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yolov5中train.py代码注释详解与使用教程

来源:互联网 收集:自由互联 发布时间:2023-01-30
目录 前言 1. parse_opt函数 2. main函数 2.1 main函数打印关键词/安装环境 2.2 main函数是否进行断点训练 2.3 main函数是否分布式训练 2.4 main函数是否进化训练/遗传算法调参 3. train函数 3.1 trai
目录
  • 前言
  • 1. parse_opt函数
  • 2. main函数
    • 2.1 main函数——打印关键词/安装环境
    • 2.2 main函数——是否进行断点训练
    • 2.3 main函数——是否分布式训练
    • 2.4 main函数——是否进化训练/遗传算法调参
  • 3. train函数
    • 3.1 train函数——基本配置信息
    • 3.2 train函数——模型加载/断点训练
    • 3.3 train函数——冻结训练/冻结层设置
    • 3.4 train函数——图片大小/batchsize设置
    • 3.5 train函数——优化器选择 / 分组优化设置
    • 3.6 train函数——学习率/ema/归一化/单机多卡
    • 3.7 train函数——数据加载 / anchor调整
    • 3.8 train函数——训练配置/多尺度训练/热身训练
    • 3.9 train函数——训练结束/打印信息/保存结果
  • 4. run函数
    • 5.全部代码注释
      • 使用教程
        • 总结 

          前言

          最近在用yolov5参加比赛,yolov5的技巧很多,仅仅用来参加比赛,着实有点浪费,所以有必要好好学习一番,在认真学习之前,首先向yolov5的作者致敬,对了我是用的版本是v6。每每看到这些大神的作品,实在是有点惭愧,要学的太多了

          1. parse_opt函数

          def parse_opt(known=False):
              """
              argparse 使用方法:
              parse = argparse.ArgumentParser()
              parse.add_argument('--s', type=int, default=2, help='flag_int')
              """
              parser = argparse.ArgumentParser()
              # weights 权重的路径./weights/yolov5s.pt.... 
              # yolov5提供4个不同深度不同宽度的预训练权重 用户可以根据自己的需求选择下载
              parser.add_argument('--weights', type=str, default=ROOT / 'yolov5s.pt', help='initial weights path')
              # cfg 配置文件(网络结构) anchor/backbone/numclasses/head,训练自己的数据集需要自己生成
              # 生成方式——例如我的yolov5s_mchar.yaml 根据自己的需求选择复制./models/下面.yaml文件,5个文件的区别在于模型的深度和宽度依次递增
              parser.add_argument('--cfg', type=str, default='', help='model.yaml path')
              # data 数据集配置文件(路径) train/val/label/, 该文件需要自己生成
              # 生成方式——例如我的/data/mchar.yaml 训练集和验证集的路径 + 类别数 + 类别名称
              parser.add_argument('--data', type=str, default=ROOT / 'data/coco128.yaml', help='dataset.yaml path')
              # hpy超参数设置文件(lr/sgd/mixup)./data/hyps/下面有5个超参数设置文件,每个文件的超参数初始值有细微区别,用户可以根据自己的需求选择其中一个
              parser.add_argument('--hyp', type=str, default=ROOT / 'data/hyps/hyp.scratch-low.yaml', help='hyperparameters path')
              # epochs 训练轮次, 默认轮次为300次
              parser.add_argument('--epochs', type=int, default=300)
              # batchsize 训练批次, 默认bs=16
              parser.add_argument('--batch-size', type=int, default=16, help='total batch size for all GPUs, -1 for autobatch')
              # imagesize 设置图片大小, 默认640*640
              parser.add_argument('--imgsz', '--img', '--img-size', type=int, default=640, help='train, val image size (pixels)')
              # rect 是否采用矩形训练,默认为False
              parser.add_argument('--rect', action='store_true', help='rectangular training')
              # resume 是否接着上次的训练结果,继续训练
              parser.add_argument('--resume', nargs='?', const=True, default=False, help='resume most recent training')
              # nosave 不保存模型  默认False(保存)  在./runs/exp*/train/weights/保存两个模型 一个是最后一次的模型 一个是最好的模型
              # best.pt/ last.pt 不建议运行代码添加 --nosave 
              parser.add_argument('--nosave', action='store_true', help='only save final checkpoint')
              # noval 最后进行测试, 设置了之后就是训练结束都测试一下, 不设置每轮都计算mAP, 建议不设置
              parser.add_argument('--noval', action='store_true', help='only validate final epoch')
              # noautoanchor 不自动调整anchor, 默认False, 自动调整anchor
              parser.add_argument('--noautoanchor', action='store_true', help='disable AutoAnchor')
              # evolve参数进化, 遗传算法调参
              parser.add_argument('--evolve', type=int, nargs='?', const=300, help='evolve hyperparameters for x generations')
              # bucket谷歌优盘 / 一般用不到
              parser.add_argument('--bucket', type=str, default='', help='gsutil bucket')
              # cache 是否提前缓存图片到内存,以加快训练速度,默认False
              parser.add_argument('--cache', type=str, nargs='?', const='ram', help='--cache images in "ram" (default) or "disk"')
              # mage-weights 使用图片采样策略,默认不使用
              parser.add_argument('--image-weights', action='store_true', help='use weighted image selection for training')
              # device 设备选择
              parser.add_argument('--device', default='', help='cuda device, i.e. 0 or 0,1,2,3 or cpu')
              # multi-scale 多测度训练
              parser.add_argument('--multi-scale', action='store_true', help='vary img-size +/- 50%%')
              # single-cls 数据集是否多类/默认True
              parser.add_argument('--single-cls', action='store_true', help='train multi-class data as single-class')
              # optimizer 优化器选择 / 提供了三种优化器
              parser.add_argument('--optimizer', type=str, choices=['SGD', 'Adam', 'AdamW'], default='SGD', help='optimizer')
              # sync-bn:是否使用跨卡同步BN,在DDP模式使用
              parser.add_argument('--sync-bn', action='store_true', help='use SyncBatchNorm, only available in DDP mode')
              # workers/dataloader的最大worker数量
              parser.add_argument('--workers', type=int, default=8, help='max dataloader workers (per RANK in DDP mode)')
              # 保存路径 / 默认保存路径 ./runs/ train
              parser.add_argument('--project', default=ROOT / 'runs/train', help='save to project/name')
              # 实验名称
              parser.add_argument('--name', default='exp', help='save to project/name')
              # 项目位置是否存在 / 默认是都不存在
              parser.add_argument('--exist-ok', action='store_true', help='existing project/name ok, do not increment')
              parser.add_argument('--quad', action='store_true', help='quad dataloader')
              # cos-lr 余弦学习率
              parser.add_argument('--cos-lr', action='store_true', help='cosine LR scheduler')
              # 标签平滑 / 默认不增强, 用户可以根据自己标签的实际情况设置这个参数,建议设置小一点 0.1 / 0.05
              parser.add_argument('--label-smoothing', type=float, default=0.0, help='Label smoothing epsilon')
              # 早停止忍耐次数 / 100次不更新就停止训练
              parser.add_argument('--patience', type=int, default=100, help='EarlyStopping patience (epochs without improvement)')
              # --freeze冻结训练 可以设置 default = [0] 数据量大的情况下,建议不设置这个参数
              parser.add_argument('--freeze', nargs='+', type=int, default=[0], help='Freeze layers: backbone=10, first3=0 1 2')
              # --save-period 多少个epoch保存一下checkpoint
              parser.add_argument('--save-period', type=int, default=-1, help='Save checkpoint every x epochs (disabled if < 1)')
              # --local_rank 进程编号 / 多卡使用
              parser.add_argument('--local_rank', type=int, default=-1, help='DDP parameter, do not modify')
          
