tf.compat.v1.estimator.tpu.TPUEstimator

Estimator with TPU support.

Inherits From: Estimator

TPUEstimator also supports training on CPU and GPU. You don't need to define a separate tf.estimator.Estimator.

TPUEstimator handles many of the details of running on TPU devices, such as replicating inputs and models for each core, and returning to host periodically to run hooks.

TPUEstimator transforms a global batch size in params to a per-shard batch size when calling the input_fn and model_fn. Users should specify global batch size in constructor, and then get the batch size for each shard in input_fn and model_fn by params['batch_size'].

  • For training, model_fn gets per-core batch size; input_fn may get per-core or per-host batch size depending on per_host_input_for_training in TPUConfig (See docstring for TPUConfig for details).

  • For evaluation and prediction, model_fn gets per-core batch size and input_fn get per-host batch size.

Evaluation

model_fn should return TPUEstimatorSpec, which expects the eval_metrics for TPU evaluation. If eval_on_tpu is False, the evaluation will execute on CPU or GPU; in this case the following discussion on TPU evaluation does not apply.

TPUEstimatorSpec.eval_metrics is a tuple of metric_fn and tensors, where tensors could be a list of any nested structure of Tensors (See TPUEstimatorSpec for details). metric_fn takes the tensors and returns a dict from metric string name to the result of calling a metric function, namely a (metric_tensor, update_op) tuple.

One can set use_tpu to False for testing. All training, evaluation, and predict will be executed on CPU. input_fn and model_fn will receive train_batch_size or eval_batch_size unmodified as params['batch_size'].

Current limitations:


  1. TPU evaluation only works on a single host (one TPU worker) except BROADCAST mode.

  2. input_fn for evaluation should NOT raise an end-of-input exception (OutOfRangeError or StopIteration). And all evaluation steps and all batches should have the same size.

Example (MNIST):

# The metric Fn which runs on CPU.
def metric_fn(labels, logits):
  predictions = tf.argmax(logits, 1)
  return {
    'accuracy': tf.compat.v1.metrics.precision(
        labels=labels, predictions=predictions),
  }

# Your model Fn which runs on TPU (eval_metrics is list in this example)
def model_fn(features, labels, mode, config, params):
  ...
  logits = ...

  if mode = tf.estimator.ModeKeys.EVAL:
    return tpu_estimator.TPUEstimatorSpec(
        mode=mode,
        loss=loss,
        eval_metrics=(metric_fn, [labels, logits]))

# or specify the eval_metrics tensors as dict.
def model_fn(features, labels, mode, config, params):
  ...
  final_layer_output = ...

  if mode = tf.estimator.ModeKeys.EVAL:
    return tpu_estimator.TPUEstimatorSpec(
        mode=mode,
        loss=loss,
        eval_metrics=(metric_fn, {
            'labels': labels,
            'logits': final_layer_output,
        }))

Prediction

Prediction on TPU is an experimental feature to support large batch inference. It is not designed for latency-critical system. In addition, due to some usability issues, for prediction with small dataset, CPU .predict, i.e., creating a new TPUEstimator instance with use_tpu=False, might be more convenient.

Current limitations:


  1. TPU prediction only works on a single host (one TPU worker).

  2. input_fn must return a Dataset instance rather than features. In fact, .train() and .evaluate() also support Dataset as return value.

Example (MNIST):

height = 32
width = 32
total_examples = 100

def predict_input_fn(params):
  batch_size = params['batch_size']

  images = tf.random.uniform(
      [total_examples, height, width, 3], minval=-1, maxval=1)

  dataset = tf.data.Dataset.from_tensor_slices(images)
  dataset = dataset.map(lambda images: {'image': images})

  dataset = dataset.batch(batch_size)
  return dataset

def model_fn(features, labels, params, mode):
   # Generate predictions, called 'output', from features['image']

  if mode == tf.estimator.ModeKeys.PREDICT:
    return tf.contrib.tpu.TPUEstimatorSpec(
        mode=mode,
        predictions={
            'predictions': output,
            'is_padding': features['is_padding']
        })

tpu_est = TPUEstimator(
    model_fn=model_fn,
    ...,
    predict_batch_size=16)

# Fully consume the generator so that TPUEstimator can shutdown the TPU
# system.
for item in tpu_est.predict(input_fn=input_fn):
  # Filter out item if the `is_padding` is 1.
  # Process the 'predictions'

Exporting

export_saved_model exports 2 metagraphs, one with saved_model.SERVING, and another with saved_model.SERVING and saved_model.TPU tags. At serving time, these tags are used to select the appropriate metagraph to load.

Before running the graph on TPU, the TPU system needs to be initialized. If TensorFlow Serving model-server is used, this is done automatically. If not, please use session.run(tpu.initialize_system()).

There are two versions of the API: 1 or 2.

In V1, the exported CPU graph is model_fn as it is. The exported TPU graph wraps tpu.rewrite() and TPUPartitionedCallOp around model_fn so model_fn is on TPU by default. To place ops on CPU, tpu.outside_compilation(host_call, logits) can be used.

Example:


def model_fn(features, labels, mode, config, params):
  ...
  logits = ...
  export_outputs = {
    'logits': export_output_lib.PredictOutput(
      {'logits': logits})
  }

  def host_call(logits):
    class_ids = math_ops.argmax(logits)
    classes = string_ops.as_string(class_ids)
    export_outputs['classes'] =
      export_output_lib.ClassificationOutput(classes=classes)

  tpu.outside_compilation(host_call, logits)

  ...

In V2, export_saved_model() sets up params['use_tpu'] flag to let the user know if the code is exporting to TPU (or not). When params['use_tpu'] is True, users need to call tpu.rewrite(), TPUPartitionedCallOp and/or batch_function(). Alternatively use inference_on_tpu() which is a convenience wrapper of the three.

  def model_fn(features, labels, mode, config, params):
    ...
    # This could be some pre-processing on CPU like calls to input layer with
    # embedding columns.
    x2 = features['x'] * 2

    def computation(input_tensor):
      return layers.dense(
          input_tensor, 1, kernel_initializer=init_ops.zeros_initializer())

    inputs = [x2]
    if params['use_tpu']:
      predictions = array_ops.identity(
          tpu_estimator.inference_on_tpu(computation, inputs,
          num_batch_threads=1, max_batch_size=2, batch_timeout_micros=100),
          name='predictions')
    else:
      predictions = array_ops.identity(
          computation(*inputs), name='predictions')
    key = signature_constants.DEFAULT_SERVING_SIGNATURE_DEF_KEY
    export_outputs = {
        key: export_lib.PredictOutput({'prediction': predictions})
    }
    ...

TIP: V2 is recommended as it is more flexible (eg: batching, etc).

model_fn Model function as required by Estimator which returns EstimatorSpec or TPUEstimatorSpec. training_hooks, 'evaluation_hooks', and prediction_hooks must not capure any TPU Tensor inside the model_fn.
model_dir Directory to save model parameters, graph and etc. This can also be used to load checkpoints from the directory into a estimator to continue training a previously saved model. If None, the model_dir in config will be used if set. If both are set, they must be same. If both are None, a temporary directory will be used.
config An tpu_config.RunConfig configuration object. Cannot be None.
params An optional dict of hyper parameters that will be passed into input_fn and model_fn. Keys are names of parameters, values are basic python types. There are reserved keys for TPUEstimator, including 'batch_size'.
use_tpu A bool indicating whether TPU support is enabled. Currently, - TPU training and evaluation respect this bit, but