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Return a device function to use when building a Graph for replicas.

Device Functions are used in with tf.device(device_function): statement to automatically assign devices to Operation objects as they are constructed, Device constraints are added from the inner-most context first, working outwards. The merging behavior adds constraints to fields that are yet unset by a more inner context. Currently the fields are (job, task, cpu/gpu).

If cluster is None, and ps_tasks is 0, the returned function is a no-op. Otherwise, the value of ps_tasks is derived from cluster.

By default, only Variable ops are placed on ps tasks, and the placement strategy is round-robin over all ps tasks. A custom ps_strategy may be used to do more intelligent placement, such as

For example,

# To build a cluster with two ps jobs on hosts ps0 and ps1, and 3 worker
# jobs on hosts worker0, worker1 and worker2.
cluster_spec = {
    "ps": ["ps0:2222", "ps1:2222"],
    "worker": ["worker0:2222", "worker1:2222", "worker2:2222"]}
  # Build your graph
  v1 = tf.Variable(...)  # assigned to /job:ps/task:0
  v2 = tf.Variable(...)  # assigned to /job:ps/task:1
  v3 = tf.Variable(...)  # assigned to /job:ps/task:0
# Run compute

ps_tasks Number of tasks in the ps job. Ignored if cluster is provided.
ps_device String. Device of the ps job. If empty no ps job is used. Defaults to ps.
worker_device String. Device of the worker job. If empty no worker job is used.
merge_devices Boolean. If True, merges or only sets a device if the device constraint is completely unset. merges device specification rather than overriding them.
cluster ClusterDef proto or ClusterSpec.
ps_ops List of strings representing Operation types that need to be placed on ps devices. If None, defaults to STANDARD_PS_OPS.
ps_strategy A callable invoked for every ps Operation (i.e. matched by ps_ops), that takes the Operation and returns the ps task index to use. If None, defaults to a round-robin strategy across all ps devices.

A function to pass to tf.device().

TypeError if cluster is not a dictionary or ClusterDef protocol buffer, or if ps_strategy is provided but not a callable.