tf.train.experimental.enable_mixed_precision_graph_rewrite

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Enable mixed precision via a graph rewrite.

tf.train.experimental.enable_mixed_precision_graph_rewrite(
    opt, loss_scale='dynamic'
)

Mixed precision is the use of both float32 and float16 data types when training a model to improve performance. This is achieved via a graph rewrite operation and a loss-scale optimizer.

Performing arithmetic operations in float16 takes advantage of specialized processing units, such as NVIDIA Tensor Cores, for much higher arithmetic throughput. However, due to the smaller representable range, performing the entire training with float16 can result in gradient underflow, that is, small gradient values becoming zeroes. Instead, performing only select arithmetic operations in float16 results in higher throughput and decreased training time when using compatible hardware accelerators while also reducing memory usage, typically without sacrificing model accuracy.

Note: While the mixed precision rewrite changes the datatype of various layers throughout the model, the same accuracy reached in float32 is expected. If a NaN gradient occurs with dynamic loss scaling, the model update for that batch is skipped. In this case, the global step count is not incremented, and the LossScaleOptimizer attempts to decrease the loss scaling value to avoid NaN values in subsequent iterations. This approach has been shown to achieve the same accuracy as float32 and, in most cases, better training throughput.

Example:

model = tf.keras.models.Sequential([
    tf.keras.layers.Dense(64, activation='relu'),
    tf.keras.layers.Dense(64, activation='softmax'),
])

opt = tf.keras.optimizers.SGD()
opt = tf.train.experimental.enable_mixed_precision_graph_rewrite(opt)
model.compile(loss="mse", optimizer=opt)

x_train = np.random.random((1024, 64))
y_train = np.random.random((1024, 64))
model.fit(x_train, y_train)

Calling enable_mixed_precision_graph_rewrite(opt) enables the graph rewrite operation before computing gradients. The function additionally returns an Optimizer (opt) wrapped with a LossScaleOptimizer. This prevents underflow in the float16 tensors during the backward pass. An optimizer of type tf.keras.optimizers.Optimizer or tf.compat.v1.train.Optimizer must be passed to this function, which will then be wrapped to use loss scaling.

The graph rewrite operation changes the dtype of certain operations in the graph from float32 to float16. There are several categories of operations that are either included or excluded by this rewrite operation. The following categories of Ops are defined inside corresponding functions under the class AutoMixedPrecisionLists in auto_mixed_precision_lists.h:

• ClearList: Ops that do not have numerically significant adverse effects. E.g. ArgMax and Floor.
• WhiteList: Ops that are considered numerically safe for execution in float16, and thus are always converted. E.g. Conv2D.
• BlackList: Ops that are numerically unsafe to execute in float16 and can negatively affect downstream nodes. E.g. Softmax.
• GrayList: Ops that are considered numerically safe for execution in float16 unless downstream from a BlackList Op. E.g. Add and AvgPool.

When this function is used, gradients should be computed and applied with the returned optimizer, either by calling opt.minimize() or opt.compute_gradients() followed by opt.apply_gradients(). If gradients are instead computed with tf.gradients or tf.GradientTape, loss scaling will not be applied, which will likely cause your model not to converge due to float16 underflow problems. To apply lossing scaling with tf.gradients or tf.GradientTape, LossScaleOptimizer.get_scaled_loss and LossScaleOptimizer.get_unscaled_gradients. See keras.mixed_precision.experimental.LossScaleOptimizer for details how to do this.

When eager execution is enabled, the mixed precision graph rewrite is only enabled within tf.functions, as outside tf.functions, there is no graph.

For NVIDIA GPUs with Tensor cores, as a general performance guide, dimensions (such as batch size, input size, output size, and channel counts) should be powers of two if under 256, or otherwise divisible by 8 if above 256. For more information, check out the NVIDIA Deep Learning Performance Guide.

Currently, mixed precision is only enabled on NVIDIA Tensor Core GPUs with Compute Capability 7.0 and above (Volta, Turing, or newer architectures). The parts of the graph on CPUs and TPUs are untouched by the graph rewrite.

Comparison with the Keras mixed precision API

Both this function and the Keras mixed precision API enable the use of mixed precision in a model. Therefore, only one of the two APIs can be used. We recommend using the Keras mixed precision API, as it is more customizable and supports Eager execution. However, it only supports models which use Keras layers, while the graph rewrite works in any model that uses tf.functions.

The core difference between the two APIs is that this function is a graph rewrite, and so it changes the graph to use mixed precision under the hood. You still build your graph in float32, and the graph rewrite will change certain ops to float16. The Keras mixed precision API directly builds the Keras Model using a mix of float16 and float32.

One core advantage of the Keras API is it supports mixed precision with Eager execution, i.e. mixed precision outside tf.functions. The graph rewrite will only affect ops within tf.functions, making it harder to debug if issues occur with mixed precision. The Keras API is also more customizable, as you can override any layer to run in float32 by passing dtype="float32" to the layer constructor. Additionally, you can query the dtype of tensors in the model by checking tensor.dtype. With the graph rewrite, all tensors appear to be float32 since the dtype is only changed under the hood.

The main advantage of the graph rewrite (this function) is that it works even if you do not use Keras layers or any other part of Keras. The Keras mixed precision API requires models which use Keras layers, as it only inserts casts inside Keras layers and models. Another advantage is that the graph rewrite never results in a TypeError, which the Keras API may introduce if you do certain operations outside Keras. For example, the following will result in a TypeError if the Keras mixed precision API is enabled, as a float16 and float32 tensor will be added: tf.keras.layers.Dense(2)(x) + tf.keras.layers.Dense(2, dtype="float32")(x)

Raises
ValueError, if the tf.keras.mixed_precision API is also used by calling tf.keras.mixed_precision.experimental.set_policy. Only one mixed precision API can be used.

Args
opt	An instance of a tf.keras.optimizers.Optimizer.
loss_scale	Either an int/float, the string "dynamic", or an instance of a tf.mixed_precision.experimental.LossScale. The loss scale to use. It is recommended to keep this as its default value of "dynamic", which will adjust the scaling automatically to prevent Inf or NaN values.

Returns
A version of opt that will use loss scaling to prevent underflow.