              # Weights & Biases arguments
              # 在线可视化工具,类似于tensorboard工具,想了解这款工具可以查看https://zhuanlan.zhihu.com/p/266337608
              parser.add_argument('--entity', default=None, help='W&B: Entity')
              # upload_dataset: 是否上传dataset到wandb tabel(将数据集作为交互式 dsviz表 在浏览器中查看、查询、筛选和分析数据集) 默认False
              parser.add_argument('--upload_dataset', nargs='?', const=True, default=False, help='W&B: Upload data, "val" option')
              # bbox_interval: 设置界框图像记录间隔 Set bounding-box image logging interval for W&B 默认-1   opt.epochs // 10
              parser.add_argument('--bbox_interval', type=int, default=-1, help='W&B: Set bounding-box image logging interval')
              # 使用数据的版本
              parser.add_argument('--artifact_alias', type=str, default='latest', help='W&B: Version of dataset artifact to use')
          
              # 传入的基本配置中没有的参数也不会报错# parse_args()和parse_known_args() 
              # parse = argparse.ArgumentParser()
              # parse.add_argument('--s', type=int, default=2, help='flag_int')
              # parser.parse_args() / parse_args()
              opt = parser.parse_known_args()[0] if known else parser.parse_args()
              return opt

          2. main函数

          2.1 main函数——打印关键词/安装环境

          def main(opt, callbacks=Callbacks()):
              ############################################### 1. Checks ##################################################
              if RANK in [-1, 0]:
                  # 输出所有训练参数 / 参数以彩色的方式表现
                  print_args(FILE.stem, opt)
                  # 检查代码版本是否更新
                  check_git_status()
                  # 检查安装是否都安装了 requirements.txt, 缺少安装包安装。
                  # 缺少安装包:建议使用 pip install -i https://pypi.tuna.tsinghua.edu.cn/simple -r requirements.txt
                  check_requirements(exclude=['thop'])

          2.2 main函数——是否进行断点训练

          ############################################### 2. Resume ##################################################
              # 初始化可视化工具wandb,wandb使用教程看https://zhuanlan.zhihu.com/p/266337608
              # 断点训练使用教程可以查看:https://blog.csdn.net/CharmsLUO/article/details/123410081
              if opt.resume and not check_wandb_resume(opt) and not opt.evolve:  # resume an interrupted run
                  # isinstance()是否是已经知道的类型
                  # 如果resume是True,则通过get_lastest_run()函数找到runs为文件夹中最近的权重文件last.pt
                  ckpt = opt.resume if isinstance(opt.resume, str) else get_latest_run()  # specified or most recent path
                  # 判断是否是文件
                  assert os.path.isfile(ckpt), 'ERROR: --resume checkpoint does not exist'
                  #  # 相关的opt参数也要替换成last.pt中的opt参数 safe_load()yaml文件加载数据
                  with open(Path(ckpt).parent.parent / 'opt.yaml', errors='ignore') as f:
                      # argparse.Namespace 可以理解为字典
                      opt = argparse.Namespace(**yaml.safe_load(f))  # replace
                  opt.cfg, opt.weights, opt.resume = '', ckpt, True  # reinstate
                  # 打印断点训练信息
                  LOGGER.info(f'Resuming training from {ckpt}')
              else:
                  # 不使用断点训练就在加载输入的参数
                  opt.data, opt.cfg, opt.hyp, opt.weights, opt.project = \
                      check_file(opt.data), check_yaml(opt.cfg), check_yaml(opt.hyp), str(opt.weights), str(opt.project)  # checks
                  assert len(opt.cfg) or len(opt.weights), 'either --cfg or --weights must be specified'
                  # opt.evolve=False,opt.name='exp'    opt.evolve=True,opt.name='evolve'
                  if opt.evolve:
                      if opt.project == str(ROOT / 'runs/train'):  # if default project name, rename to runs/evolve
                          opt.project = str(ROOT / 'runs/evolve')
                      opt.exist_ok, opt.resume = opt.resume, False  # pass resume to exist_ok and disable resume
                  # 保存相关信息
                  opt.save_dir = str(increment_path(Path(opt.project) / opt.name, exist_ok=opt.exist_ok))
          

