【大模型学习 | 量化】pytorch量化基础知识（1）

pytorch量化

[!note]

• 官方定义：performing computations and storing tensors at lower bitwidths than floating point precision.
• 支持INT8量化，可以降低4倍的模型大小以及显存需求，加速2-4倍的推理速度
• 通俗理解：降低权重和激活值的精度（FP32→INT8），从而提高模型大小以及显存需求。

一、前置知识

1.1 算子融合

​ 将多个连续层的计算操作合并为单个复合算子，减少对内存的访问次数

e.g. 例如将Conv → BN → ReLU, 融合为ConvBnReLU

操作流程	内存访问次数	计算强度
未融合（3个算子）	6次	低
已融合（1个算子）	2次	高

​ NVIDA GPU:

// 未融合：多次启动核函数
conv_kernel<<<...>>>(input, weight, temp1);
bias_kernel<<<...>>>(temp1, bias, temp2);
relu_kernel<<<...>>>(temp2, output);// 已融合：单核函数完成所有操作
fused_kernel<<<...>>>(input, weight, bias, output) {float val = conv2d(input, weight);val += bias;output = max(val, 0.0f);
}

二、量化知识

2.1 对称量化 & 非对称量化

⚙️ 区别

• 对称量化（Symmetric Quantization）

$$X_{int}=round(\frac{X_{float}}{scale}),scale=\frac{max(|X|)}{2^{n-1}-1}$$

• 非对称量化（Affine Quantization）

$$X_{int}=round(\frac{X_{float}}{scale})+zero\_point,scale=\frac{ma{x}_x-mi{n}_{)}x}{2^n-1}$$

$$zero\_point=round(\frac{-min(x)}{scale})$$

特性	对称量化（Symmetric Quantization）	非对称量化（Affine Quantization）
零点位置	固定为0	动态计算（zero_point）
数值范围	[-127, 127] (int8)	[0, 255] (uint8)
计算开销	更低（无需zero_point计算）	更高
精度损失	对偏斜分布敏感	更鲁棒，能更好处理数据分布偏斜的情况
典型应用	权重量化（正负均衡）	激活值量化
硬件支持	广泛支持（如GPU/TPU）	需要额外处理zero_point

🤖 工程实现角度：为什么 PTQ 常用非对称，QAT 用对称？

模式	推荐默认	背后原因
PTQ	权重：对称 激活：非对称	因为激活是不可训练的静态量化，非对称能更好地适应非负分布
QAT	权重：对称 激活：对称（人为设定）	因为激活是可训练的，你可以通过训练让它“对称”起来，精度损失更可控

2.2 PTQ & QAT

[!note]

PTQ 是直接对训练后的模型参数进行量化，因此适合于快速部署；QAT是通过插入伪量化节点，在训练过程中模拟量化误差以达到更高的精度，因此需要重新训练。

⚙️ 区别

特性	PTQ（训练后量化）	QAT（量化感知训练）
训练阶段	仅FP32训练	插入伪量化节点训练
反向传播	❌ 不支持	✅ 通过STE支持
精度损失	较大（尤其小模型）	通常更小
计算开销	低（仅需校准）	高（需完整训练）
典型用途	快速部署	高精度要求的场景

[!tip]

QAT伪量化节点

• 作用：在训练时模拟量化的误差。在每一层训练时，权重、激活值依然是FT32，但在每一层的传播中，值被“量化再还原”，模拟了量化过程。
• 由于量化过程有round函数，是不可微的，因此需要Straight-Through Estimator（STE）近似梯度的 FakeQuant 模块

三、Pytorch实现量化的三种方式

参考链接：Quantization — PyTorch 2.7 documentation

特性	Eager Mode QAT	FX Graph QAT	Export QAT
实现方式	动态图模式	符号化重写	编译器优化
控制流支持	✅	❌	✅
算子融合	❌（只能手动融合）	✅	✅🌟
典型API	prepare_qat	prepare_fx	export
Type	只支持module	支持 module & function	支持 module & function

[!note]

无论是PTQ 还是 QAT ， 每一种实现方式都需要 prepare_fx 和 convert_fx

model_prepared = quantize_fx.prepare_fx(model, qconfig_mapping, example_inputs)
model_quantized = quantize_fx.convert_fx(model_prepared)

🎯 核心功能：在模型的每一个 qconfig_mapping 指定的量化位置（如 Conv2d、Linear）处，插入对应的 observer 或 fake_quant 节点。

📦 插入两类模块：

类型	对应 prepare 的用途	说明
Observer	用于 PTQ	统计 min/max 用来 校准计算 scale 和 zero_point
FakeQuantize	用于 QAT	模拟量化误差，保留梯度流动，支持训练

3.1 Eager Mode Quantization

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 quantizedself.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 pointself.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)

3.2 FX Graph Mode Quantization （maintenance）

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 copymodel_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)

3.3 PyTorch 2 Export Quantization

import torch
from torch.ao.quantization.quantize_pt2e import prepare_pt2e
from torch.export import export_for_training
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 = export_for_training(m, *example_inputs).module()
# 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)