low result with only 1 GPU

[Harley-Sun]

Hi Yixiao, I have tried to run code with only 1 2080ti, but got a very low result（eg. 20% mAP drop on market1501）, so what causes the performance drop? I can't undersand. Could you please explain for me?

[yxgeee]

ReID tasks are very sensitive to the number of GPUs, and I think this is caused by the batch size for BN. For example, in default, I use a batch size of 64 with 4 GPUs, indicating that 16 images are used for calculating mean and std in BN. If you use only one GPU and still 64 images in a batch, 64 images are used for BN. Maybe you could try to decrease the batch size from 64 to 16. But I am not sure if the current lr is the optimal setup for one GPU. You need to tune it. For convenience, I recommend using the default settings (4 GPUs, 64 bs) for reproducing the results.