train.py

import argparse
import logging
import math
import os
import random
import shutil
import time
from pathlib import Path

import numpy as np
import torch.distributed as dist
import torch.nn.functional as F
import torch.optim as optim
import torch.optim.lr_scheduler as lr_scheduler
import torch.utils.data
import yaml
from torch.cuda import amp
from torch.nn.parallel import DistributedDataParallel as DDP
from torch.utils.tensorboard import SummaryWriter
from tqdm import tqdm

import test  # import test.py to get mAP after each epoch
from models.yolo import Model
from utils.datasets import create_dataloader
from utils.general import (
    torch_distributed_zero_first, labels_to_class_weights, plot_labels, check_anchors, labels_to_image_weights,
    compute_loss, plot_images, fitness, strip_optimizer, plot_results, get_latest_run, check_dataset, check_file,
    check_git_status, check_img_size, increment_dir, print_mutation, plot_evolution, set_logging, init_seeds)
from utils.google_utils import attempt_download
from utils.torch_utils import ModelEMA, select_device, intersect_dicts

logger = logging.getLogger(__name__)


def train(hyp, opt, device, tb_writer=None):
    logger.info(f'Hyperparameters {hyp}')
    """
    获取记录训练日志的路径:
    训练日志包括：权重、tensorboard文件、超参数hyp、设置的训练参数opt(也就是epochs,batch_size等),result.txt
    result.txt包括: 占GPU内存、训练集的GIOU loss, objectness loss, classification loss, 总loss, 
    targets的数量, 输入图片分辨率, 准确率TP/(TP+FP),召回率TP/P ; 
    测试集的mAP50, mAP@0.5:0.95, GIOU loss, objectness loss, classification loss.
    还会保存batch<3的ground truth
    """
    # 如果设置进化算法则不会传入tb_writer(则为None)，设置一个evolve文件夹作为日志目录
    log_dir = Path(tb_writer.log_dir) if tb_writer else Path(opt.logdir) / 'evolve'  # logging directory

    # 设置生成文件的保存路径
    wdir = log_dir / 'weights'  # weights directory
    os.makedirs(wdir, exist_ok=True)
    last = wdir / 'last.pt'
    best = wdir / 'best.pt'
    results_file = str(log_dir / 'results.txt')

    # 获取轮次、批次、总批次(涉及到分布式训练)、权重、进程序号(主要用于分布式训练)
    epochs, batch_size, total_batch_size, weights, rank = \
        opt.epochs, opt.batch_size, opt.total_batch_size, opt.weights, opt.global_rank

    # Save run settings
    # 保存hyp和opt
    with open(log_dir / 'hyp.yaml', 'w') as f:
        yaml.dump(hyp, f, sort_keys=False)
    with open(log_dir / 'opt.yaml', 'w') as f:
        yaml.dump(vars(opt), f, sort_keys=False)

    # Configure
    # 获取数据路径
    cuda = device.type != 'cpu'
    # 设置随机种子
    # 需要在每一个进程设置相同的随机种子，以便所有模型权重都初始化为相同的值，即确保神经网络每次初始化都相同
    init_seeds(2 + rank)
    # 加载数据配置信息
    with open(opt.data) as f:
        data_dict = yaml.load(f, Loader=yaml.FullLoader)  # data dict

    # torch_distributed_zero_first同步所有进程
    # check_dataset检查数据集，如果没找到数据集则下载数据集(仅适用于项目中自带的yaml文件数据集)
    with torch_distributed_zero_first(rank):
        check_dataset(data_dict)  # check

    # 获取训练集、测试集图片路径
    train_path = data_dict['train']
    test_path = data_dict['val']

    # 获取类别数量和类别名字
    # 如果设置了opt.single_cls则为一类
    nc, names = (1, ['item']) if opt.single_cls else (int(data_dict['nc']), data_dict['names'])  # 保存data.yaml中的number classes, names
    assert len(names) == nc, '%g names found for nc=%g dataset in %s' % (len(names), nc, opt.data)  # check

