Selecting tensorflow object detection API training hyper parameters

Question

I am setting up an object detection pipeline based on recently released tensorflow object detection API. I am using the arXiv as guidance. I am looking to understand the below for training on my own dataset.

1. It is not clear how they selected the learning rate schedules and how that would change based on the number of GPUs available for training. How do the training rate schedule change based on number of GPU's available for training? The paper mentions 9 GPUs are used. How should I change the training rate if I only want to use 1 GPU?

2. The released sample training config file for Pascal VOC using Faster R-CNN has initial learning rate = 0.0001. This is 10x lower than what was published in the original Faster-RCNN paper. Is this due to an assumption on the number of GPU's available for training or due to a different reason?

3. When I start training from the COCO detection checkpoint, how should the training loss decrease? Looking at tensorboard, on my dataset training loss is low - between 0.8 to 1.2 per iteration (with batch size of 1). Below image shows the various losses from tensorboard. . Is this expected behavior?
   

Answer 1

For questions 1 and 2: our implementation differs in a few small details compared to the original paper and internally we train all of our detectors with asynchronous SGD with ~10 GPUs. Our learning rates are calibrated for this setting (which you will also have if you decide to train via Cloud ML Engine as in the Pets walkthrough). If you use another setting, you will have to do a bit of hyperparameter exploration. For a single GPU, leaving the learning rate alone probably won't hurt performance, but you may be able to get faster convergence by increasing it.

For question 3: Training losses decrease erratically and you can only see the decrease if you smooth the plots quite a bit over time. Moreover, it's hard to explicitly say how well you are doing with respect to eval metrics just by looking at the training losses. I recommend looking at the mAP plots over time as well as the image visualizations to really get an idea of whether your model has "lifted off".

Hope this helps.