          2.3 main函数——是否分布式训练

          # ############################################## 3.DDP mode ###############################################
              # 选择设备cpu/cuda
              device = select_device(opt.device, batch_size=opt.batch_size)
              # 多卡训练GPU
              if LOCAL_RANK != -1:
                  msg = 'is not compatible with YOLOv5 Multi-GPU DDP training'
                  assert not opt.image_weights, f'--image-weights {msg}'
                  assert not opt.evolve, f'--evolve {msg}'
                  assert opt.batch_size != -1, f'AutoBatch with --batch-size -1 {msg}, please pass a valid --batch-size'
                  assert opt.batch_size % WORLD_SIZE == 0, f'--batch-size {opt.batch_size} must be multiple of WORLD_SIZE'
                  assert torch.cuda.device_count() > LOCAL_RANK, 'insufficient CUDA devices for DDP command'
                  # 根据编号选择设备
                  #使用torch.cuda.set_device()可以更方便地将模型和数据加载到对应GPU上, 直接定义模型之前加入一行代码即可
                  # torch.cuda.set_device(gpu_id) #单卡
                  # torch.cuda.set_device('cuda:'+str(gpu_ids)) #可指定多卡
                  torch.cuda.set_device(LOCAL_RANK)
                  device = torch.device('cuda', LOCAL_RANK)
                  # 初始化多进程
                  dist.init_process_group(backend="nccl" if dist.is_nccl_available() else "gloo")

          2.4 main函数——是否进化训练/遗传算法调参

          ################################################ 4. Train #################################################
              # 不设置evolve直接调用train训练
              if not opt.evolve:
                  train(opt.hyp, opt, device, callbacks)
                  # 分布式训练 WORLD_SIZE=主机的数量
                  # 如果是使用多卡训练, 那么销毁进程组
                  if WORLD_SIZE > 1 and RANK == 0:
                      LOGGER.info('Destroying process group... ')
                      # 使用多卡训练, 那么销毁进程组
                      dist.destroy_process_group()
          
              # Evolve hyperparameters (optional)
              # 遗传净化算法/一边训练一遍进化
              # 了解遗传算法可以查看我的博客:
              else:
                  # Hyperparameter evolution metadata (mutation scale 0-1, lower_limit, upper_limit)
                  # 超参数列表(突变范围 - 最小值 - 最大值)
                  meta = {'lr0': (1, 1e-5, 1e-1),  # initial learning rate (SGD=1E-2, Adam=1E-3)
                          'lrf': (1, 0.01, 1.0),  # final OneCycleLR learning rate (lr0 * lrf)
                          'momentum': (0.3, 0.6, 0.98),  # SGD momentum/Adam beta1
                          'weight_decay': (1, 0.0, 0.001),  # optimizer weight decay
                          'warmup_epochs': (1, 0.0, 5.0),  # warmup epochs (fractions ok)
                          'warmup_momentum': (1, 0.0, 0.95),  # warmup initial momentum
                          'warmup_bias_lr': (1, 0.0, 0.2),  # warmup initial bias lr
                          'box': (1, 0.02, 0.2),  # box loss gain
                          'cls': (1, 0.2, 4.0),  # cls loss gain
                          'cls_pw': (1, 0.5, 2.0),  # cls BCELoss positive_weight
                          'obj': (1, 0.2, 4.0),  # obj loss gain (scale with pixels)
                          'obj_pw': (1, 0.5, 2.0),  # obj BCELoss positive_weight
                          'iou_t': (0, 0.1, 0.7),  # IoU training threshold
                          'anchor_t': (1, 2.0, 8.0),  # anchor-multiple threshold
                          'anchors': (2, 2.0, 10.0),  # anchors per output grid (0 to ignore)
                          'fl_gamma': (0, 0.0, 2.0),  # focal loss gamma (efficientDet default gamma=1.5)
                          'hsv_h': (1, 0.0, 0.1),  # image HSV-Hue augmentation (fraction)
                          'hsv_s': (1, 0.0, 0.9),  # image HSV-Saturation augmentation (fraction)
                          'hsv_v': (1, 0.0, 0.9),  # image HSV-Value augmentation (fraction)
                          'degrees': (1, 0.0, 45.0),  # image rotation (+/- deg)
                          'translate': (1, 0.0, 0.9),  # image translation (+/- fraction)
                          'scale': (1, 0.0, 0.9),  # image scale (+/- gain)
                          'shear': (1, 0.0, 10.0),  # image shear (+/- deg)
                          'perspective': (0, 0.0, 0.001),  # image perspective (+/- fraction), range 0-0.001
                          'flipud': (1, 0.0, 1.0),  # image flip up-down (probability)
                          'fliplr': (0, 0.0, 1.0),  # image flip left-right (probability)
                          'mosaic': (1, 0.0, 1.0),  # image mixup (probability)
                          'mixup': (1, 0.0, 1.0),  # image mixup (probability)
                          'copy_paste': (1, 0.0, 1.0)}  # segment copy-paste (probability)
          
                  with open(opt.hyp, errors='ignore') as f:
                      # 加载yaml超参数
                      hyp = yaml.safe_load(f)  # load hyps dict
                      if 'anchors' not in hyp:  # anchors commented in hyp.yaml
                          hyp['anchors'] = 3
                  opt.noval, opt.nosave, save_dir = True, True, Path(opt.save_dir)  # only val/save final epoch
                  # ei = [isinstance(x, (int, float)) for x in hyp.values()]  # evolvable indices
                  # 保存进化的超参数列表
                  evolve_yaml, evolve_csv = save_dir / 'hyp_evolve.yaml', save_dir / 'evolve.csv'
                  if opt.bucket:
                      os.system(f'gsutil cp gs://{opt.bucket}/evolve.csv {evolve_csv}')  # download evolve.csv if exists
                  """
                  遗传算法调参:遵循适者生存、优胜劣汰的法则,即寻优过程中保留有用的,去除无用的。
                  遗传算法需要提前设置4个参数: 群体大小/进化代数/交叉概率/变异概率
          
                  """
          