    # Model
    # 判断weights字符串是否以'.pt'为结尾。若是，则说明本次训练需要预训练模型
    pretrained = weights.endswith('.pt')
    if pretrained:
        # 加载模型，从google云盘中自动下载模型
        # 但通常会下载失败，建议提前下载下来放进weights目录
        with torch_distributed_zero_first(rank):
            attempt_download(weights)  # download if not found locally
        ckpt = torch.load(weights, map_location=device)  # load checkpoint 导入权重文件
        """
            这里模型创建，可通过opt.cfg，也可通过ckpt['model'].yaml
            这里的区别在于是否是resume，resume时会将opt.cfg设为空，
            则按照ckpt['model'].yaml创建模型；
            这也影响着下面是否除去anchor的key(也就是不加载anchor)，
            如果resume，则加载权重中保存的anchor来继续训练；
            主要是预训练权重里面保存了默认coco数据集对应的anchor，
            如果用户自定义了anchor，再加载预训练权重进行训练，会覆盖掉用户自定义的anchor；
            所以这里主要是设定一个，如果加载预训练权重进行训练的话，就去除掉权重中的anchor，采用用户自定义的；
            如果是resume的话，就是不去除anchor，就权重和anchor一起加载， 接着训练；
            参考https://github.com/ultralytics/yolov5/issues/459
            所以下面设置了intersect_dicts，该函数就是忽略掉exclude中的键对应的值
        """
        '''
        ckpt:
             {'epoch': -1, 
              'best_fitness': array([    0.49124]),
              'training_results': None, 
              'model': Model( 
                             ...
                            )
              'optimizer': None
              }
        '''
        if hyp.get('anchors'):  # 用户自定义的anchors优先级大于权重文件中自带的anchors
            ckpt['model'].yaml['anchors'] = round(hyp['anchors'])  # force autoanchor
        # 创建并初始化yolo模型
        model = Model(opt.cfg or ckpt['model'].yaml, ch=3, nc=nc).to(device)  # create
        '''
        model = 
                Model( 
                       (model): Sequential(
                                           (0): Focus(...)
                                                 ...
                                           (24): Detect(...)
                                            )
                      )
        '''
        # 如果opt.cfg存在，或重新设置了'anchors'，则将预训练权重文件中的'anchors'参数清除，使用用户自定义的‘anchors’信息
        exclude = ['anchor'] if opt.cfg or hyp.get('anchors') else []  # exclude keys
        # state_dict变量存放训练过程中需要学习的权重和偏执系数，state_dict 是一个python的字典格式,以字典的格式存储,然后以字典的格式被加载,而且只加载key匹配的项
        # 将ckpt中的‘model’中的”可训练“的每一层的参数建立映射关系（如 'conv1.weight'： 数值...）存在state_dict中
        state_dict = ckpt['model'].float().state_dict()  # to FP32
        # 加载除了与exclude以外，所有与key匹配的项的参数  即将权重文件中的参数导入对应层中
        state_dict = intersect_dicts(state_dict, model.state_dict(), exclude=exclude)  # intersect
        # 将最终模型参数导入yolo模型
        model.load_state_dict(state_dict, strict=False)  # load
        logger.info('Transferred %g/%g items from %s' % (len(state_dict), len(model.state_dict()), weights))  # report
    else:
        # 不进行预训练，则直接创建并初始化yolo模型
        model = Model(opt.cfg, ch=3, nc=nc).to(device)  # create

    # Optimizer
    """
    nbs人为模拟的batch_size; 
    就比如默认的话上面设置的opt.batch_size为16,这个nbs就为64，
    也就是模型梯度累积了64/16=4(accumulate)次之后
    再更新一次模型，变相的扩大了batch_size
    """
    nbs = 64  # nominal batch size
    accumulate = max(round(nbs / total_batch_size), 1)  # accumulate loss before optimizing
    # 根据accumulate设置权重衰减系数
    hyp['weight_decay'] *= total_batch_size * accumulate / nbs  # scale weight_decay

    pg0, pg1, pg2 = [], [], []  # optimizer parameter groups
    # 将模型分成三组(w权重参数（非bn层）, bias, 其他所有参数)优化
    for k, v in model.named_parameters():  # named_parameters：网络层的名字和参数的迭代器
        '''
        (0): Focus(
                   (conv): Conv(
                                 (conv): Conv2d(12, 80, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1), bias=False)
                                 (bn): BatchNorm2d(80, eps=0.001, momentum=0.03, affine=True, track_running_stats=True)
                                 (act): Hardswish()
                                )
                   )
        k: 网络层可训练参数的名字所属  如： model.0.conv.conv.weight 或 model.0.conv.bn.weight 或 model.0.conv.bn.bias  （Focus层举例）
        v： 对应网络层的具体参数   如：对应model.0.conv.conv.weight的 size为(80，12，3，3)的参数数据 即 卷积核的数量为80，深度为12，size为3×3
        '''
        v.requires_grad = True  # 设置当前参数在训练时保留梯度信息
        if '.bias' in k:
            pg2.append(v)  # biases  (所有的偏置参数)
        elif '.weight' in k and '.bn' not in k:
            pg1.append(v)  # apply weight decay (非bn层的权重参数w)
        else:
            pg0.append(v)  # all else  （网络层的其他参数）

    # 选用优化器，并设置pg0组的优化方式
    if opt.adam:
        optimizer = optim.Adam(pg0, lr=hyp['lr0'], betas=(hyp['momentum'], 0.999))  # adjust beta1 to momentum
    else:
        optimizer = optim.SGD(pg0, lr=hyp['lr0'], momentum=hyp['momentum'], nesterov=True)

    # 设置权重参数weights（非bn层）的优化方式
    optimizer.add_param_group({'params': pg1, 'weight_decay': hyp['weight_decay']})  # add pg1 with weight_decay
    # 设置偏置参数bias的优化方式
    optimizer.add_param_group({'params': pg2})  # add pg2 (biases)
    logger.info('Optimizer groups: %g .bias, %g conv.weight, %g other' % (len(pg2), len(pg1), len(pg0)))
    del pg0, pg1, pg2
    
