减少模型参数---模型剪枝(Pruning Deep Neural Networks)_模型剪枝原理

简介

模型剪枝就是根据神经元的贡献程度对网络中的神经元进行排名，可以从网络中移除排名较低的神经元，从而形成一个更小、更快的网络模型。

基本思想示意图：

模型剪枝根据神经元权重的L1/L2范数来进行排序。剪枝后，准确率会下降，网络通常是训练-剪枝-训练-剪枝（trained-pruned-trained-pruned）迭代恢复的。如果我们一次剪枝太多，网络可能会被破坏得无法恢复。所以在实践中，这是一个迭代过程——通常被称为迭代剪枝（iterative pruning）：剪枝/训练/重复。

训练时使用L1正则化能对参数进行稀疏作用

L1：稀疏与特征选择；L2：平滑特征

代码实现

预训练：

 原始网络模型需要满足 Conv2d+BatchNorm2d+ReLU 作为一个整体

训练时在BatchNorm层增加L1正则进行稀疏训练，得到每个特征图对应的gamma值，即γ越小，其对应的特征图越不重要，为了使得γ 能有特征选择的作用，引入L1正则来控制γ

def updateBN(model, s):
    for m in model.modules():
        if isinstance(m, nn.BatchNorm2d):
            # L1 大于0为1 小于0为-1 0还是0
            m.weight.grad.data.add_(s*torch.sign(m.weight.data))
 
'''
'''
#在训练函数中调用
'''
'''
loss.backward()
#剪枝优化
sr = 0.0001
if sr:
    updateBN(self.model,sr)
self.optimizer.step()

 剪枝：

加载预训练模型，进行剪枝，然后保存剪枝后的模型

需要指定--percent 剪枝比例、--model 预训练的模型、--save 保存剪枝后的模型名称

import os
import argparse
import torch
import torch.nn as nn
from torch.autograd import Variable
from torchvision import datasets, transforms
 
#from vgg import vgg
from model.model import ASPNET
import numpy as np
 
# Prune settings
parser = argparse.ArgumentParser(description='PyTorch Slimming CIFAR prune')
parser.add_argument('--dataset', type=str, default='cifar10',
                    help='training dataset (default: cifar10)')
parser.add_argument('--test-batch-size', type=int, default=1000, metavar='N',
                    help='input batch size for testing (default: 1000)')
parser.add_argument('--no-cuda', action='store_true', default=False,
                    help='disables CUDA training')
parser.add_argument('--percent', type=float, default=0.5,
                    help='scale sparse rate (default: 0.5)')
parser.add_argument('--model', default='', type=str, metavar='PATH',
                    help='path to raw trained model (default: none)')
parser.add_argument('--save', default='', type=str, metavar='PATH',
                    help='path to save prune model (default: none)')
args = parser.parse_args()
args.cuda = not args.no_cuda and torch.cuda.is_available()
 
#model = vgg()
model = ASPNET()
if args.cuda:
    model.cuda()
if args.model:
    if os.path.isfile(args.model):
        print("=> loading checkpoint '{}'".format(args.model))
        checkpoint = torch.load(args.model)
        args.start_epoch = checkpoint['epoch']
        best_prec1 = checkpoint['monitor_best']
        model.load_state_dict(checkpoint['state_dict'])
        print("=> loaded checkpoint '{}' (epoch {}) Prec1: {:f}"
              .format(args.model, checkpoint['epoch'], best_prec1))
    else:
        print("=> no checkpoint found at '{}'".format(args.resume))
 
print(model)
total = 0 # 每层特征图个数 总和
for m in model.modules():
    if isinstance(m, nn.BatchNorm2d):
        total += m.weight.data.shape[0]
 
bn = torch.zeros(total) # 拿到每一个gamma值 每个特征图都会对应一个γ、β
index = 0
for m in model.modules():
    if isinstance(m, nn.BatchNorm2d):
        size = m.weight.data.shape[0]
        bn[index:(index+size)] = m.weight.data.abs().clone()
        index += size
 