                  # 默认选择进化300代
                  for _ in range(opt.evolve):  # generations to evolve
                      if evolve_csv.exists():  # if evolve.csv exists: select best hyps and mutate
                          # Select parent(s)
                          # 进化方式--single / --weight
                          parent = 'single'  # parent selection method: 'single' or 'weighted'
                          # 加载evolve.txt文件
                          x = np.loadtxt(evolve_csv, ndmin=2, delimiter=',', skiprows=1)
                          # 选取进化结果代数
                          n = min(5, len(x))  # number of previous results to consider
                          x = x[np.argsort(-fitness(x))][:n]  # top n mutations
                           # 根据resluts计算hyp权重
                          w = fitness(x) - fitness(x).min() + 1E-6  # weights (sum > 0)
                          # 根据不同进化方式获得base hyp
                          if parent == 'single' or len(x) == 1:
                              # x = x[random.randint(0, n - 1)]  # random selection
                              x = x[random.choices(range(n), weights=w)[0]]  # weighted selection
                          elif parent == 'weighted':
                              x = (x * w.reshape(n, 1)).sum(0) / w.sum()  # weighted combination
          
                          # Mutate
                          # # 获取突变初始值
                          mp, s = 0.8, 0.2  # mutation probability, sigma
                          npr = np.random
                          npr.seed(int(time.time()))
                          g = np.array([meta[k][0] for k in hyp.keys()])  # gains 0-1
                          ng = len(meta)
                          v = np.ones(ng)
                          # 设置突变
                          while all(v == 1):  # mutate until a change occurs (prevent duplicates)
                              # 将突变添加到base hyp上
                              # [i+7]是因为x中前7个数字为results的指标(P,R,mAP,F1,test_loss=(box,obj,cls)),之后才是超参数hyp
                              v = (g * (npr.random(ng) < mp) * npr.randn(ng) * npr.random() * s + 1).clip(0.3, 3.0)
                          for i, k in enumerate(hyp.keys()):  # plt.hist(v.ravel(), 300)
                              hyp[k] = float(x[i + 7] * v[i])  # mutate
          
                      # Constrain to limits
                      # 限制超参再规定范围
                      for k, v in meta.items():
                          hyp[k] = max(hyp[k], v[1])  # lower limit
                          hyp[k] = min(hyp[k], v[2])  # upper limit
                          hyp[k] = round(hyp[k], 5)  # significant digits
          
                      # Train mutation
                      # 训练 使用突变后的参超 测试其效果
                      results = train(hyp.copy(), opt, device, callbacks)
                      callbacks = Callbacks()
                      # Write mutation results
                      # Write mutation results
                      # 将结果写入results 并将对应的hyp写到evolve.txt evolve.txt中每一行为一次进化的结果
                      # 每行前七个数字 (P, R, mAP, F1, test_losses(GIOU, obj, cls)) 之后为hyp
                      # 保存hyp到yaml文件
                      print_mutation(results, hyp.copy(), save_dir, opt.bucket)
          
                  # Plot results
                  # 将结果可视化 / 输出保存信息
                  plot_evolve(evolve_csv)
                  LOGGER.info(f'Hyperparameter evolution finished {opt.evolve} generations\n'
                              f"Results saved to {colorstr('bold', save_dir)}\n"
                              f'Usage example: $ python train.py --hyp {evolve_yaml}')
          

          3. train函数

          3.1 train函数——基本配置信息

          ################################################ 1. 传入参数/基本配置 #############################################
              # opt传入的参数
              save_dir, epochs, batch_size, weights, single_cls, evolve, data, cfg, resume, noval, nosave, workers, freeze = \
                  Path(opt.save_dir), opt.epochs, opt.batch_size, opt.weights, opt.single_cls, opt.evolve, opt.data, opt.cfg, \
                  opt.resume, opt.noval, opt.nosave, opt.workers, opt.freeze
          
              # Directories
              w = save_dir / 'weights'  # weights dir
              # 新建文件夹 weights train evolve
              (w.parent if evolve else w).mkdir(parents=True, exist_ok=True)  # make dir
              # 保存训练结果的目录  如runs/train/exp*/weights/last.pt
              last, best = w / 'last.pt', w / 'best.pt'
          
              # Hyperparameters # isinstance()是否是已知类型
              if isinstance(hyp, str):
                  with open(hyp, errors='ignore') as f:
                      # 加载yaml文件
                      hyp = yaml.safe_load(f)  # load hyps dict
              # 打印超参数 彩色字体
              LOGGER.info(colorstr('hyperparameters: ') + ', '.join(f'{k}={v}' for k, v in hyp.items()))
          
              # Save run settings
              # 如果不使用进化训练
              if not evolve:
                  # safe_dump() python值转化为yaml序列化
                  with open(save_dir / 'hyp.yaml', 'w') as f:
                      yaml.safe_dump(hyp, f, sort_keys=False)
                  with open(save_dir / 'opt.yaml', 'w') as f:
                      # vars(opt) 的作用是把数据类型是Namespace的数据转换为字典的形式。
                      yaml.safe_dump(vars(opt), f, sort_keys=False)
          
              # Loggers
              data_dict = None
              if RANK in [-1, 0]:
                  loggers = Loggers(save_dir, weights, opt, hyp, LOGGER)  # loggers instance
                  if loggers.wandb:
                      data_dict = loggers.wandb.data_dict
                      if resume:
                          weights, epochs, hyp, batch_size = opt.weights, opt.epochs, opt.hyp, opt.batch_size
          
                  # Register actions
                  for k in methods(loggers):
                      callbacks.register_action(k, callback=getattr(loggers, k))
          
              # Config 画图
              plots = not evolve  # create plots
              # GPU / CPU
              cuda = device.type != 'cpu'
              # 随机种子
              init_seeds(1 + RANK)
              # 存在子进程-分布式训练
              with torch_distributed_zero_first(LOCAL_RANK):
                  data_dict = data_dict or check_dataset(data)  # check if None
              # 训练集和验证集的位路径
              train_path, val_path = data_dict['train'], data_dict['val']
              # 设置类别 是否单类
              nc = 1 if single_cls else int(data_dict['nc'])  # number of classes
              # 类别对应的名称
              names = ['item'] if single_cls and len(data_dict['names']) != 1 else data_dict['names']  # class names
              # 判断类别长度和文件是否对应
              assert len(names) == nc, f'{len(names)} names found for nc={nc} dataset in {data}'  # check
              # 当前数据集是否是coco数据集(80个类别) 
              is_coco = isinstance(val_path, str) and val_path.endswith('coco/val2017.txt')  # COCO dataset
          

