# EMA#

class composer.algorithms.EMA(half_life='1000ba', smoothing=None, ema_start='0.0dur', update_interval=None)[source]#

Maintains a shadow model with weights that follow the exponential moving average of the trained model weights.

Weights are updated according to

$W_{ema_model}^{(t+1)} = smoothing\times W_{ema_model}^{(t)}+(1-smoothing)\times W_{model}^{(t)}$

Where the smoothing is determined from half_life according to

$smoothing = \exp\left[-\frac{\log(2)}{t_{1/2}}\right]$

Model evaluation is done with the moving average weights, which can result in better generalization. Because of the shadow models, EMA triples the model’s memory consumption. Note that this does not mean that the total memory required doubles, since stored activations and the optimizer state are not duplicated. EMA also uses a small amount of extra compute to update the moving average weights.

See the Method Card for more details.

Parameters
• half_life (str, optional) – The time string specifying the half life for terms in the average. A longer half life means old information is remembered longer, a shorter half life means old information is discared sooner. A half life of 0 means no averaging is done, an infinite half life means no update is done. Currently only units of epoch (‘ep’) and batch (‘ba’). Time must be an integer value in the units specified. Cannot be used if smoothing is also specified. Default: "1000ba".

• smoothing (float, optional) – The coefficient representing the degree to which older observations are kept. Must be in the interval $$(0, 1)$$. Cannot be used if half_life also specified. This value will not be adjusted if update_interval is changed. Default: None.

• ema_start (str, optional) – The time string denoting the amount of training completed before EMA begins. Currently only units of duration (‘dur’), batch (‘ba’) and epoch (‘ep’) are supported. Default: '0.0dur'.

• update_interval (str, optional) – The time string specifying the period at which updates are done. For example, an update_interval='1ep' means updates are done every epoch, while update_interval='10ba' means updates are done once every ten batches. Units must match the units used to specify half_life if not using smoothing. If not specified, update_interval will default to 1 in the units of half_life, or "1ba" if smoothing is specified. Time must be an integer value in the units specified. Default: None.

Example

from composer.algorithms import EMA
algorithm = EMA(half_life='1000ba', update_interval='1ba')
trainer = Trainer(
model=model,