100行代码使用torch.fx极简量化教程
网上很多关于量化的文章，要么就是跑一跑官方残缺的例子，要么就是过旧的API，早已经不潮流。现在比较fashion的方式，是使用 torch.fx来做量化。本文将使用100行代码，极简的教你入门比较标准的量化步骤。这些步骤不是简单的告诉你torch.fx有什么卵用，大家都知道它有什么卵用，只是怎么用，用在哪里，哪里不能用，这些问题需要解答。本文100行代码，麻雀虽小五脏俱全，不管你量化什么模型，一顿套用就是了，出了问题我背锅。
很多古老的文章，还在用手动插入stub来做量化节点，这就好比在21世纪还在飞鸽传书。我们必然会包含一下几个完整的内容：
- fx怎么插入量化节点，不要吓倒，这就一行代码；
- 量化的模型怎么保存权重到本地；
- 怎么把量化后的权重再load回来；
- 怎么做calibration，做跟不做区别多大；
- fx到底有没有局限性；
以上问题，本文都将囊括。
量化前期知识
此处省略三万字，具体大家清百度。没啥好讲的。
量化现状
如果你要问我现在最好的量化工具是什么，我的回答是没有。真的，不管是 nni，还是 nvidia的 pytorch_quantization ，还是nncf so on，不是说这些东西不好，而是在做的各位都是垃圾。
这些东西本质上是在做一件事情，至少从量化角度上看是这样的，但是到最后不具备通用性，当你看到 pytorch_quanzation 这个工具保存的模型体积根float32一样的时候，就会开始怀疑人生了，这tm是人干的事儿？这就好比普通人想要中杯，他便要说这是大杯。
轮子不好用，那就只能自己造轮子了。只能说，torch.fxyyds. 用了都说好，谁用谁知道。
100行代码
talk is cheap，我们直接上代码。需要注意的是，torch.fx最好使用最新的stable版本，老版本API或有不同之处，我测试的是 `1.11`。
由于pytorch的自带的 imagnet系列模型，我们没有办法做calibration，我们用小一些的Cifra10, 不需要下载，pytorch自己可以处理，但是这就需要我们自己finetune一下。
先把finetune的代码备好：
这只是用来fintune一个我们准备去量化，并且校准的模型：
import torch
import torch.nn as nn
import torch.nn.functional as F
import copy
import torchvision
from torchvision import transforms
from torchvision.models.resnet import resnet50, resnet18
from torch.quantization.quantize_fx import prepare_fx, convert_fx
from torch.ao.quantization.fx.graph_module import ObservedGraphModule
from torch.quantization import (
get_default_qconfig,
)
from torch import optim
import os
import time
def train_model(model, train_loader, test_loader, device):
# The training configurations were not carefully selected.
learning_rate = 1e-2
num_epochs = 20
criterion = nn.CrossEntropyLoss()
model.to(device)
# It seems that SGD optimizer is better than Adam optimizer for ResNet18 training on CIFAR10.
optimizer = optim.SGD(
model.parameters(), lr=learning_rate, momentum=0.9, weight_decay=1e-5
)
# optimizer = optim.Adam(model.parameters(), lr=learning_rate, betas=(0.9, 0.999), eps=1e-08, weight_decay=0, amsgrad=False)
for epoch in range(num_epochs):
# Training
model.train()
running_loss = 0
running_corrects = 0
for inputs, labels in train_loader:
inputs = inputs.to(device)
labels = labels.to(device)
# zero the parameter gradients
optimizer.zero_grad()
# forward + backward + optimize
outputs = model(inputs)
_, preds = torch.max(outputs, 1)
loss = criterion(outputs, labels)
loss.backward()
optimizer.step()
# statistics
running_loss += loss.item() * inputs.size(0)
running_corrects += torch.sum(preds == labels.data)
train_loss = running_loss / len(train_loader.dataset)
train_accuracy = running_corrects / len(train_loader.dataset)
# Evaluation
model.eval()
eval_loss, eval_accuracy = evaluate_model(
model=model, test_loader=test_loader, device=device, criterion=criterion
)
print(
"Epoch: {:02d} Train Loss: {:.3f} Train Acc: {:.3f} Eval Loss: {:.3f} Eval Acc: {:.3f}".format(
epoch, train_loss, train_accuracy, eval_loss, eval_accuracy
)
)
return model
def prepare_dataloader(num_workers=8, train_batch_size=128, eval_batch_size=256):
train_transform = transforms.Compose(
[
transforms.RandomCrop(32, padding=4),
transforms.RandomHorizontalFlip(),
transforms.ToTensor(),
transforms.Normalize((0.4914, 0.4822, 0.4465), (0.2023, 0.1994, 0.2010)),
]
)
test_transform = transforms.Compose(
[
transforms.ToTensor(),
transforms.Normalize((0.4914, 0.4822, 0.4465), (0.2023, 0.1994, 0.2010)),
]
)
train_set = torchvision.datasets.CIFAR10(
root="data", train=True, download=True, transform=train_transform
)
# We will use test set for validation and test in this project.
# Do not use test set for validation in practice!
test_set = torchvision.datasets.CIFAR10(
root="data", train=False, download=True, transform=test_transform
)
train_sampler = torch.utils.data.RandomSampler(train_set)
test_sampler = torch.utils.data.SequentialSampler(test_set)
train_loader = torch.utils.data.DataLoader(
dataset=train_set,
batch_size=train_batch_size,
sampler=train_sampler,
num_workers=num_workers,
)
test_loader = torch.utils.data.DataLoader(
dataset=test_set,
batch_size=eval_batch_size,
sampler=test_sampler,
num_workers=num_workers,
)
return train_loader, test_loader
然后训练一波模型：
if __name__ == "__main__":
train_loader, test_loader = prepare_dataloader()
# first finetune model on cifar, we don't have imagnet so using cifar as test
model = resnet18(pretrained=True)
model.fc = nn.Linear(512, 10)
if os.path.exists("r18_row.pth"):
model.load_state_dict(torch.load("r18_row.pth", map_location="cpu"))
else:
train_model(model, train_loader, test_loader, torch.device("cuda"))
print("train finished.")
