# Troubles Training a Faster R-CNN RPN using a Resnet 101 backbone in Pytorch

Training Problems for a RPN

I am trying to train a network for region proposals as in the anchor box-concept from Faster R-CNN on the Pascal VOC 2012 training data.

I am using a pretrained Resnet 101 backbone with three layers popped off. The popped off layers are the conv5_x layer, average pooling layer, and softmax layer.

As a result my convolutional feature map fed to the RPN heads for images of size 600*600 results is of spatial resolution 37 by 37 with 1024 channels.

I have set the gradients of only block conv4_x to be trainable. From there I am using the torchvision.models.detection rpn code to use the rpn.AnchorGenerator, rpn.RPNHead, and ultimately rpn.RegionProposalNetwork classes. There are two losses that are returned by the call to forward, the objectness loss, and the regression loss.

The issue I am having is that my model is training very, very slowly (as in the loss is improving very slowly). In Girschick's original paper he says he trains over 80K minibatches (roughly 8 epochs since the Pascal VOC 2012 dataset has about 11000 images), where each mini batch is a single image with 256 anchor boxes, but my network from epoch to epoch improves its loss VERY SLOWLY, and I am training for 30 + epochs.

Below is my class code for the network.

class ResnetRegionProposalNetwork(torch.nn.Module):
def __init__(self):
super(ResnetRegionProposalNetwork, self).__init__()
self.resnet_backbone = torch.nn.Sequential(*list(models.resnet101(pretrained=True).children())[:-3])
non_trainable_backbone_layers = 5
counter = 0
for child in self.resnet_backbone:
if counter < non_trainable_backbone_layers:
for param in child.parameters():
counter += 1
else:
break

anchor_sizes = ((32,), (64,), (128,), (256,), (512,))
aspect_ratios = ((0.5, 1.0, 2.0),) * len(anchor_sizes)
self.rpn_anchor_generator = rpn.AnchorGenerator(
anchor_sizes, aspect_ratios
)
out_channels = 1024
out_channels, self.rpn_anchor_generator.num_anchors_per_location()[0]
)

rpn_pre_nms_top_n = {"training": 2000, "testing": 1000}
rpn_post_nms_top_n = {"training": 2000, "testing": 1000}
rpn_nms_thresh = 0.7
rpn_fg_iou_thresh = 0.7
rpn_bg_iou_thresh = 0.2
rpn_batch_size_per_image = 256
rpn_positive_fraction = 0.5

self.rpn = rpn.RegionProposalNetwork(
rpn_fg_iou_thresh, rpn_bg_iou_thresh,
rpn_batch_size_per_image, rpn_positive_fraction,
rpn_pre_nms_top_n, rpn_post_nms_top_n, rpn_nms_thresh)

def forward(self,
images,       # type: ImageList
targets=None  # type: Optional[List[Dict[str, Tensor]]]
):
feature_maps = self.resnet_backbone(images)
features = {"0": feature_maps}
image_sizes = getImageSizes(images)
image_list = il.ImageList(images, image_sizes)
return self.rpn(image_list, features, targets)


I am using the adam optimizer with the following parameters: optimizer = torch.optim.Adam(filter(lambda p: p.requires_grad, ResnetRPN.parameters()), lr=0.01, betas=(0.9, 0.999), eps=1e-08, weight_decay=0, amsgrad=False)

My training loop is here:

for epoch_num in range(epochs): # will train epoch number of times per execution of this program
loss_per_epoch = 0.0
current_epoch = epoch + epoch_num
saveModelDuringTraining(current_epoch, ResnetRPN, optimizer, running_loss)
batch_number = 0
for image_batch, ground_truth_box_batch in dl_iterator:
#print(batch_number)
boxes, losses = ResnetRPN(image_batch, ground_truth_box_batch)
losses = losses["loss_objectness"] + losses["loss_rpn_box_reg"]
losses.backward()
optimizer.step()
running_loss += float(losses)
batch_number += 1
if batch_number % 100 == 0:  # print the loss on every batch of 100 images
print('[%d, %5d] loss: %.3f' %
(current_epoch + 1, batch_number + 1, running_loss))
string_to_print = "\n epoch number:" + str(epoch + 1) + ", batch number:" \
+ str(batch_number + 1) + ", running loss: " + str(running_loss)
printToFile(string_to_print)
loss_per_epoch += running_loss
running_loss = 0.0
print("finished Epoch with epoch loss " + str(loss_per_epoch))
printToFile("Finished Epoch: " + str(epoch + 1) + " with epoch loss: " + str(loss_per_epoch))
loss_per_epoch = 0.0


I am considering trying the following ideas to fix the network training very slowly:

• trying various learning rates (although I have already tried 0.01, 0.001, 0.003 with similar results
• various batch sizes (so far the best results have been batches of 4 (4 images * 256 anchors per image)
• freezing more/less layers of the Resnet-101 backbone
• using a different optimizer altogether
• different weightings of the loss function

Any hints or things obviously wrong with my approach MUCH APPRECIATED. I would be happy to give any more information to anyone who can help.

Edit: My network is training on a fast GPU, with the images and bounding boxes as torch tensors.

Here we are assuming that there is a pattern in your data. My questions will be:

• Are you sure that there is a common pattern to detect in your data?
• Are the labels well assigned? Sometimes even if data is good you can have some errors while creating the label that throw away hours of work.

For the rest of your code, it seems okay. In order to debug it properly you can try:

• Find a problem that is really similar and you know that the model will be successful. Some toy dataset should make it. Then train the same architecture and see if the loss drops. This way you will check if what you have done in pytorch is right.
• Use another algorithm or some else implementation. It might be that ResNet101 is not suitable for the problem.

Hope it helps :)

• I like your idea of training on a toy dataset, I will try this approach. I will also compare the training results on VGG-16 to see if ResNet101 is the issue. Oct 11, 2020 at 18:25
• I wonder what a toy dataset would be for learning ROIs however? Any ideas appreciated. Oct 11, 2020 at 18:37
• @IntegrateThis what does the literature say about toydata for ROI? Oct 12, 2020 at 6:05

So far I have tried a few things that have helped a lot:

• First, embarassingly I was inputting images in BGR format to a network trained on RGB format.
• Second, trying the optimizer:
optimizer = torch.optim.SGD(filter(lambda p: p.requires_grad, ResnetRPN.parameters()), lr=0.001, momentum=0.9, weight_decay=0.0005)


Perhaps the Adam optimizer is not good for convolutional neural networks ??

as in the original paper, in addition to a learning rate scheduler that after 24 epochs decreases the learning rate to 0.0001.

As for which layers to freeze, I am going to try pretty much everything including:

• only training the RPN heads
• freezing 1 layer and no longer removing any of the Resnet101 sequential blocks
• training the entire thing from scratch without pre-trained weights
• training the entire thing from scratch with pre-trained weights

Moreover, the normalization of the input images was tuned for the Imagenet dataset, which has different channel means and standard deviations than the Pascal VOC 2012 dataset.

Further, to test just the RPN I have written a class of 4 comparison RPNS which generate random boxes:

• random boxes in the image of any width, height, centre position
• random boxes from each of the four image quadrants of random width and height from an array dimensions = [4, 16, 32, 64, 128, 256, 512]
• random anchor boxes without learned displacements as in the anchor boxes used in Faster RCNN
• Finding the mean and (std) of the x_min, y_min and width, and height of bounding boxes in the Pascal VOC 2012 training set, and randomly sampling from a normal distribution of each of these values (and using math.floor, math.ceil to make them valid boxes)

My network is at the very least outperforming the ROIS performed by these comparison RPNs, which I am measuring by computing the max IOU for each box per image with the 300 ROIS generated per image by the RPNS.

I am also going to train my network on MS COCO 2014 train_val data. I hope this info helps someone.