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Summary by Alexander Jung 7 years ago
* Weight Normalization (WN) is a normalization technique, similar to Batch Normalization (BN).
* It normalizes each layer's weights.
### Differences to BN
* WN normalizes based on each weight vector's orientation and magnitude. BN normalizes based on each weight's mean and variance in a batch.
* WN works on each example on its own. BN works on whole batches.
* WN is more deterministic than BN (due to not working an batches).
* WN is better suited for noisy environment (RNNs, LSTMs, reinforcement learning, generative models). (Due to being more deterministic.)
* WN is computationally simpler than BN.
### How its done
* WN is a module added on top of a linear or convolutional layer.
* If that layer's weights are `w` then WN learns two parameters `g` (scalar) and `v` (vector, identical dimension to `w`) so that `w = gv / ||v||` is fullfilled (`||v||` = euclidean norm of v).
* `g` is the magnitude of the weights, `v` are their orientation.
* `v` is initialized to zero mean and a standard deviation of 0.05.
* For networks without recursions (i.e. not RNN/LSTM/GRU):
* Right after initialization, they feed a single batch through the network.
* For each neuron/weight, they calculate the mean and standard deviation after the WN layer.
* They then adjust the bias to `-mean/stdDev` and `g` to `1/stdDev`.
* That makes the network start with each feature being roughly zero-mean and unit-variance.
* The same method can also be applied to networks without WN.
### Results:
* They define BN-MEAN as a variant of BN which only normalizes to zero-mean (not unit-variance).
* CIFAR-10 image classification (no data augmentation, some dropout, some white noise):
* WN, BN, BN-MEAN all learn similarly fast. Network without normalization learns slower, but catches up towards the end.
* BN learns "more" per example, but is about 16% slower (time-wise) than WN.
* WN reaches about same test error as no normalization (both ~8.4%), BN achieves better results (~8.0%).
* WN + BN-MEAN achieves best results with 7.31%.
* Optimizer: Adam
* Convolutional VAE on MNIST and CIFAR-10:
* WN learns more per example und plateaus at better values than network without normalization. (BN was not tested.)
* Optimizer: Adamax
* DRAW on MNIST (heavy on LSTMs):
* WN learns significantly more example than network without normalization.
* Also ends up with better results. (Normal network might catch up though if run longer.)
* Deep Reinforcement Learning (Space Invaders):
* WN seemed to overall acquire a bit more reward per epoch than network without normalization. Variance (in acquired reward) however also grew.
* Results not as clear as in DRAW.
* Optimizer: Adamax
### Extensions
* They argue that initializing `g` to `exp(cs)` (`c` constant, `s` learned) might be better, but they didn't get better test results with that.
* Due to some gradient effects, `||v||` currently grows monotonically with every weight update. (Not necessarily when using optimizers that use separate learning rates per parameters.)
* That grow effect leads the network to be more robust to different learning rates.
* Setting a small hard limit/constraint for `||v||` can lead to better test set performance (parameter updates are larger, introducing more noise).
*Performance of WN on CIFAR-10 compared to BN, BN-MEAN and no normalization.*
*Performance of WN for DRAW (left) and deep reinforcement learning (right).*