    
    # Freeze
    #freeze = ['', ]  # parameter names to freeze (full or partial)
    freeze = ['model.%s.' % x for x in range(10)]  # 冻结带有'model.0.'-'model.9.'的所有参数 即冻结0-9层的backbone
    if any(freeze):
        for k, v in model.named_parameters():
            if any(x in k for x in freeze):
                print('freezing %s' % k)
                v.requires_grad = False
                

    # 设置学习率衰减，这里为余弦退火方式进行衰减
    # 就是根据以下公式lf,epoch和超参数hyp['lrf']进行衰减
    # Scheduler https://arxiv.org/pdf/1812.01187.pdf
    # https://pytorch.org/docs/stable/_modules/torch/optim/lr_scheduler.html#OneCycleLR
    lf = lambda x: ((1 + math.cos(x * math.pi / epochs)) / 2) * (1 - hyp['lrf']) + hyp['lrf']  # cosine  匿名余弦退火函数
    scheduler = lr_scheduler.LambdaLR(optimizer, lr_lambda=lf)
    # plot_lr_scheduler(optimizer, scheduler, epochs)

    # Resume
    # 初始化开始训练的epoch和最好的结果
    # best_fitness是以[0.0, 0.0, 0.1, 0.9]为系数并乘以[精确度, 召回率, mAP@0.5, mAP@0.5:0.95]再求和所得
    # 根据best_fitness来保存best.pt
    start_epoch, best_fitness = 0, 0.0
    if pretrained:
        # Optimizer
        # 加载优化器与best_fitness
        if ckpt['optimizer'] is not None:
            optimizer.load_state_dict(ckpt['optimizer'])
            best_fitness = ckpt['best_fitness']

        # Results
        # 加载训练结果result.txt
        if ckpt.get('training_results') is not None:
            with open(results_file, 'w') as file:
                file.write(ckpt['training_results'])  # write results.txt

        # Epochs
        # 加载上次断点模型中训练的轮次，并在此基础上继续训练
        start_epoch = ckpt['epoch'] + 1

        # 如果使用断点重训的同时发现 start_epoch= 0，则说明上次训练正常结束，不存在断点
        if opt.resume:
            assert start_epoch > 0, '%s training to %g epochs is finished, nothing to resume.' % (weights, epochs)
            shutil.copytree(wdir, wdir.parent / f'weights_backup_epoch{start_epoch - 1}')  # save previous weights

        # 如果新设置epochs小于加载的epoch，则视新设置的epochs为需要再训练的轮次数而不再是总的轮次数
        if epochs < start_epoch:
            logger.info('%s has been trained for %g epochs. Fine-tuning for %g additional epochs.' %
                        (weights, ckpt['epoch'], epochs))
            epochs += ckpt['epoch']  # finetune additional epochs

        del ckpt, state_dict

    # Image sizes
    # 获取模型总步长和模型输入图片分辨率
    gs = int(max(model.stride))  # grid size (max stride)
    # 检查输入图片分辨率确保能够整除总步长gs
    imgsz, imgsz_test = [check_img_size(x, gs) for x in opt.img_size]  # verify imgsz are gs-multiples

    # DP mode
    # 分布式训练,参照:https://github.com/ultralytics/yolov5/issues/475
    # DataParallel模式,仅支持单机多卡,不支持混合精度训练
    # rank为进程编号, 这里应该设置为rank=-1则使用DataParallel模式
    # 如果 当前运行设备为gpu 且 进程编号=-1 且gpu数量大于1时 才会进行分布式训练 ，将model对象放入DataParallel容器即可进行分布式训练
    if cuda and rank == -1 and torch.cuda.device_count() > 1:
        model = torch.nn.DataParallel(model)

    # SyncBatchNorm
    # 实现多GPU之间的BatchNorm
    if opt.sync_bn and cuda and rank != -1:
        model = torch.nn.SyncBatchNorm.convert_sync_batchnorm(model).to(device)
        logger.info('Using SyncBatchNorm()')

    # Exponential moving average
    '''
    EMA ： YOLOv5优化策略之一
    EMA + SGD可提高模型鲁棒性
    为模型创建EMA指数滑动平均,如果GPU进程数大于1,则不创建
    '''
    ema = ModelEMA(model) if rank in [-1, 0] else None

    # DDP mode
    # 如果rank不等于-1,则使用DistributedDataParallel模式
    # local_rank为gpu编号,rank为进程,例如rank=3，local_rank=0 表示第 3 个进程内的第 1 块 GPU。
    if cuda and rank != -1:
        model = DDP(model, device_ids=[opt.local_rank], output_device=opt.local_rank)

    # Trainloader
    # class dataloader 和 dataset .
    dataloader, dataset = create_dataloader(train_path, imgsz, batch_size, gs, opt,
                                            hyp=hyp, augment=True, cache=opt.cache_images, rect=opt.rect,
                                            rank=rank, world_size=opt.world_size, workers=opt.workers)