          3.2 train函数——模型加载/断点训练

          ################################################### 2. Model ###########################################
              # 检查文件后缀是否是.pt
              check_suffix(weights, '.pt')  # check weights
              # 加载预训练权重 yolov5提供了5个不同的预训练权重,大家可以根据自己的模型选择预训练权重
              pretrained = weights.endswith('.pt')
              if pretrained:
                  # # torch_distributed_zero_first(RANK): 用于同步不同进程对数据读取的上下文管理器
                  with torch_distributed_zero_first(LOCAL_RANK):
                      # 如果本地不存在就从网站上下载
                      weights = attempt_download(weights)  # download if not found locally
                  # 加载模型以及参数
                  ckpt = torch.load(weights, map_location='cpu')  # load checkpoint to CPU to avoid CUDA memory leak
                  """
                  两种加载模型的方式: opt.cfg / ckpt['model'].yaml
                  使用resume-断点训练: 选择ckpt['model']yaml创建模型, 且不加载anchor
                  使用断点训练时,保存的模型会保存anchor,所以不需要加载
          
                  """
                  model = Model(cfg or ckpt['model'].yaml, ch=3, nc=nc, anchors=hyp.get('anchors')).to(device)  # create
                  exclude = ['anchor'] if (cfg or hyp.get('anchors')) and not resume else []  # exclude keys
                  csd = ckpt['model'].float().state_dict()  # checkpoint state_dict as FP32
                  # 筛选字典中的键值对  把exclude删除
                  csd = intersect_dicts(csd, model.state_dict(), exclude=exclude)  # intersect
                  model.load_state_dict(csd, strict=False)  # load
                  LOGGER.info(f'Transferred {len(csd)}/{len(model.state_dict())} items from {weights}')  # report
              else:
                  # 不适用预训练权重
                  model = Model(cfg, ch=3, nc=nc, anchors=hyp.get('anchors')).to(device)  # create
          

          3.3 train函数——冻结训练/冻结层设置

          ################################################ 3. Freeze/冻结训练 #########################################
              # 冻结训练的网络层
              freeze = [f'model.{x}.' for x in (freeze if len(freeze) > 1 else range(freeze[0]))]  # layers to freeze
              for k, v in model.named_parameters():
                  v.requires_grad = True  # train all layers
                  if any(x in k for x in freeze):
                      LOGGER.info(f'freezing {k}')
                      # 冻结训练的层梯度不更新
                      v.requires_grad = False
          

          3.4 train函数——图片大小/batchsize设置

          # Image size
              gs = max(int(model.stride.max()), 32)  # grid size (max stride)
              # 检查图片的大小
              imgsz = check_img_size(opt.imgsz, gs, floor=gs * 2)  # verify imgsz is gs-multiple
          
              # Batch size
              if RANK == -1 and batch_size == -1:  # single-GPU only, estimate best batch size
                  batch_size = check_train_batch_size(model, imgsz)
                  loggers.on_params_update({"batch_size": batch_size})
          

          3.5 train函数——优化器选择 / 分组优化设置

          ############################################ 4. Optimizer/优化器 ###########################################
              """
              nbs = 64
              batchsize = 16
              accumulate = 64 / 16 = 4
              模型梯度累计accumulate次之后就更新一次模型 相当于使用更大batch_size
              """
              nbs = 64  # nominal batch size
              accumulate = max(round(nbs / batch_size), 1)  # accumulate loss before optimizing
              # 权重衰减参数
              hyp['weight_decay'] *= batch_size * accumulate / nbs  # scale weight_decay
              # 打印日志
              LOGGER.info(f"Scaled weight_decay = {hyp['weight_decay']}")
          
              # 将模型参数分为三组(weights、biases、bn)来进行分组优化
              g0, g1, g2 = [], [], []  # optimizer parameter groups
              for v in model.modules():
                  if hasattr(v, 'bias') and isinstance(v.bias, nn.Parameter):  # bias
                      g2.append(v.bias)
                  if isinstance(v, nn.BatchNorm2d):  # weight (no decay)
                      g0.append(v.weight)
                  elif hasattr(v, 'weight') and isinstance(v.weight, nn.Parameter):  # weight (with decay)
                      g1.append(v.weight)
              # 选择优化器 / 提供了三个优化器——g0
              if opt.optimizer == 'Adam':
                  optimizer = Adam(g0, lr=hyp['lr0'], betas=(hyp['momentum'], 0.999))  # adjust beta1 to momentum
              elif opt.optimizer == 'AdamW':
                  optimizer = AdamW(g0, lr=hyp['lr0'], betas=(hyp['momentum'], 0.999))  # adjust beta1 to momentum
              else:
                  optimizer = SGD(g0, lr=hyp['lr0'], momentum=hyp['momentum'], nesterov=True)
              # 设置优化的方式——g1 / g2
              optimizer.add_param_group({'params': g1, 'weight_decay': hyp['weight_decay']})  # add g1 with weight_decay
              optimizer.add_param_group({'params': g2})  # add g2 (biases)
              # 打印log日志 优化信息
              LOGGER.info(f"{colorstr('optimizer:')} {type(optimizer).__name__} with parameter groups "
                          f"{len(g0)} weight (no decay), {len(g1)} weight, {len(g2)} bias")
              # 删除变量
              del g0, g1, g2
          

          3.6 train函数——学习率/ema/归一化/单机多卡

          ############################################ 5. Scheduler ##############################################
              # 是否余弦学习率调整方式
              if opt.cos_lr:
                  lf = one_cycle(1, hyp['lrf'], epochs)  # cosine 1->hyp['lrf']
              else:
                  lf = lambda x: (1 - x / epochs) * (1.0 - hyp['lrf']) + hyp['lrf']  # linear
              scheduler = lr_scheduler.LambdaLR(optimizer, lr_lambda=lf)  # plot_lr_scheduler(optimizer, scheduler, epochs)
          
              # EMA
              # 使用EMA(指数移动平均)对模型的参数做平均, 一种给予近期数据更高权重的平均方法, 以求提高测试指标并增加模型鲁棒。
              ema = ModelEMA(model) if RANK in [-1, 0] else None
          
