Thanks everyone.
My dataset contains 15 million images. I have convert them into lmdb format and concat them
At first I set shuffle = False，envery iteration’s IO take no extra cost.
Inorder to improve the performance , I set it into True and use num_workers.
train_data = ConcatDataset([train_data_1,train_data_2])
    train_loader = DataLoader(dataset=train_data, batch_size=64,num_workers=32, shuffle=True,pin_memory=False)
for i in range(epochs):
for j,data in enumerate(train_loader):
                print(i,"ITER",j,"IO END",datetime.now())
                continue

But IO takes too much time.

Is there something I can do to makes IO faster?