    # 获取标签中最大的类别值，并于类别数作比较, 如果小于类别数则表示有问题
    mlc = np.concatenate(dataset.labels, 0)[:, 0].max()  # max label class
    nb = len(dataloader)  # number of batches
    assert mlc < nc, 'Label class %g exceeds nc=%g in %s. Possible class labels are 0-%g' % (mlc, nc, opt.data, nc - 1)

    '''
    dataloader和testloader不同之处在于：
         1. testloader：没有数据增强，rect=True（大概是测试图片保留了原图的长宽比）
         2. dataloader：数据增强，保留了矩形框训练。
    '''
    # Process 0
    if rank in [-1, 0]:
        # local_rank is set to -1. Because only the first process is expected to do evaluation.
        # testloader
        ema.updates = start_epoch * nb // accumulate  # set EMA updates
        # testloader = create_dataloader(test_path, imgsz_test, total_batch_size, gs, opt,
        #                                hyp=hyp, augment=False, cache=opt.cache_images and not opt.notest, rect=True,
        #                                rank=-1, world_size=opt.world_size, workers=opt.workers)[0]  # testloader

        if not opt.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))
            plot_labels(labels, save_dir=log_dir)
            if tb_writer:
                # tb_writer.add_hparams(hyp, {})  # causes duplicate https://github.com/ultralytics/yolov5/pull/384
                tb_writer.add_histogram('classes', c, 0)

            # Anchors
            if not opt.noautoanchor:
                check_anchors(dataset, model=model, thr=hyp['anchor_t'], imgsz=imgsz)

    # Model parameters
    # 根据自己数据集的类别数设置分类损失的系数
    hyp['cls'] *= nc / 80.  # scale coco-tuned hyp['cls'] to current dataset
    # 设置类别数，超参数
    model.nc = nc  # attach number of classes to model
    model.hyp = hyp  # attach hyperparameters to model
    """
    设置giou的值在objectness loss中做标签的系数, 使用代码如下
    tobj[b, a, gj, gi] = (1.0 - model.gr) + model.gr * giou.detach().clamp(0).type(tobj.dtype)
    这里model.gr=1，也就是说完全使用标签框与预测框的giou值来作为该预测框的objectness标签
    """
    model.gr = 1.0  # iou loss ratio (obj_loss = 1.0 or iou)
    # 根据labels初始化图片采样权重（图像类别所占比例高的采样频率低）
    model.class_weights = labels_to_class_weights(dataset.labels, nc).to(device)  # attach class weights
    # 获取类别的名字
    model.names = names

    # Start training
    t0 = time.time()
    # 获取warm-up训练的迭代次数
    nw = max(round(hyp['warmup_epochs'] * nb), 1e3)  # number of warmup iterations, max(3 epochs, 1k iterations)
    # nw = min(nw, (epochs - start_epoch) / 2 * nb)  # limit warmup to < 1/2 of training
    # 初始化mAP和results
    maps = np.zeros(nc)  # mAP per class
    results = (0, 0, 0, 0, 0, 0, 0, 0)  # P, R, mAP@.5, mAP@.5-.95, val_loss(box, obj, cls, angleloss)
    """
        设置学习率衰减所进行到的轮次，
        目的是打断训练后，--resume接着训练也能正常的衔接之前的训练进行学习率衰减
    """
    scheduler.last_epoch = start_epoch - 1  # do not move
    # 通过torch1.6自带的api设置混合精度训练
    scaler = amp.GradScaler(enabled=cuda)
    """
    打印训练和测试输入图片分辨率
    加载图片时调用的cpu进程数
    从哪个epoch开始训练
    """
    logger.info('Image sizes %g train, %g test\nUsing %g dataloader workers\nLogging results to %s\n'
                'Starting training for %g epochs...' % (imgsz, imgsz_test, dataloader.num_workers, log_dir, epochs))

    # 训练
    for epoch in range(start_epoch, epochs):  # epoch ------------------------------------------------------------------
        # model设置为训练模式，其中training属性表示BatchNorm与Dropout层在训练阶段和测试阶段中采取的策略不同，通过判断training值来决定前向传播策略
        model.train()

        # Update image weights (optional)
        # 加载图片权重（可选）
        if opt.image_weights:
            # Generate indices
            """
            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
            """
            if rank in [-1, 0]:
                cw = model.class_weights.cpu().numpy() * (1 - maps) ** 2  # 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

            # Broadcast if DDP
            # 如果是DDP模式,则广播采样策略
            if rank != -1:
                indices = (torch.tensor(dataset.indices) if rank == 0 else torch.zeros(dataset.n)).int()
                dist.broadcast(indices, 0)
                if rank != 0:
                    dataset.indices = indices.cpu().numpy()

        # Update mosaic border
        # b = int(random.uniform(0.25 * imgsz, 0.75 * imgsz + gs) // gs * gs)
        # dataset.mosaic_border = [b - imgsz, -b]  # height, width borders