              # Resume
              start_epoch, best_fitness = 0, 0.0
              if pretrained:
                  # Optimizer
                  if ckpt['optimizer'] is not None:
                      optimizer.load_state_dict(ckpt['optimizer'])
                      best_fitness = ckpt['best_fitness']
          
                  # EMA
                  if ema and ckpt.get('ema'):
                      ema.ema.load_state_dict(ckpt['ema'].float().state_dict())
                      ema.updates = ckpt['updates']
          
                  # Epochs
                  start_epoch = ckpt['epoch'] + 1
                  if resume:
                      assert start_epoch > 0, f'{weights} training to {epochs} epochs is finished, nothing to resume.'
                  if epochs < start_epoch:
                      LOGGER.info(f"{weights} has been trained for {ckpt['epoch']} epochs. Fine-tuning for {epochs} more epochs.")
                      epochs += ckpt['epoch']  # finetune additional epochs
          
                  del ckpt, csd
          
              # DP mode
              # DP: 单机多卡模式
              if cuda and RANK == -1 and torch.cuda.device_count() > 1:
                  LOGGER.warning('WARNING: DP not recommended, use torch.distributed.run for best DDP Multi-GPU results.\n'
                                 'See Multi-GPU Tutorial at https://github.com/ultralytics/yolov5/issues/475 to get started.')
                  model = torch.nn.DataParallel(model)
          
              # SyncBatchNorm 多卡归一化
              if opt.sync_bn and cuda and RANK != -1:
                  model = torch.nn.SyncBatchNorm.convert_sync_batchnorm(model).to(device)
                  # 打印信息
                  LOGGER.info('Using SyncBatchNorm()')

          3.7 train函数——数据加载 / anchor调整

          # ############################################## 6. Trainloader / 数据加载 ######################################
              # 训练集数据加载
              train_loader, dataset = create_dataloader(train_path, imgsz, batch_size // WORLD_SIZE, gs, single_cls,
                                                        hyp=hyp, augment=True, cache=None if opt.cache == 'val' else opt.cache,
                                                        rect=opt.rect, rank=LOCAL_RANK, workers=workers,
                                                        image_weights=opt.image_weights, quad=opt.quad,
                                                        prefix=colorstr('train: '), shuffle=True)
              # 标签编号最大值
              mlc = int(np.concatenate(dataset.labels, 0)[:, 0].max())  # max label class
              # 类别总数
              nb = len(train_loader)  # number of batches
              # 判断编号是否正确
              assert mlc < nc, f'Label class {mlc} exceeds nc={nc} in {data}. Possible class labels are 0-{nc - 1}'
          
              # Process 0
              # 验证集数据集加载
              if RANK in [-1, 0]:
                  val_loader = create_dataloader(val_path, imgsz, batch_size // WORLD_SIZE * 2, gs, single_cls,
                                                 hyp=hyp, cache=None if noval else opt.cache,
                                                 rect=True, rank=-1, workers=workers * 2, pad=0.5,
                                                 prefix=colorstr('val: '))[0]
                  # 没有使用断点训练
                  if not resume:
                      labels = np.concatenate(dataset.labels, 0)
                      # c = torch.tensor(labels[:, 0])  # classes
                      # cf = torch.bincount(c.long(), minlength=nc) + 1.  # frequency
                      # model._initialize_biases(cf.to(device))
                      if plots:
                          # 画出标签信息
                          plot_labels(labels, names, save_dir)
          
                      # Anchors
                      # 自适应anchor / anchor可以理解为程序预测的box
                      # 根据k-mean算法聚类生成新的锚框
                      if not opt.noautoanchor:
                          # 参数dataset代表的是训练集,hyp['anchor_t']是从配置文件hpy.scratch.yaml读取的超参数 anchor_t:4.0
                          # 当配置文件中的anchor计算bpr(best possible recall)小于0.98时才会重新计算anchor。
                          # best possible recall最大值1,如果bpr小于0.98,程序会根据数据集的label自动学习anchor的尺寸
                          check_anchors(dataset, model=model, thr=hyp['anchor_t'], imgsz=imgsz)
                      # 半进度
                      model.half().float()  # pre-reduce anchor precision
                  callbacks.run('on_pretrain_routine_end')
          

          3.8 train函数——训练配置/多尺度训练/热身训练

          # #################################################### 7. 训练 ###############################################
              # DDP mode
              # DDP:多机多卡
              if cuda and RANK != -1:
                  model = DDP(model, device_ids=[LOCAL_RANK], output_device=LOCAL_RANK)
          
              # Model attributes
              nl = de_parallel(model).model[-1].nl  # number of detection layers (to scale hyps)
              hyp['box'] *= 3 / nl  # scale to layers
              hyp['cls'] *= nc / 80 * 3 / nl  # scale to classes and layers
              hyp['obj'] *= (imgsz / 640) ** 2 * 3 / nl  # scale to image size and layers
              # 标签平滑
              hyp['label_smoothing'] = opt.label_smoothing
              model.nc = nc  # attach number of classes to model
              model.hyp = hyp  # attach hyperparameters to model
              # 从训练样本标签得到类别权重(和类别中的目标数即类别频率成反比)
              model.class_weights = labels_to_class_weights(dataset.labels, nc).to(device) * nc  # attach class weights
              model.names = names
          
              # Start training
              t0 = time.time()
              # # 获取热身迭代的次数iterations: 3
              nw = max(round(hyp['warmup_epochs'] * nb), 100)  # number of warmup iterations, max(3 epochs, 100 iterations)
              # nw = min(nw, (epochs - start_epoch) / 2 * nb)  # limit warmup to < 1/2 of training
              last_opt_step = -1
              # # 初始化maps(每个类别的map)和results
              maps = np.zeros(nc)  # mAP per class
              results = (0, 0, 0, 0, 0, 0, 0)  # P, R, mAP@.5, mAP@.5-.95, val_loss(box, obj, cls)
              # 设置学习率衰减所进行到的轮次,即使打断训练,使用resume接着训练也能正常衔接之前的训练进行学习率衰减
              scheduler.last_epoch = start_epoch - 1  # do not move
              # 设置amp混合精度训练
              scaler = amp.GradScaler(enabled=cuda)
              # 早停止,不更新结束训练
              stopper = EarlyStopping(patience=opt.patience)
              # 初始化损失函数
              compute_loss = ComputeLoss(model)  # init loss class
              # 打印信息
              LOGGER.info(f'Image sizes {imgsz} train, {imgsz} val\n'
                          f'Using {train_loader.num_workers * WORLD_SIZE} dataloader workers\n'
                          f"Logging results to {colorstr('bold', save_dir)}\n"
                          f'Starting training for {epochs} epochs...')
              # 开始走起训练
              for epoch in range(start_epoch, epochs):  # epoch ------------------------------------------------------------------
                  model.train()
          
