tf.train.experimental.enable_mixed_precision_graph_rewrite

Enable mixed precision via a graph rewrite. (deprecated)

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.

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.
  • AllowList: Ops that are considered numerically safe for execution in float16, and thus are always converted. E.g. Conv2D.
  • DenyList: 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 DenyList 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 a