        # 初始化训练时打印的平均损失信息
        mloss = torch.zeros(5, device=device)  # mean losses
        if rank != -1:
            # DDP模式下打乱数据, ddp.sampler的随机采样数据是基于epoch+seed作为随机种子，
            # 每次epoch不同，随机种子就不同
            dataloader.sampler.set_epoch(epoch)
        pbar = enumerate(dataloader)
        logger.info(('\n' + '%10s' * 9) % ('Epoch', 'gpu_mem', 'box', 'obj', 'cls', 'angle', 'total', 'targets', 'img_size'))
        if rank in [-1, 0]:
            # tqdm 创建进度条，方便训练时 信息的展示
            pbar = tqdm(pbar, total=nb)  # progress bar
        optimizer.zero_grad()
        for i, (imgs, targets, paths, _) in pbar:  # batch ------------------------------------------------------------
            '''
            i: batch_index, 第i个batch
            imgs : torch.Size([batch_size, 3, resized_height, resized_weight])
            targets : torch.Size = (该batch中的目标数量, [该image属于该batch的第几个图片, class, xywh, θ])       
            paths : List['img1_path','img2_path',......,'img-1_path']  len(paths)=batch_size
            shapes :  size= batch_size, 不进行mosaic时进行矩形训练时才有值
            '''
            # ni计算迭代的次数iteration
            ni = i + nb * epoch  # number integrated batches (since train start)
            imgs = imgs.to(device, non_blocking=True).float() / 255.0  # uint8 to float32, 0-255 to 0.0-1.0

            # Warmup
            """
            warmup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
            """
            if ni <= nw:
                xi = [0, nw]  # x interp
                # model.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 / total_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
                    """
                    # 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
            # 设置多尺度训练，从imgsz * 0.5, imgsz * 1.5 + gs随机选取尺寸
            if opt.multi_scale:
                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)
                    # 采用上采样下采样函数interpolate完成imgs尺寸的转变，模式设置为双线性插值
                    imgs = F.interpolate(imgs, size=ns, mode='bilinear', align_corners=False)

            # Forward
            # 前向传播
            with amp.autocast(enabled=cuda):
                '''
                训练时返回x
                x list: [small_forward, medium_forward, large_forward]  eg:small_forward.size=( batch_size, 3种scale框, size1, size2, no)
                '''
                pred = model(imgs)  # forward
                # Loss
                # 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(lbox, lobj, lcls, langle, loss)
                loss, loss_items = compute_loss(pred, targets.to(device), model, csl_label_flag=True)  # loss scaled by batch_size
                if rank != -1:
                    # 平均不同gpu之间的梯度
                    loss *= opt.world_size  # gradient averaged between devices in DDP mode

            # 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 % accumulate == 0:
                scaler.step(optimizer)  # optimizer.step
                scaler.update()
                optimizer.zero_grad()
                if ema:
                    ema.update(model)

            # Print
            if rank in [-1, 0]:
                # mloss  (lbox, lobj, lcls, langle, loss)
                # 打印显存，进行的轮次，损失，target的数量和图片的size等信息
                mloss = (mloss * i + loss_items) / (i + 1)  # update mean losses
                mem = '%.3gG' % (torch.cuda.memory_reserved() / 1E9 if torch.cuda.is_available() else 0)  # (GB)
                s = ('%10s' * 2 + '%10.4g' * 7) % (
                    '%g/%g' % (epoch, epochs - 1), mem, *mloss, targets.shape[0], imgs.shape[-1])
                # 进度条显示以上信息
                pbar.set_description(s)

                # Plot
                # 将前三次迭代batch的标签框在图片上画出来并保存
                if ni < 3:
                    f = str(log_dir / ('train_batch%g.jpg' % ni))  # filename
                    result = plot_images(images=imgs, targets=targets, paths=paths, fname=f)
                    if tb_writer and result is not None:
                        tb_writer.add_image(f, result, dataformats='HWC', global_step=epoch)  # 存储的格式为[H, W, C]
                        # tb_writer.add_graph(model, imgs)  # add model to tensorboard

            # end batch ------------------------------------------------------------------------------------------------

        # Scheduler
        lr = [x['lr'] for x in optimizer.param_groups]  # for tensorboard
        scheduler.step()

        # DDP process 0 or single-GPU
        if rank in [-1, 0]:
            # mAP
            if ema:
                # 更新EMA的属性
                # 添加include的属性
                ema.update_attr(model, include=['yaml', 'nc', 'hyp', 'gr', 'names', 'stride'])
            final_epoch = epoch + 1 == epochs
            # # 判断该epoch是否为最后一轮
            # if not opt.notest or final_epoch:  # Calculate mAP
            #     # 对测试集进行测试，计算mAP等指标
            #     # 测试时使用的是EMA模型
            #     results, maps, times = test.test(opt.data,
            #                                      batch_size=total_batch_size,
            #                                      imgsz=imgsz_test,
            #                                      model=ema.ema,
            #                                      single_cls=opt.single_cls,
            #                                      dataloader=testloader,
            #                                      save_dir=log_dir,
            #                                      plots=epoch == 0 or final_epoch)  # plot first and last