                  # Update image weights (optional, single-GPU only)
                  # opt.image_weights
                  if opt.image_weights:
                      """
                      如果设置进行图片采样策略,
                      则根据前面初始化的图片采样权重model.class_weights以及maps配合每张图片包含的类别数
                      通过random.choices生成图片索引indices从而进行采样
                      """
                      cw = model.class_weights.cpu().numpy() * (1 - maps) ** 2 / nc  # class weights
                      iw = labels_to_image_weights(dataset.labels, nc=nc, class_weights=cw)  # image weights
                      dataset.indices = random.choices(range(dataset.n), weights=iw, k=dataset.n)  # rand weighted idx
          
                  # Update mosaic border (optional)
                  # b = int(random.uniform(0.25 * imgsz, 0.75 * imgsz + gs) // gs * gs)
                  # dataset.mosaic_border = [b - imgsz, -b]  # height, width borders
          
                  mloss = torch.zeros(3, device=device)  # mean losses
                  if RANK != -1:
                      train_loader.sampler.set_epoch(epoch)
                  pbar = enumerate(train_loader)
                  LOGGER.info(('\n' + '%10s' * 7) % ('Epoch', 'gpu_mem', 'box', 'obj', 'cls', 'labels', 'img_size'))
                  if RANK in [-1, 0]:
                      # 进度条显示
                      pbar = tqdm(pbar, total=nb, bar_format='{l_bar}{bar:10}{r_bar}{bar:-10b}')  # progress bar
                  # 梯度清零
                  optimizer.zero_grad()
                  for i, (imgs, targets, paths, _) in pbar:  # batch -------------------------------------------------------------
                      ni = i + nb * epoch  # number integrated batches (since train start)
                      imgs = imgs.to(device, non_blocking=True).float() / 255  # uint8 to float32, 0-255 to 0.0-1.0
          
                      """
                      热身训练(前nw次迭代)
                      在前nw次迭代中, 根据以下方式选取accumulate和学习率
                      """
                      # Warmup
                      if ni <= nw:
                          xi = [0, nw]  # x interp
                          # compute_loss.gr = np.interp(ni, xi, [0.0, 1.0])  # iou loss ratio (obj_loss = 1.0 or iou)
                          accumulate = max(1, np.interp(ni, xi, [1, nbs / batch_size]).round())
                          for j, x in enumerate(optimizer.param_groups):
                              """
                              bias的学习率从0.1下降到基准学习率lr*lf(epoch),
                              其他的参数学习率从0增加到lr*lf(epoch).
                              lf为上面设置的余弦退火的衰减函数
                              动量momentum也从0.9慢慢变到hyp['momentum'](default=0.937)
                              """
          
                              # bias lr falls from 0.1 to lr0, all other lrs rise from 0.0 to lr0
                              x['lr'] = np.interp(ni, xi, [hyp['warmup_bias_lr'] if j == 2 else 0.0, x['initial_lr'] * lf(epoch)])
                              if 'momentum' in x:
                                  x['momentum'] = np.interp(ni, xi, [hyp['warmup_momentum'], hyp['momentum']])
          
                      # Multi-scale
                      if opt.multi_scale:
                          """
                          Multi-scale  设置多尺度训练,从imgsz * 0.5, imgsz * 1.5 + gs随机选取尺寸
                          """
                          sz = random.randrange(imgsz * 0.5, imgsz * 1.5 + gs) // gs * gs  # size
                          sf = sz / max(imgs.shape[2:])  # scale factor
                          if sf != 1:
                              ns = [math.ceil(x * sf / gs) * gs for x in imgs.shape[2:]]  # new shape (stretched to gs-multiple)
                              imgs = nn.functional.interpolate(imgs, size=ns, mode='bilinear', align_corners=False)
          
                      # Forward / 前向传播
                      with amp.autocast(enabled=cuda):
                          pred = model(imgs)  # forward
                          # # 计算损失,包括分类损失,objectness损失,框的回归损失
                          # loss为总损失值,loss_items为一个元组,包含分类损失,objectness损失,框的回归损失和总损失
                          loss, loss_items = compute_loss(pred, targets.to(device))  # loss scaled by batch_size
                          if RANK != -1:
                              # 平均不同gpu之间的梯度
                              loss *= WORLD_SIZE  # gradient averaged between devices in DDP mode
                          if opt.quad:
                              loss *= 4.
          
                      # Backward
                      scaler.scale(loss).backward()
          
                      # Optimize  # 模型反向传播accumulate次之后再根据累积的梯度更新一次参数
                      if ni - last_opt_step >= accumulate:
                          scaler.step(optimizer)  # optimizer.step
                          scaler.update()
                          optimizer.zero_grad()
                          if ema:
                              ema.update(model)
                          last_opt_step = ni
          
                      # Log
                      if RANK in [-1, 0]:
                          mloss = (mloss * i + loss_items) / (i + 1)  # update mean losses
                          mem = f'{torch.cuda.memory_reserved() / 1E9 if torch.cuda.is_available() else 0:.3g}G'  # (GB)
                          pbar.set_description(('%10s' * 2 + '%10.4g' * 5) % (
                              f'{epoch}/{epochs - 1}', mem, *mloss, targets.shape[0], imgs.shape[-1]))
                          callbacks.run('on_train_batch_end', ni, model, imgs, targets, paths, plots, opt.sync_bn)
                          if callbacks.stop_training:
                              return
                      # end batch ------------------------------------------------------------------------------------------------
          
