I have tried to display the data loading cost of each iteration in the training phase and validation phase, the observation as follow:

the first iteration of the first epoch cost so much time: 8s around for training dataset and 6s around for the validation dataset ( training dataset has 18944 images in 256x256 and validation dataset has 4144 images in 256x256) it seems normal due to the different size.
time cost of other iteration ( except the first iter of the first epoch ) all at the same level.
I think the display in training phase and validation phase is ok because batch_size is the same.

so, what could cause the utilization of validation phase close to zero?
Looking forward to your help, thank you!