            # Write
            # 将测试指标写入result.txt
            with open(results_file, 'a') as f:
                f.write(s + '%10.4g' * 8 % results + '\n')  # P, R, mAP@.5, mAP@.5-.95, val_loss(box, obj, cls)
            if len(opt.name) and opt.bucket:
                os.system('gsutil cp %s gs://%s/results/results%s.txt' % (results_file, opt.bucket, opt.name))

            # Tensorboard
            # 添加指标，损失等信息到tensorboard显示
            if tb_writer:
                tags = ['train/box_loss', 'train/obj_loss', 'train/cls_loss', 'train/angle_loss',  # train loss
                        'metrics/precision', 'metrics/recall', 'metrics/mAP_0.5', 'metrics/mAP_0.5:0.95',
                        'val/box_loss', 'val/obj_loss', 'val/cls_loss', 'val/angle_loss',  # val loss
                        'x/lr0', 'x/lr1', 'x/lr2']  # params
                for x, tag in zip(list(mloss[:-1]) + list(results) + lr, tags):
                    tb_writer.add_scalar(tag, x, epoch)

            # Update best mAP
            # 更新best_fitness
            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

            # Save model
            """
            保存模型，还保存了epoch，results，optimizer等信息，
            optimizer信息在最后一轮完成后不会进行保存  未完成训练则保存该信息
            model保存的是EMA的模型
            """
            save = (not opt.nosave) or (final_epoch and not opt.evolve)
            if save:
                with open(results_file, 'r') as f:  # create checkpoint
                    ckpt = {'epoch': epoch,
                            'best_fitness': best_fitness,
                            'training_results': f.read(),
                            'model': ema.ema,
                            'optimizer': None if final_epoch else optimizer.state_dict()}

                # Save last, best and delete
                torch.save(ckpt, last)
                if best_fitness == fi:
                    torch.save(ckpt, best)
                del ckpt
        # end epoch ----------------------------------------------------------------------------------------------------
    # end training

    if rank in [-1, 0]:
        # Strip optimizers
        """
        模型训练完后，strip_optimizer函数将optimizer从ckpt中去除；
        并且对模型进行model.half(), 将Float32的模型->Float16，
        可以减少模型大小，提高inference速度
        """
        n = opt.name if opt.name.isnumeric() else ''
        fresults, flast, fbest = log_dir / f'results{n}.txt', wdir / f'last{n}.pt', wdir / f'best{n}.pt'
        for f1, f2 in zip([wdir / 'last.pt', wdir / 'best.pt', results_file], [flast, fbest, fresults]):
            if os.path.exists(f1):
                os.rename(f1, f2)  # rename
                if str(f2).endswith('.pt'):  # is *.pt
                    strip_optimizer(f2)  # strip optimizer
                    # 上传结果到谷歌云盘
                    os.system('gsutil cp %s gs://%s/weights' % (f2, opt.bucket)) if opt.bucket else None  # upload

        # Finish
        # 可视化results.txt文件
        if not opt.evolve:
            plot_results(save_dir=log_dir)  # save as results.png
        logger.info('%g epochs completed in %.3f hours.\n' % (epoch - start_epoch + 1, (time.time() - t0) / 3600))

