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Class MirroredStrategy

Inherits From: Strategy

Defined in tensorflow/contrib/distribute/python/mirrored_strategy.py.

Mirrors vars to distribute across multiple devices and machines.

*** contrib version ***

This strategy uses one replica per device and sync replication for its multi-GPU version.

When cluster_spec is given by the configure method., it turns into the mulit-worker version that works on multiple workers with in-graph replication. Note: configure will be called by higher-level APIs if running in distributed environment.

There are several important concepts for distributed TensorFlow, e.g. client, job, 'task', cluster, in-graph replication and 'synchronous training' and they have already been defined in the TensorFlow's documentation. The distribution strategy inherits these concepts as well and in addition to that we also clarify several more concepts:

  • In-graph replication: the client creates a single tf.Graph that specifies tasks for devices on all workers. The client then creates a client session which will talk to the master service of a worker. Then the master will partition the graph and distribute the work to all participating workers.
  • Worker: A worker is a TensorFlow task that usually maps to one physical machine. We will have multiple workers with different task index. They all do similar things except for one worker checkpointing model variables, writing summaries, etc. in addition to its ordinary work.

The multi-worker version of this class maps one replica to one device on a worker. It mirrors all model variables on all replicas. For example, if you have two workers and each worker has 4 GPUs, it will create 8 copies of the model variables on these 8 GPUs. Then like in MirroredStrategy, each replica performs their computation with their own copy of variables unless in cross-replica model where variable or tensor reduction happens.


  • devices: a list of device strings.
  • num_gpus: number of GPUs. For local training, either specify devices or num_gpus. In distributed training, this must be specified as number of GPUs on each worker.
  • num_gpus_per_worker: number of GPUs per worker. This is the same as num_gpus and only one of num_gpus and num_gpus_per_worker can be specified.
  • cross_device_ops: optional, a descedant of CrossDeviceOps. If this is not set, the configure method will try to find the best one.
  • auto_shard_dataset: whether to auto-shard the dataset when there are multiple workers.
  • cross_tower_ops: Deprecated alias for cross_device_ops.



Initialize self. See help(type(self)) for accurate signature.



tf.distribute.StrategyExtended with additional methods.


Returns number of replicas over which gradients are aggregated.






Any final actions to be done at the end of all computations.

In eager mode, it executes any finalize actions as a side effect. In graph mode, it creates the finalize ops and returns them.

For example, TPU shutdown ops.


A list of ops to execute.



Any initialization to be done before running any computations.

In eager mode, it executes any initialization as a side effect. In graph mode, it creates the initialization ops and returns them.

For example, TPU initialize_system ops.


A list of ops to execute.



Runs ops in fn on each replica, with inputs from input_iterator.

When eager execution is enabled, executes ops specified by fn on each replica. Otherwise, builds a graph to execute the ops on each replica.

Each replica will take a single, different input from the inputs provided by one get_next call on the input iterator.

fn may call tf.distribute.get_replica_context() to access members such as replica_id_in_sync_group.

IMPORTANT: Depending on the DistributionStrategy being used, and whether eager execution is enabled, fn may be called one or more times (once for each replica).


  • fn: function to run. The inputs to the function must match the outputs of input_iterator.get_next(). The output must be a tf.nest of Tensors.
  • input_iterator: (Optional) input iterator from which the inputs are taken.


Merged return value of fn across replicas. The structure of the return value is the same as the return value from fn. Each element in the structure can either be PerReplica (if the values are unsynchronized), Mirrored (if the values are kept in sync), or Tensor (if running on a single replica).



Makes an iterator for input provided via input_dataset.

Data from the given dataset will be distributed evenly across all the compute replicas. We will assume that the input dataset is batched by the global batch size. With this assumption, we will make a best effort to divide each batch across all the replicas (one or more workers). If this effort fails, an error will be thrown, and the user should instead use make_input_fn_iterator which provides more control to the user, and does not try to divide a batch across replicas.

The user could also use make_input_fn_iterator if they want to customize which input is fed to which replica/worker etc.


  • dataset: tf.data.Dataset that will be distributed evenly across all replicas.


An tf.distribute.InputIterator which returns inputs for each step of the computation. User should call initialize on the returned iterator.



Returns an iterator split across replicas created from an input function.

The input_fn should take an tf.distribute.InputContext object where information about input sharding can be accessed:

def input_fn(input_context):
  d = tf.data.Dataset.from_tensors([[1.]]).repeat()
  return d.shard(input_context.num_input_pipelines,
with strategy.scope():
  iterator = strategy.make_input_fn_iterator(
  replica_results = strategy.extended.call_for_each_replica(
      replica_fn, iterator.get_next())



An iterator object that can be initialized and fetched next element.



Reduce value across replicas.


  • reduce_op: A tf.distribute.ReduceOp value specifying how values should be combined.
  • value: A "per replica" value to be combined into a single tensor.


A Tensor.



Returns a context manager selecting this Strategy as current.

Inside a with strategy.scope(): code block, this thread will use a variable creator set by strategy, and will enter its "cross-replica context".


A context manager.



Returns a copy of config_proto modified for use with this strategy.

The updated config has something needed to run a strategy, e.g. configuration to run collective ops, or device filters to improve distributed training performance.



The updated copy of the config_proto.