torch.save(model.state_dict(), "r18_row.pth")
接下来就是核心代码：
def quant_fx(model):
model.eval()
qconfig = get_default_qconfig("fbgemm")
qconfig_dict = {
"": qconfig,
# 'object_type': []
}
model_to_quantize = copy.deepcopy(model)
prepared_model = prepare_fx(model_to_quantize, qconfig_dict)
print("prepared model: ", prepared_model)
quantized_model = convert_fx(prepared_model)
print("quantized model: ", quantized_model)
torch.save(model.state_dict(), "r18.pth")
torch.save(quantized_model.state_dict(), "r18_quant.pth")
懂了吗？很快阿，啪一下，一个int8的量化模型就生成了。
没错，其实都不用100行，15行就够了。torch.fx 就是这么的牛逼！
我们做一个evaluation，来验证一下，在不校准的情况下，精度如何：
def evaluate_model(model, test_loader, device=torch.device("cpu"), criterion=None):
t0 = time.time()
model.eval()
model.to(device)
running_loss = 0
running_corrects = 0
for inputs, labels in test_loader:
inputs = inputs.to(device)
labels = labels.to(device)
outputs = model(inputs)
_, preds = torch.max(outputs, 1)
if criterion is not None:
loss = criterion(outputs, labels).item()
else:
loss = 0
# statistics
running_loss += loss * inputs.size(0)
running_corrects += torch.sum(preds == labels.data)
eval_loss = running_loss / len(test_loader.dataset)
eval_accuracy = running_corrects / len(test_loader.dataset)
t1 = time.time()
print(f"eval loss: {eval_loss}, eval acc: {eval_accuracy}, cost: {t1 - t0}")
return eval_loss, eval_accuracy
这是evaluation的结果：
eval loss: 0.0, eval acc: 0.8476999998092651, cost: 2.8914074897766113 eval loss: 0.0, eval acc: 0.15240000188350677, cost: 1.240293264389038
可以看到，精度下降严重。此时需要进行一下校准，我直接放校准函数：
def calib_quant_model(model, calib_dataloader):
assert isinstance(
model, ObservedGraphModule
), "model must be a perpared fx ObservedGraphModule."
model.eval()
with torch.inference_mode():
for inputs, labels in calib_dataloader:
model(inputs)
print("calib done.")
that's all. 就这么简单。
如果你有其他非分类模型，也可以直接把dataloader丢进来。请注意，这里的标签并没有用到。只需要统计数据的分布即可。
非常简单。
最后我们再次eval一下：
def quant_calib_and_eval(model):
# test only on CPU
model.to(torch.device("cpu"))
model.eval()
qconfig = get_default_qconfig("fbgemm")
qconfig_dict = {
"": qconfig,
# 'object_type': []
}
model2 = copy.deepcopy(model)
model_prepared = prepare_fx(model2, qconfig_dict)
model_int8 = convert_fx(model_prepared)
model_int8.load_state_dict(torch.load("r18_quant.pth"))
model_int8.eval()
a = torch.randn([1, 3, 224, 224])
o1 = model(a)
o2 = model_int8(a)
diff = torch.allclose(o1, o2, 1e-4)
print(diff)
print(o1.shape, o2.shape)
print(o1, o2)
get_output_from_logits(o1)
get_output_from_logits(o2)
train_loader, test_loader = prepare_dataloader()
evaluate_model(model, test_loader)
evaluate_model(model_int8, test_loader)
# calib quant model
model2 = copy.deepcopy(model)
model_prepared = prepare_fx(model2, qconfig_dict)
model_int8 = convert_fx(model_prepared)
torch.save(model_int8.state_dict(), "r18.pth")
model_int8.eval()
model_prepared = prepare_fx(model2, qconfig_dict)
calib_quant_model(model_prepared, test_loader)
model_int8 = convert_fx(model_prepared)
torch.save(model_int8.state_dict(), "r18_quant_calib.pth")
evaluate_model(model_int8, test_loader)
得到结果：
eval loss: 0.0, eval acc: 0.8476999998092651, cost: 2.8914074897766113 eval loss: 0.0, eval acc: 0.15240000188350677, cost: 1.240293264389038 calib done. eval loss: 0.0, eval acc: 0.8442999720573425, cost: 1.2966759204864502
精度瞬间恢复了。速度快了超过一半。
总结
ok，我们用几十行代码就完成这个量化过程。并且使用校准，恢复了精度。由此可见fx的强大之处。
抛出一个问题，欢迎留言区解答：
torch.fx量化的模型，如果export 到onnx并使用其他前推引擎推理。
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