    # 释放显存
    dist.destroy_process_group() if rank not in [-1, 0] else None
    torch.cuda.empty_cache()
    return results


if __name__ == '__main__':
    """
        opt参数解析：
        weights:加载的权重文件
        cfg:模型配置文件，网络结构
        data:数据集配置文件，数据集路径，类名等
        hyp:超参数文件
        epochs:训练总轮次
        batch-size:批次大小
        img-size:输入图片分辨率大小
        rect:是否采用矩形训练，默认False
        resume:接着打断训练上次的结果接着训练
        nosave:不保存模型，默认False
        notest:不进行test，默认False
        noautoanchor:不自动调整anchor，默认False
        evolve:是否进行超参数进化，默认False
        bucket:谷歌云盘bucket，一般不会用到
        cache-images:是否提前缓存图片到内存，以加快训练速度，默认False
        name:数据集名字，如果设置：results.txt to results_name.txt，默认无
        device:训练的设备，cpu；0(表示一个gpu设备cuda:0)；0,1,2,3(多个gpu设备)
        multi-scale:是否进行多尺度训练，默认False
        single-cls:数据集是否只有一个类别，默认False
        adam:是否使用adam优化器
        sync-bn:是否使用跨卡同步BN,在DDP模式使用
        local_rank:gpu编号
        logdir:存放日志的目录
        workers:dataloader的最大worker数量
    """
    parser = argparse.ArgumentParser()
    parser.add_argument('--weights', type=str, default='./weights/yolov5m.pt',help='initil weights path')
    parser.add_argument('--cfg', type=str, default='', help='model.yaml path')
    parser.add_argument('--data', type=str, default='data/DOTA_ROTATED.yaml', help='data.yaml path')
    parser.add_argument('--hyp', type=str, default='data/hyp.scratch.yaml', help='hyperparameters path')
    parser.add_argument('--epochs', type=int, default=150)
    parser.add_argument('--batch-size', type=int, default=4, help='total batch size for all GPUs')
    parser.add_argument('--img-size', nargs='+', type=int, default=[1024, 1024], help='[train, test] image sizes')
    parser.add_argument('--rect', action='store_true', help='rectangular training')
    parser.add_argument('--resume', nargs='?', const=True, default=False, help='resume most recent training')
    parser.add_argument('--nosave', action='store_true', help='only save final checkpoint')
    parser.add_argument('--notest', action='store_true', default=True, help='only test final epoch')
    parser.add_argument('--noautoanchor', action='store_true', help='disable autoanchor check')
    parser.add_argument('--evolve', action='store_true', help='evolve hyperparameters')
    parser.add_argument('--bucket', type=str, default='', help='gsutil bucket')
    parser.add_argument('--cache-images', action='store_true', default=False, help='cache images for faster training')
    parser.add_argument('--image-weights', action='store_true', help='use weighted image selection for training')
    parser.add_argument('--name', default='', help='renames results.txt to results_name.txt if supplied')
    parser.add_argument('--device', default='0', help='cuda device, i.e. 0 or 0,1,2,3 or cpu')
    parser.add_argument('--multi-scale', action='store_true', help='vary img-size +/- 50%%')
    parser.add_argument('--single-cls', action='store_true', help='train as single-class dataset')
    parser.add_argument('--adam', action='store_true', help='use torch.optim.Adam() optimizer')
    parser.add_argument('--sync-bn', action='store_true', help='use SyncBatchNorm, only available in DDP mode')
    parser.add_argument('--local_rank', type=int, default=-1, help='DDP parameter, do not modify')
    parser.add_argument('--logdir', type=str, default='runs/', help='logging directory')
    parser.add_argument('--workers', type=int, default=8, help='maximum number of dataloader workers')
    opt = parser.parse_args()

    # Set DDP variables
    """
        设置DDP(Distributed Data Parallel，分布式数据并行)模式的参数
        world_size:表示全局进程个数，可认为是gpu的数量
        global_rank:进程编号
    """
    opt.total_batch_size = opt.batch_size
    opt.world_size = int(os.environ['WORLD_SIZE']) if 'WORLD_SIZE' in os.environ else 1
    opt.global_rank = int(os.environ['RANK']) if 'RANK' in os.environ else -1
    set_logging(opt.global_rank)
    if opt.global_rank in [-1, 0]:
        check_git_status()

    # Resume
    '''
        是否从断点开始训练
    '''
    if opt.resume:  # resume an interrupted run
        # 如果resume的参数是str,则表示传入的是最新的断点模型的路径地址,直接导入last模型进行训练
        # get_latest_run()函数获取runs文件夹中最近的last.pt，从断点处导入继续训练
        ckpt = opt.resume if isinstance(opt.resume, str) else get_latest_run()  # specified or most recent path
        log_dir = Path(ckpt).parent.parent  # runs/exp0
        assert os.path.isfile(ckpt), 'ERROR: --resume checkpoint does not exist'
        # opt参数也全部替换
        with open(log_dir / 'opt.yaml') as f:
            opt = argparse.Namespace(**yaml.load(f, Loader=yaml.FullLoader))  # replace

        # opt.cfg设置为'' 对应着train函数里面的操作(加载权重时是否加载权重里的anchor)
        opt.cfg, opt.weights, opt.resume = '', ckpt, True
        logger.info('Resuming training from %s' % ckpt)

    else:
        # # 获取超参数列表
        # opt.hyp = opt.hyp or ('hyp.finetune.yaml' if opt.weights else 'hyp.scratch.yaml')
        # 检查数据集参数信息文件、cfg配置文件、训练超参数信息文件是否存在以及是否存在多个文件
        opt.data, opt.cfg, opt.hyp = check_file(opt.data), check_file(opt.cfg), check_file(opt.hyp)  # check files
        assert len(opt.cfg) or len(opt.weights), 'either --cfg or --weights must be specified'
        # 扩展image_size为[image_size, image_size]一个是训练size，一个是测试size
        opt.img_size.extend([opt.img_size[-1]] * (2 - len(opt.img_size)))  # extend to 2 sizes (train, test)
        log_dir = increment_dir(Path(opt.logdir) / 'exp', opt.name)  # runs/exp1  训练日志存放路径
    # 选择设备
    device = select_device(opt.device, batch_size=opt.batch_size)

