Pytorch训练网络过程中loss突然变为0的解决方案

问题

// loss 突然变成0
python train.py -b=8
INFO: Using device cpu
INFO: Network:
        1 input channels
        7 output channels (classes)
        Bilinear upscaling
INFO: Creating dataset with 868 examples
INFO: Starting training:
        Epochs:          5
        Batch size:      8
        Learning rate:   0.001
        Training size:   782
        Validation size: 86
        Checkpoints:     True
        Device:          cpu
        Images scaling:  1

Epoch 1/5:  10%|██████████████▏                                                                                                                            | 80/782 [01:33<13:21,  1.14s/img, loss (batch)=0.886I
NFO: Validation cross entropy: 1.86862473487854
Epoch 1/5:  20%|███████████████████████████▊                                                                                                            | 160/782 [03:34<11:51,  1.14s/img, loss (batch)=2.35e-7I
NFO: Validation cross entropy: 5.887489884504049e-10
Epoch 1/5:  31%|███████████████████████████████████████████▌                                                                                                  | 240/782 [05:41<11:29,  1.27s/img, loss (batch)=0I
NFO: Validation cross entropy: 0.0
Epoch 1/5:  41%|██████████████████████████████████████████████████████████                                                                                    | 320/782 [07:49<09:16,  1.20s/img, loss (batch)=0I
NFO: Validation cross entropy: 0.0
Epoch 1/5:  51%|████████████████████████████████████████████████████████████████████████▋                                                                     | 400/782 [09:55<07:31,  1.18s/img, loss (batch)=0I
NFO: Validation cross entropy: 0.0
Epoch 1/5:  61%|███████████████████████████████████████████████████████████████████████████████████████▏                                                      | 480/782 [12:02<05:58,  1.19s/img, loss (batch)=0I
NFO: Validation cross entropy: 0.0
Epoch 1/5:  72%|█████████████████████████████████████████████████████████████████████████████████████████████████████▋                                        | 560/782 [14:04<04:16,  1.15s/img, loss (batch)=0I
NFO: Validation cross entropy: 0.0
Epoch 1/5:  82%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████▏                         | 640/782 [16:11<02:49,  1.20s/img, loss (batch)=0I
NFO: Validation cross entropy: 0.0
Epoch 1/5:  92%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████▋           | 720/782 [18:21<01:18,  1.26s/img, loss (batch)=0I
NFO: Validation cross entropy: 0.0
Epoch 1/5:  94%|█████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████▋        | 736/782 [19:17<01:12,  1.57s/img, loss (batch)=0]
Traceback (most recent call last):
  File "train.py", line 182, in <module>
    val_percent=args.val / 100)
  File "train.py", line 66, in train_net
    for batch in train_loader:
  File "/public/home/lidd/.conda/envs/lgg2/lib/python3.6/site-packages/torch/utils/data/dataloader.py", line 819, in __next__
    return self._process_data(data)
  File "/public/home/lidd/.conda/envs/lgg2/lib/python3.6/site-packages/torch/utils/data/dataloader.py", line 846, in _process_data
    data.reraise()
  File "/public/home/lidd/.conda/envs/lgg2/lib/python3.6/site-packages/torch/_utils.py", line 385, in reraise
    raise self.exc_type(msg)
RuntimeError: Caught RuntimeError in DataLoader worker process 4.
Original Traceback (most recent call last):
  File "/public/home/lidd/.conda/envs/lgg2/lib/python3.6/site-packages/torch/utils/data/_utils/worker.py", line 178, in _worker_loop
    data = fetcher.fetch(index)
  File "/public/home/lidd/.conda/envs/lgg2/lib/python3.6/site-packages/torch/utils/data/_utils/fetch.py", line 47, in fetch
    return self.collate_fn(data)
  File "/public/home/lidd/.conda/envs/lgg2/lib/python3.6/site-packages/torch/utils/data/_utils/collate.py", line 74, in default_collate
    return {key: default_collate([d[key] for d in batch]) for key in elem}
  File "/public/home/lidd/.conda/envs/lgg2/lib/python3.6/site-packages/torch/utils/data/_utils/collate.py", line 74, in <dictcomp>
    return {key: default_collate([d[key] for d in batch]) for key in elem}
  File "/public/home/lidd/.conda/envs/lgg2/lib/python3.6/site-packages/torch/utils/data/_utils/collate.py", line 55, in default_collate
    return torch.stack(batch, 0, out=out)
RuntimeError: Expected object of scalar type Double but got scalar type Byte for sequence element 4 in sequence argument at position #1 'tensors'

交叉熵损失函数是衡量输出与标签之间的损失，通过求导确定梯度下降的方向。

loss突然变为0，有两种可能性。

一是因为预测输出为0，二是因为标签为0。

如果是因为标签为0，那么一开始loss就可能为0.

检查参数初始化

检查前向传播的网络

检查loss的计算格式

检查梯度下降

是否出现梯度消失。

实际上是标签出了错误

补充：pytorch训练出现loss=na

遇到一个很坑的情况，在pytorch训练过程中出现loss=nan的情况

有以下几种可能：

1.学习率太高。

2.loss函数有问题

3.对于回归问题，可能出现了除0 的计算，加一个很小的余项可能可以解决

4.数据本身，是否存在Nan、inf，可以用np.isnan(),np.isinf()检查一下input和target

5.target本身应该是能够被loss函数计算的，比如sigmoid激活函数的target应该大于0，同样的需要检查数据集

以上为个人经验，希望能给大家一个参考，也希望大家多多支持我们。如有错误或未考虑完全的地方，望不吝赐教。