y, i = torch.sort(bn)
thre_index = int(total * args.percent)
thre = y[thre_index]
 
pruned = 0
cfg = []
cfg_mask = []
for k, m in enumerate(model.modules()):
    if isinstance(m, nn.BatchNorm2d):
        weight_copy = m.weight.data.clone()
        mask = weight_copy.abs().gt(thre).float().cuda() #.gt 比较前者是否大于后者
        pruned = pruned + mask.shape[0] - torch.sum(mask)
        m.weight.data.mul_(mask) # BN层gamma置0
        m.bias.data.mul_(mask) #
        cfg.append(int(torch.sum(mask)))
        cfg_mask.append(mask.clone())
        print('layer index: {:d} \t total channel: {:d} \t remaining channel: {:d}'.
            format(k, mask.shape[0], int(torch.sum(mask))))
    elif isinstance(m, nn.MaxPool2d):
        cfg.append('M')
 
pruned_ratio = pruned/total
 
print('Pre-processing Successful!')
 
 
# 执行剪枝
print(cfg)
#newmodel = vgg(cfg=cfg) # 剪枝后的模型
newmodel = ASPNET(net_name=cfg) # 剪枝后的模型
newmodel.cuda()
# 为剪枝后的模型赋值权重
layer_id_in_cfg = 0
start_mask = torch.ones(1) #输入
end_mask = cfg_mask[layer_id_in_cfg] #输出
for [m0, m1] in zip(model.modules(), newmodel.modules()):
    if isinstance(m0, nn.BatchNorm2d): 
        idx1 = np.squeeze(np.argwhere(np.asarray(end_mask.cpu().numpy()))) # 赋值
        m1.weight.data = m0.weight.data[idx1].clone()
        m1.bias.data = m0.bias.data[idx1].clone()
        m1.running_mean = m0.running_mean[idx1].clone()
        m1.running_var = m0.running_var[idx1].clone()
        layer_id_in_cfg += 1
        start_mask = end_mask.clone() #下一层的
        if layer_id_in_cfg < len(cfg_mask):  # do not change in Final FC
            end_mask = cfg_mask[layer_id_in_cfg] #输出
    elif isinstance(m0, nn.Conv2d):
        idx0 = np.squeeze(np.argwhere(np.asarray(start_mask.cpu().numpy())))
        idx1 = np.squeeze(np.argwhere(np.asarray(end_mask.cpu().numpy())))
        print(idx0)
        print(idx1)
        if idx0.size == 1:
            idx0 = np.resize(idx0, (1,))
        if idx1.size == 1:
            idx1 = np.resize(idx1, (1,))
        #print('In shape: {:d} Out shape:{:d}'.format(idx0.shape[0], idx1.shape[0]))
        w = m0.weight.data[:, idx0, :, :].clone() #拿到原始训练好权重
        w = w[idx1, :, :, :].clone()
        m1.weight.data = w.clone() # 将所需权重赋值到剪枝后的模型
        # m1.bias.data = m0.bias.data[idx1].clone()
    elif isinstance(m0, nn.Linear):
        #idx0 = np.squeeze(np.argwhere(np.asarray(start_mask.cpu().numpy())))
        #m1.weight.data = m0.weight.data[:, idx0].clone()
        m1.weight.data = m0.weight.data.clone()
 
 
torch.save({'cfg': cfg, 'state_dict': newmodel.state_dict()}, args.save)
 
print(newmodel)

剪枝前参数大小  27102，剪枝后参数大小 7185

使用剪枝后的模型再训练：

使用剪枝后的网络架构，同时加载剪枝后的模型参数进行初始化

refine = 剪枝后的模型
if refine:
  checkpoint = torch.load(refine)
  print(checkpoint['cfg'])
  model = ASPNET(net_name=checkpoint['cfg'])#使用剪枝后的网络架构
  model.cuda()
  model.load_state_dict(checkpoint['state_dict'])