    # DDP mode
    if opt.local_rank != -1:  # -1 表示cpu
        assert torch.cuda.device_count() > opt.local_rank
        # 根据gpu编号选择设备
        torch.cuda.set_device(opt.local_rank)
        device = torch.device('cuda', opt.local_rank)
        # 初始化进程组
        dist.init_process_group(backend='nccl', init_method='env://')  # distributed backend 一般来说使用NCCL对于GPU分布式训练，使用gloo对CPU进行分布式训练
        assert opt.batch_size % opt.world_size == 0, '--batch-size must be multiple of CUDA device count'
        # 将总批次按照进程数分配给各个gpu
        opt.batch_size = opt.total_batch_size // opt.world_size

    # 打印opt参数信息
    logger.info(opt)
    # 加载超参数列表
    with open(opt.hyp) as f:
        hyp = yaml.load(f, Loader=yaml.FullLoader)  # load hyps

    # Train
    # 如果不进行超参数进化，则直接调用train()函数，开始训练
    if not opt.evolve:
        tb_writer = None
        if opt.global_rank in [-1, 0]:
            # 创建tensorboard
            logger.info('Start Tensorboard with "tensorboard --logdir %s", view at http://localhost:6006/' % opt.logdir)
            tb_writer = SummaryWriter(log_dir=log_dir)  # runs/exp0

        train(hyp, opt, device, tb_writer)

    # Evolve hyperparameters (optional)
    else:
        # Hyperparameter evolution metadata (mutation scale 0-1, lower_limit, upper_limit)
        # 超参数进化列表,括号里分别为(突变规模, 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
                'angle': (1, 0.2, 4.0),
                'angle_pw': (1, 0.5, 2.0),
                '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)

        assert opt.local_rank == -1, 'DDP mode not implemented for --evolve'
        opt.notest, opt.nosave = True, True  # only test/save final epoch
        # ei = [isinstance(x, (int, float)) for x in hyp.values()]  # evolvable indices
        yaml_file = Path('runs/evolve/hyp_evolved.yaml')  # save best result here
        if opt.bucket:
            os.system('gsutil cp gs://%s/evolve.txt .' % opt.bucket)  # download evolve.txt if exists

        # 默认进化300次
        """
            这里的进化算法是：根据之前训练时的hyp来确定一个base hyp再进行突变；
            如何根据？通过之前每次进化得到的results来确定之前每个hyp的权重
            有了每个hyp和每个hyp的权重之后有两种进化方式；
            1.根据每个hyp的权重随机选择一个之前的hyp作为base hyp，random.choices(range(n), weights=w)
            2.根据每个hyp的权重对之前所有的hyp进行融合获得一个base hyp，(x * w.reshape(n, 1)).sum(0) / w.sum()
            evolve.txt会记录每次进化之后的results+hyp
            每次进化时，hyp会根据之前的results进行从大到小的排序；
            再根据fitness函数计算之前每次进化得到的hyp的权重
            再确定哪一种进化方式，从而进行进化
        """
        for _ in range(300):  # generations to evolve
            if os.path.exists('evolve.txt'):  # if evolve.txt exists: select best hyps and mutate
                # Select parent(s)
                # 选择进化方式
                parent = 'single'  # parent selection method: 'single' or 'weighted'
                # 加载evolve.txt
                x = np.loadtxt('evolve.txt', ndmin=2)
                # 选取至多前5次进化的结果
                n = min(5, len(x))  # number of previous results to consider
                x = x[np.argsort(-fitness(x))][:n]  # top n mutations
                # 根据results计算hyp的权重
                w = fitness(x) - fitness(x).min()  # weights
                # 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
                # 超参数进化
                mp, s = 0.8, 0.2  # mutation probability, sigma
                npr = np.random
                npr.seed(int(time.time()))
                # 获取突变初始值
                g = np.array([x[0] for x in meta.values()])  # gains 0-1
                ng = len(meta)
                v = np.ones(ng)
                while all(v == 1):  # mutate until a change occurs (prevent duplicates)
                    v = (g * (npr.random(ng) < mp) * npr.randn(ng) * npr.random() * s + 1).clip(0.3, 3.0)
                # 将突变添加到base hyp上
                # [i+7]是因为x中前七个数字为results的指标(P, R, mAP, F1, test_losses=(GIoU, obj, cls))，之后才是超参数hyp
                for i, k in enumerate(hyp.keys()):  # plt.hist(v.ravel(), 300)
                    hyp[k] = float(x[i + 7] * v[i])  # mutate

            # Constrain to limits
            # 修剪hyp在规定范围里
            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
            results = train(hyp.copy(), opt, device)

            # Write mutation results
            """
            写入results和对应的hyp到evolve.txt
            evolve.txt文件每一行为一次进化的结果
            一行中前七个数字为(P, R, mAP, F1, test_losses=(GIoU, obj, cls))，之后为hyp
            保存hyp到yaml文件
            """
            print_mutation(hyp.copy(), results, yaml_file, opt.bucket)

        # Plot results
        plot_evolution(yaml_file)
        print('Hyperparameter evolution complete. Best results saved as: %s\nCommand to train a new model with these '
              'hyperparameters: $ python train.py --hyp %s' % (yaml_file, yaml_file))