # `quantize_per_tensor` call
        x = self.quant(x)
        x = self.conv1(x)
        # during the convert step, this will be replaced with a
        # `dequantize` call
        x = self.dequant(x)
        x = self.conv2(x)
        return x

m = M()
m.qconfig = some_qconfig
# turn off quantization for conv2
m.conv2.qconfig = None

Saving and Loading Quantized models¶

When calling torch.load on a quantized model, if you see an error like:

AttributeError: 'LinearPackedParams' object has no attribute '_modules'

This is because directly saving and loading a quantized model using torch.save and torch.load is not supported. To save/load quantized models, the following ways can be used:

Saving/Loading the quantized model state_dict

An example:

class M(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.linear = nn.Linear(5, 5)
        self.relu = nn.ReLU()

    def forward(self, x):
        x = self.linear(x)
        x = self.relu(x)
        return x

m = M().eval()
prepare_orig = prepare_fx(m, {'' : default_qconfig})
prepare_orig(torch.rand(5, 5))
quantized_orig = convert_fx(prepare_orig)

# Save/load using state_dict
b = io.BytesIO()
torch.save(quantized_orig.state_dict(), b)

m2 = M().eval()
prepared = prepare_fx(m2, {'' : default_qconfig})
quantized = convert_fx(prepared)
b.seek(0)
quantized.load_state_dict(torch.load(b))

Saving/Loading scripted quantized models using torch.jit.save and torch.jit.load

An example:

# Note: using the same model M from previous example
m = M().eval()
prepare_orig = prepare_fx(m, {'' : default_qconfig})
prepare_orig(torch.rand(5, 5))
quantized_orig = convert_fx(prepare_orig)

# save/load using scripted model
scripted = torch.jit.script(quantized_orig)
b = io.BytesIO()
torch.jit.save(scripted, b)
b.seek(0)
scripted_quantized = torch.jit.load(b)

Symbolic Trace Error when using FX Graph Mode Quantization¶

Symbolic traceability is a requirement for (Prototype - maintenance mode) FX Graph Mode Quantization, so if you pass a PyTorch Model that is not symbolically traceable to torch.ao.quantization.prepare_fx or torch.ao.quantization.prepare_qat_fx, we might see an error like the following:

torch.fx.proxy.TraceError: symbolically traced variables cannot be used as inputs to control flow

Please take a look at Limitations of Symbolic Tracing and use - User Guide on Using FX Graph Mode Quantization to workaround the problem.

Quantization¶

Introduction to Quantization¶

Quantization API Summary¶

Eager Mode Quantization¶

Post Training Dynamic Quantization¶

Post Training Static Quantization¶

Quantization Aware Training for Static Quantization¶

Model Preparation for Eager Mode Static Quantization¶

(Prototype - maintenance mode) FX Graph Mode Quantization¶

(Prototype) PyTorch 2 Export Quantization¶

Quantization Stack¶

Quantized Model¶

Quantized Tensor¶

Quantize and Dequantize¶

Quantized Operators/Modules¶

Quantized Engine¶

Quantization Flow¶

Observer and FakeQuantize¶

QConfig¶

General Quantization Flow¶

Quantization Support Matrix¶

Quantization Mode Support¶

Quantization Flow Support¶

Backend/Hardware Support¶

Note for native CPU backends¶

Operator Support¶

Quantization API Reference¶

Quantization Backend Configuration¶

Quantization Accuracy Debugging¶

Quantization Customizations¶

Quantization Custom Module API¶

Best Practices¶

Frequently Asked Questions¶

Common Errors¶

Passing a non-quantized Tensor into a quantized kernel¶

Passing a quantized Tensor into a non-quantized kernel¶

Saving and Loading Quantized models¶

Symbolic Trace Error when using FX Graph Mode Quantization¶

Docs

Tutorials

Resources