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Keras API reference /
Mixed precision /
LossScaleOptimizer

`BaseLossScaleOptimizer`

class```
tf.keras.mixed_precision.LossScaleOptimizer()
```

An optimizer that applies loss scaling to prevent numeric underflow.

Loss scaling is a technique to prevent numeric underflow in intermediate gradients when float16 is used. To prevent underflow, the loss is multiplied (or "scaled") by a certain factor called the "loss scale", which causes intermediate gradients to be scaled by the loss scale as well. The final gradients are divided (or "unscaled") by the loss scale to bring them back to their original value.

`LossScaleOptimizer`

wraps another optimizer and applies loss scaling to it.
By default, the loss scale is dynamically updated over time so you do not
have to choose the loss scale. The `minimize`

method automatically scales
the loss, unscales the gradients, and updates the loss scale so all you have
to do is wrap your optimizer with a `LossScaleOptimizer`

if you use
`minimize`

. For example:

```
>>> opt = tf.keras.optimizers.SGD(0.25)
>>> opt = tf.keras.mixed_precision.LossScaleOptimizer(opt)
>>> var = tf.Variable(1.)
>>> loss_fn = lambda: var ** 2
>>> # 'minimize' applies loss scaling and updates the loss sale.
>>> opt.minimize(loss_fn, var_list=var)
>>> var.numpy()
0.5
```

If a `tf.GradientTape`

is used to compute gradients instead of `minimize`

,
you must scale the loss and gradients manually. This can be done with the
`LossScaleOptimizer.get_scaled_loss`

and
`LossScaleOptimizer.get_unscaled_gradients`

methods. For example:

```
>>> with tf.GradientTape() as tape:
... loss = loss_fn()
... scaled_loss = opt.get_scaled_loss(loss)
>>> scaled_grad = tape.gradient(scaled_loss, var)
>>> (grad,) = opt.get_unscaled_gradients([scaled_grad])
>>> opt.apply_gradients([(grad, var)]) # Loss scale is updated here
>>> var.numpy()
0.25
```

Warning: If you forget to call `get_scaled_loss`

or `get_unscaled_gradients`

(or both) when using a `tf.GradientTape`

, the model will likely converge to
a worse quality. Please make sure you call each function exactly once.

When mixed precision with float16 is used, there is typically no risk of underflow affecting model quality if loss scaling is properly used. See the mixed precision guide for more information on how to use mixed precision.

**Arguments**

**inner_optimizer**: The`tf.keras.optimizers.Optimizer`

or`tf.keras.optimizers.experimental.Optimizer`

instance to wrap.**dynamic**: Bool indicating whether dynamic loss scaling is used. Defaults to True. If True, the loss scale will be dynamically updated over time using an algorithm that keeps the loss scale at approximately its optimal value. If False, a single fixed loss scale is used and`initial_scale`

must be specified, which is used as the loss scale. Recommended to keep as True, as choosing a fixed loss scale can be tricky. Currently, there is a small performance overhead to dynamic loss scaling compared to fixed loss scaling.**initial_scale**: The initial loss scale. If`dynamic`

is True, this defaults to`2 ** 15`

. If`dynamic`

is False, this must be specified and acts as the sole loss scale, as the loss scale does not change over time. When dynamic loss scaling is used, is better for this to be a very high number, because a loss scale that is too high gets lowered far more quickly than a loss scale that is too low gets raised.**dynamic_growth_steps**: With dynamic loss scaling, every`dynamic_growth_steps`

steps with finite gradients, the loss scale is doubled. Defaults to 2000. If a nonfinite gradient is encountered, the count is reset back to zero, gradients are skipped that step, and the loss scale is halved. The count can be queried with`LossScaleOptimizer.dynamic_counter`

. This argument can only be specified if`dynamic`

is True.

`LossScaleOptimizer`

will occasionally skip applying gradients to the
variables, in which case the trainable variables will not change that step.
This is done because the dynamic loss scale will sometimes be raised too
high, causing overflow in the gradients. Typically, the first 2 to 15 steps
of the model are skipped as the initial loss scale is very high, but
afterwards steps will only be skipped on average 0.05% of the time (the
fraction of steps skipped is `1 / dynamic_growth_steps`

).

`LossScaleOptimizer`

delegates all public `Optimizer`

methods to the inner
optimizer. Additionally, in methods `minimize`

and `get_gradients`

, it
scales the loss and unscales the gradients. In methods `minimize`

and
`apply_gradients`

, it additionally updates the loss scale and skips applying
gradients if any gradient has a nonfinite value.

If wrapping a `tf.keras.optimizers.Optimizer`

, hyperparameters can be
accessed and set on the LossScaleOptimizer, which will be delegated to the
wrapped optimizer.

```
>>> opt = tf.keras.optimizers.Adam(beta_1=0.8, epsilon=1e-5)
>>> opt = tf.keras.mixed_precision.LossScaleOptimizer(opt)
>>> opt.beta_1 # Equivalent to `opt.inner_optimizer.beta_1`
0.8
>>> opt.beta_1 = 0.7 # Equivalent to `opt.inner_optimizer.beta_1 = 0.7`
>>> opt.beta_1
0.7
>>> opt.inner_optimizer.beta_1
0.7
```

However, accessing or setting non-hyperparameters is not delegated to the
LossScaleOptimizer. In an Adam optimizer, `beta_1`

is a hyperparameter but
`epsilon`

is not, as the Adam optimizer only calls `Optimizer._set_hyper`

on
`beta_1`

.

```
>>> opt.inner_optimizer.epsilon
1e-5
>>> opt.epsilon
Traceback (most recent call last):
...
AttributeError: 'LossScaleOptimizer' object has no attribute 'epsilon'
>>> opt.epsilon = 1e-4 # This does NOT set epsilon on `opt.inner_optimizer`
>>> opt.inner_optimizer.epsilon
>>> 1e-5
```

In the above example, despite epsilon being set on the LossScaleOptimizer, the old epsilon value will still be used when training as epsilon was not set on the inner optimizer.