                  # Scheduler 进行学习率衰减
                  lr = [x['lr'] for x in optimizer.param_groups]  # for loggers
                  scheduler.step()
          
                  if RANK in [-1, 0]:
                      # mAP
                      callbacks.run('on_train_epoch_end', epoch=epoch)
                      # 将model中的属性赋值给ema
                      ema.update_attr(model, include=['yaml', 'nc', 'hyp', 'names', 'stride', 'class_weights'])
                      # 判断当前的epoch是否是最后一轮
                      final_epoch = (epoch + 1 == epochs) or stopper.possible_stop
                      # notest: 是否只测试最后一轮  True: 只测试最后一轮   False: 每轮训练完都测试mAP
                      if not noval or final_epoch:  # Calculate mAP
                          """
                          测试使用的是ema(指数移动平均 对模型的参数做平均)的模型
                          results: [1] Precision 所有类别的平均precision(最大f1时)
                                   [1] Recall 所有类别的平均recall
                                   [1] map@0.5 所有类别的平均mAP@0.5
                                   [1] map@0.5:0.95 所有类别的平均mAP@0.5:0.95
                                   [1] box_loss 验证集回归损失, obj_loss 验证集置信度损失, cls_loss 验证集分类损失
                          maps: [80] 所有类别的mAP@0.5:0.95
                          """
                          results, maps, _ = val.run(data_dict,
                                                     batch_size=batch_size // WORLD_SIZE * 2,
                                                     imgsz=imgsz,
                                                     model=ema.ema,
                                                     single_cls=single_cls,
                                                     dataloader=val_loader,
                                                     save_dir=save_dir,
                                                     plots=False,
                                                     callbacks=callbacks,
                                                     compute_loss=compute_loss)
          
                      # Update best mAP
                      # Update best mAP 这里的best mAP其实是[P, R, mAP@.5, mAP@.5-.95]的一个加权值
                      # fi: [P, R, mAP@.5, mAP@.5-.95]的一个加权值 = 0.1*mAP@.5 + 0.9*mAP@.5-.95
                      fi = fitness(np.array(results).reshape(1, -1))  # weighted combination of [P, R, mAP@.5, mAP@.5-.95]
                      if fi > best_fitness:
                          best_fitness = fi
                      log_vals = list(mloss) + list(results) + lr
                      callbacks.run('on_fit_epoch_end', log_vals, epoch, best_fitness, fi)
          
                      # Save model
                      """
                      保存带checkpoint的模型用于inference或resuming training
                      保存模型, 还保存了epoch, results, optimizer等信息
                      optimizer将不会在最后一轮完成后保存
                      model保存的是EMA的模型
                      """
                      if (not nosave) or (final_epoch and not evolve):  # if save
                          ckpt = {'epoch': epoch,
                                  'best_fitness': best_fitness,
                                  'model': deepcopy(de_parallel(model)).half(),
                                  'ema': deepcopy(ema.ema).half(),
                                  'updates': ema.updates,
                                  'optimizer': optimizer.state_dict(),
                                  'wandb_id': loggers.wandb.wandb_run.id if loggers.wandb else None,
                                  'date': datetime.now().isoformat()}
          
                          # Save last, best and delete
                          torch.save(ckpt, last)
                          if best_fitness == fi:
                              torch.save(ckpt, best)
                          if (epoch > 0) and (opt.save_period > 0) and (epoch % opt.save_period == 0):
                              torch.save(ckpt, w / f'epoch{epoch}.pt')
                          del ckpt
                          callbacks.run('on_model_save', last, epoch, final_epoch, best_fitness, fi)
          
                      # Stop Single-GPU
                      if RANK == -1 and stopper(epoch=epoch, fitness=fi):
                          break
          
                      # Stop DDP TODO: known issues shttps://github.com/ultralytics/yolov5/pull/4576
                      # stop = stopper(epoch=epoch, fitness=fi)
                      # if RANK == 0:
                      #    dist.broadcast_object_list([stop], 0)  # broadcast 'stop' to all ranks
          
                  # Stop DPP
                  # with torch_distributed_zero_first(RANK):
                  # if stop:
                  #    break  # must break all DDP ranks
          

          3.9 train函数——训练结束/打印信息/保存结果

          ############################################### 8. 打印训练信息 ##########################################
              if RANK in [-1, 0]:
                  LOGGER.info(f'\n{epoch - start_epoch + 1} epochs completed in {(time.time() - t0) / 3600:.3f} hours.')
                  for f in last, best:
                      if f.exists():
                          # 模型训练完后, strip_optimizer函数将optimizer从ckpt中删除
                          # 并对模型进行model.half() 将Float32->Float16 这样可以减少模型大小, 提高inference速度
                          strip_optimizer(f)  # strip optimizers
                          if f is best:
                              LOGGER.info(f'\nValidating {f}...')
                              results, _, _ = val.run(data_dict,
                                                      batch_size=batch_size // WORLD_SIZE * 2,
                                                      imgsz=imgsz,
                                                      model=attempt_load(f, device).half(),
                                                      iou_thres=0.65 if is_coco else 0.60,  # best pycocotools results at 0.65
                                                      single_cls=single_cls,
                                                      dataloader=val_loader,
                                                      save_dir=save_dir,
                                                      save_json=is_coco,
                                                      verbose=True,
                                                      plots=True,
                                                      callbacks=callbacks,
                                                      compute_loss=compute_loss)  # val best model with plots
                              if is_coco:
                                  callbacks.run('on_fit_epoch_end', list(mloss) + list(results) + lr, epoch, best_fitness, fi)
                  # 回调函数
                  callbacks.run('on_train_end', last, best, plots, epoch, results)
                  LOGGER.info(f"Results saved to {colorstr('bold', save_dir)}")
              # 释放显存
              torch.cuda.empty_cache()
              return results

          4. run函数

          def run(**kwargs):
              # 执行这个脚本/ 调用train函数 / 开启训练
              # Usage: import train; train.run(data='coco128.yaml', imgsz=320, weights='yolov5m.pt')
              opt = parse_opt(True)
              for k, v in kwargs.items():
                  # setattr() 赋值属性,属性不存在则创建一个赋值
                  setattr(opt, k, v)
              main(opt)
              return opt
          

          5.全部代码注释

          # YOLOv5  
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