# ⏮️ Selective Backprop#

Computer Vision

Selective Backprop prioritizes examples with high loss at each iteration, skipping backpropagation on examples with low loss. This speeds up training with limited impact on generalization.

Four examples are forward propagated through the network. Selective backprop only backpropagates the two examples that have the highest loss.

## Suggested Hyperparameters#

We recommend setting start=0.5 and end=0.9. This performs Selective Backprop halfway through training and stops performing Selective Backprop at 90% of the way through training. We found that the network performs better when it has time at both the beginning and end of training where it is exposed to all training examples.

We recommend setting keep=0.5, which keeps half of the examples on each step where Selective Backprop is performed. We found a value of 0.5 to represent a good tradeoff that greatly improves speed at limited cost to quality. This is likely to be the hyperparameter most worth tuning.

We recommend setting interrupt=2, which performs a standard training batch after every two batches of Selective Backprop. We found that including some unmodified batches is worth the tradeoff in speed, although a value of 0 is also worth considering.

We recommend setting scale_factor=0.5, which downsamples the height and width image examples by 50% on the first forward pass (the one that selects which examples to train on). This mitigates the cost of that additional forward pass.

❗ The Network Must Be Able to Handle Lower Resolution Images to Use scale_factor

Using the scale_factor hyperparameter requires a network and data preparation pipeline capable of handling lower resolution images. If your pipeline and network are not capable of doing so, set this hyperparameter to 1.0.

## Technical Details#

The goal of Selective Backprop is to reduce the number of examples the model sees to only those with high loss. This lets the model learn on fewer examples, speeding up forward and backward propagation with limited impact on final model quality. To determine the per-example loss and which examples to skip, an additional, initial forward pass must be performed. The loss values from this pass are then used to weight the examples, and the network is trained on a sample of examples selected based on those weights.

🚧 Requires an Additional Forward Pass on Each Step

Selective backprop must perform two forward passes on each training step. The first forward pass computes the loss for each example. The main training step then occurs, with a forward and backward pass for any examples selected after the first forward pass. This additional forward pass can slow down training depending on the number of examples that are dropped. The forward pass accounts for approximately one third of the cost of each training step, so at least a third of the examples must hypothetically be dropped for selective backprop to improve throughput.

✅ The Cost of the Additional Forward Pass Can Be Mitigated

For some data types, including images, it is possible to mitigate the cost of this additional forward pass. The first forward pass does not need to be as precise as the second forward pass, since it is only selecting how to weight the examples, not how to update the network. As such, this first forward pass can be approximate. Our implementation of Selective Backprop for image datasets provides the option to perform the first forward pass at lower resolution (see the scale_factor hyperparameter), reducing the burden imposed by this additional forward pass.

Depending on the precise hyperparameters chosen, we see decreases in training time of around 10% without any degradation in performance. Larger values are possible, but these run into speed-accuracy tradeoffs. Namely, the more data that is eliminated for longer periods of training, the larger the potential impact on model performance. The default hyperparameters listed above have worked well for us and strike a good balance between attaining speedup and maintaining model quality. We found several techniques for mitigating accuracy degradation, including starting Selective Backprop mid-way through training (see the start hyperparameter), disabling it before the end of training to allow fine-tuning with the standard training regime (see the end hyperparameter), and mixing in occasional iterations where all data is used (see the interrupt hyperparameter).

We have explored Selective Backprop primarily on image recognition tasks such as ImageNet and CIFAR-10. For both of these, we see large improvements in training time with little degradation in accuracy. The table below shows some examples using the default hyperparameters from above. For CIFAR-10, ResNet-56 was trained on 1 x NVIDIA 3080 GPU for 160 epochs. For ImageNet, ResNet-50 was trained on 8 x NVIDIA 3090 GPUs for 90 epochs.

Dataset

Run

Validation Accuracy

Time to Train

ImageNet

Baseline

76.46%

5h 43m 8s

+ Selective Backprop

76.46%

5h 22m 14s

CIFAR-10

Baseline

93.16%

35m 33s

+ Selective Backprop

93.32%

32m 36s

✅ Selective Backprop Improves the Tradeoff Between Quality and Training Speed

In our experiments, Selective Backprop improves the attainable tradeoffs between training speed and the final quality of the trained model. In some cases, it leads to slightly lower quality than the original model for the same number of training steps. However, Selective Backprop increases training speed so much (via improved throughput) that it is possible to train for more steps, recover accuracy, and still complete training in less time.