tf.compat / v1 / v1.tpu.cross_replica_sum

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Builds part of a computation outside any current TPU replicate scope.

tf.compat.v1.tpu.outside_compilation(
    computation, *args, **kwargs
)

tf.tpu.outside_compilation() is used to run ops in computation on CPU instead of running on TPU. For example, users can run ops that are not supported on TPU's (e.g. tf.summary.write()) by explicitly placing those ops on CPU's. Below usage of outside compilation will place ops in computation_with_string_ops on CPU.

Example usage:

def computation_with_string_ops(x):
  # strings types are not supported on TPU's and below ops must
  # run on CPU instead.
  output = tf.strings.format('1{}', x)
  return tf.strings.to_number(output)

def tpu_computation():
  # Expected output is 11.
  output = tf.tpu.outside_compilation(computation_with_string_ops, 1)

Outside compilation should be called inside TPUReplicateContext. That is, tf.tpu.outside_compilation() should be called inside a function that is passed to tpu.split_compile_and_replicate() -- this is implied when outside compilation is invoked inside a function passed to TPUStrategy run(). If invoked outside of TPUReplicateContext, then this simply returns the result of computation, and therefore, would be a no-op. Note that outside compilation is different from tf.distribute.experimental.TPUStrategy.merge_call() as logic in outside compilation is replicated and executed separately for each replica. On the other hand, merge_call() requires a merge_fn to aggregate the inputs from different replicas and is executed only once.

For variables placed in TPU device, which includes variables created inside TPUStrategy scope, outside compilation logic must not include variable read/write. For variables placed on host, which is the case when variables created via TPUEstimator, variable read/write is only allowed if the variable is not accessed by any other ops in the TPU computation. Variable read/write from outside compilation cluster is not visible from TPU computation and vice versa. Therefore, if outside compilation logic contains such host variables read/write ops and if the variables are accessed by TPU computation as well, then this may lead to deadlock.

Internally, tf.tpu.outside_compilation() adds outside compilation attributes to all ops in computation. During later graph pass, these ops with outside compilation attribute is extracted out and replicated into a host-side graph. Inputs to this extract host-side graph is sent from TPU computation graph to host graph via a pair of XlaSendToHost and XlaRecvFromHost ops. Note that using tf.tpu.outside_compilation() may result in tensor transfer between TPU and CPU, leading to non-trivial performance impact.